U.S. patent application number 11/168778 was filed with the patent office on 2006-06-08 for novel well logging method for the determination of catalytic activity.
This patent application is currently assigned to Petroleum Habitats, L.L.C.. Invention is credited to Frank D. Mango.
Application Number | 20060117841 11/168778 |
Document ID | / |
Family ID | 46322187 |
Filed Date | 2006-06-08 |
United States Patent
Application |
20060117841 |
Kind Code |
A1 |
Mango; Frank D. |
June 8, 2006 |
Novel well logging method for the determination of catalytic
activity
Abstract
The present invention relates to assays for ascribing catalytic
activity to rock samples by virtue of zero-valent transition metals
potentially being present within the sample. Embodiments of the
present invention are generally directed to novel assays for
measuring intrinsic paleocatalytic activities (k) of sedimentary
rocks for converting oil to gas and projecting the activities to
the subsurface based on the measured linear relationship between
ln(k) and temperature (T).
Inventors: |
Mango; Frank D.; (Houston,
TX) |
Correspondence
Address: |
MARK S. SOLOMON;WINSTEAD, SECHREST & MINICK P.C.
910 TRAVIS
SUITE 2400
HOUSTON
TX
77002
US
|
Assignee: |
Petroleum Habitats, L.L.C.
|
Family ID: |
46322187 |
Appl. No.: |
11/168778 |
Filed: |
June 28, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11006159 |
Dec 7, 2004 |
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11168778 |
Jun 28, 2005 |
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Current U.S.
Class: |
73/152.11 |
Current CPC
Class: |
G01N 31/10 20130101;
G01N 33/24 20130101 |
Class at
Publication: |
073/152.11 |
International
Class: |
E21B 49/00 20060101
E21B049/00 |
Claims
1. A well logging method comprising: a) obtaining sedimentary rock
samples during the course of well logging, wherein said sedimentary
rock sample potentially comprises zero-valent transition metal; and
b) analyzing said sedimentary rock samples by a method comprising:
i) a sample preparation step for exposing fresh surface of a
quantity of the sedimentary rock sample; ii) a reaction step for
exposing the sedimentary rock sample to hydrogen gas and a
hydrocarbon material in an exposure environment under assay
exposure conditions, the hydrocarbon material comprising
hydrocarbon species having two or more carbons and wherein said
exposure leads to the catalytic generation of methane gas if
zero-valent transition metal is present within the sedimentary rock
sample; and iii) an analysis step for ascertaining the presence of
methane catalytically-generated by zero-valent transition metal
potentially present in said rock sample and, if present, for
ascribing an intrinsic catalytic activity to said sedimentary rock,
wherein said analyzing yields a well log with intrinsic catalytic
rock activity as a function of well depth.
2. The well logging method of claim 1, wherein the sample
preparation step of said analyzing involves a grinding process for
exposing fresh surface of a quantity of sedimentary rock
sample.
3. The well logging method of claim 1, wherein the sample
preparation step of said analyzing is carried out under inert
conditions.
4. The well logging method of claim 1, wherein the exposure
environment of the reaction step of said analyzing is carried out
under conditions selected from the group consisting of static,
flow, and combinations thereof.
5. The well logging method of claim 1, wherein the assay exposure
conditions of said analyzing method comprise a temperature of
between about 200.degree. C. and about 450.degree. C., and a
hydrogen partial pressure of between about 0.1 torr and about 500
torr.
6. The well logging method of claim 1, wherein the reaction step of
said analyzing method comprises a duration of between about 1
minute and about 30 days.
7. The well logging method of claim 1, wherein the hydrocarbon
material used in the reaction step of said analyzing method
comprises hydrocarbon species having 2-25 carbon atoms.
8. The well logging method of claim 1, wherein said analyzing
method further comprises a separation step for separating any
catalytically-generated methane from any other hydrocarbon species
potentially present after the reaction step, wherein said
separating is done by a method selected from the group consisting
of condensing the other hydrocarbon species on a cold trap,
chromatographic means, and combinations thereof.
9. The well logging method of claim 1, wherein the analysis step of
said analyzing method involves detecting the presence of
catalytically-generated methane using a detection device selected
from the group consisting of a flame ionization detector, a
mass-selective detector, a spectroscopic detector, an electron
capture detector, a thermal conductivity detector, a residual gas
analyzer, and combinations thereof.
10. The well logging method of claim 1, wherein ascribing an
intrinsic catalytic activity to said sedimentary rock in the
analysis step of said analyzing method further comprises
determining a rate constant, k, associated with a given set of
reaction conditions and a reaction duration, as utilized in the
reaction step.
11. A well logging method comprising: a) obtaining sedimentary rock
samples potentially comprising a quantity of at least one
zero-valent transition metal during the course of well logging; and
b) analyzing said sedimentary rock samples by a method comprising
the steps of: i) processing the sedimentary rock sample to provide
freshly exposed surface; ii) exposing the sedimentary rock sample
to hydrogen gas and a quantity of hydrocarbon material in an
exposure environment under a set of assay exposure conditions, the
hydrocarbon material comprising hydrocarbon species having at least
two carbons, such that methane is catalytically generated if
zero-valent transition metal is present within the sedimentary rock
sample; and iii) analyzing the exposure environment for any
catalytically-generated methane, generated as a result of said
exposing, in order to ascertain the presence of zero-valent
transition metal within said sedimentary rock sample and, if
present, ascribing an intrinsic catalytic activity to the
sedimentary rock for the set of assay exposure conditions, wherein
said analyzing yields a well log with intrinsic catalytic rock
activity as a function of well depth.
12. The well logging method of claim 11, wherein the processing
step involves a grinding process for exposing fresh surface of the
sedimentary rock sample.
13. The well logging method of claim 11, wherein the processing
step is carried out under inert conditions.
14. The well logging method of claim 11, wherein the exposure
environment is selected from the group consisting of a static
system, a flow system, and combinations thereof.
15. The well logging method of claim 11, wherein the assay exposure
conditions comprise a temperature of between about 200.degree. C.
and about 350.degree. C., and a hydrogen partial pressure of
between about 0.1 torr and about 500 torr.
16. The well logging method of claim 11, wherein the step of
exposing comprises a duration of between about 1 minute and 30
days.
17. The well logging method of claim 11, wherein the hydrocarbon
material used in the exposing step comprises hydrocarbon species
having 2-25 carbon atoms.
18. The well logging method of claim 11, further comprising a step
of separating any catalytically-generated methane from any other
hydrocarbon species potentially present after the exposing step,
wherein the separating is done by a method selected from the group
consisting of condensing the other hydrocarbon species on a cold
trap, a chromatographic means, and combinations thereof.
19. The well logging method of claim 11, wherein the step of
analyzing involves detecting the presence of
catalytically-generated methane using a detection device selected
from the group consisting of a flame ionization detector, a
mass-selective detector, a spectroscopic detector, an electron
capture detector, a thermal conductivity detector, a residual gas
analyzer, and combinations thereof.
20. The well logging method of claim 11, wherein ascribing an
intrinsic catalytic activity to said sedimentary rock in the step
of analyzing further comprises determining a rate constant, k,
associated with a given set of reaction conditions and a reaction
duration, as utilized in the reaction step.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This Application is a continuation-in-part of U.S. patent
application Ser. No. 11/006,159, filed Dec. 7, 2004.
TECHNICAL FIELD
[0002] The present invention relates generally to well logging, and
specifically to assays for ascribing catalytic activity to rock
samples, during the course of well logging, by virtue of
zero-valent transition metals potentially being present within the
sample. An understanding of such catalytic activity is useful in
predicting the distribution of oil and gas in sedimentary basins
and thus has revolutionary potential in oil and gas
exploration.
BACKGROUND INFORMATION
[0003] Oil progresses to natural gas in deep sedimentary basins.
This process, hereafter referred to as "oil-to-gas," is believed to
be the major source of natural gas in the earth (Hunt, Petroleum
Geochemistry and Geology, 2.sup.nd ed., W. H. Freeman, New York.,
Chapter 7, 1996). Knowing when and how this process occurs is the
key to predicting the distribution of oil and gas with depth. The
conventional view is that oil thermally cracks to gas (thermal gas)
at temperatures between 150.degree. C. and 200.degree. C., the
observed temperature range where most oil-to-gas occurs. Various
kinetic models (thermal models) based on this theory have had only
marginal success, however, and there are glaring contradictions.
Oil, for example, is found in deep reservoirs (>20,000 ft) at
temperatures where it should not exist (Paine et al., "Geology of
natural gas in South Louisiana," American Association of Petroleum
Geologists, Memoir 9, Volume 1, Natural Gases of North America,
Beebe, B. W., Editor, 376-581, 1968; Price, "Thermal stability of
hydrocarbons in nature: Limits, evidence, characteristics, and
possible controls," Geochimica et Cosmochimica Acta, 57:3261-3280,
1993), and giant deposits of so-called thermal gas exist in shallow
reservoirs that cannot be explained by the thermal model without
invoking long-range migration from deeper horizons (Littke et al.,
"Gas generation and accumulation in the West Siberian basin," AAPG
Bull., 83:1642-1665, 1999).
[0004] There is now mounting scientific evidence against the
thermal models. From a series of laboratory experiments under
realistic conditions (Domine et al., "Towards a new method of
geochemical kinetic modeling: implications for the stability of
crude oils," Organic Geochemistry, 28:597-612, 1998; Domine et al.,
"Up to what temperature is petroleum stable? New insights from 5200
free radical reaction model," Organic Geochemistry, 33:1487-1499,
2002), evidence now suggests that oil should not crack to gas over
geologic time at temperatures between 150.degree. C. and
200.degree. C., the range within which most so-called thermal gas
is formed, a conclusion supported by numerous other studies
(Mallinson et al., "Detailed chemical kinetics study of the role of
pressure in butane pyrolysis," Industrial & Engineering
Chemistry, Research, 31:37-45, 1992; Burnham et al., "Unraveling
the kinetics of petroleum destruction by using 1,2.sup.13C
isotopically labeled dopants," Energy & Fuels, 9:190-191, 1995;
Jackson et al., "Temperature and pressure dependence of
n-hexadecane cracking," Organic Geochemistry, 23:941-953, 1995).
Moreover, the gas produced in oil cracking is severely depleted in
methane and does not resemble natural gas as it is distributed in
the earth (Mango, "The origin of light hydrocarbons," Geochimica et
Cosmochimica Acta, 64:1265-1277, 2001).
[0005] Catalysis by transition metals is an alternative explanation
for oil-to-gas (Mango, "Transition metal catalysis in the
generation of petroleum and natural gas," Geochimica et
Cosmochimica Acta. 56:553-555, 1992), and there is experimental
evidence supporting it. Crude oils are converted to gas over
zero-valent transition metals (ZVTM) (e.g., Ni, Co, and Fe) under
moderate laboratory conditions (150-200.degree. C.) and the
products are identical to natural gas in molecular and isotopic
composition (Mango and Hightower, "The catalytic decomposition of
petroleum into natural gas," Geochimica et Cosmochimica Acta,
61:5347-5350, 1997; Mango and Elrod, "The carbon isotopic
composition of catalytic gas: A comparative analysis with natural
gas," Geochimica et Cosmochimica Acta, 63:1097-1106, 1998; Mango,
"The origin of light hydrocarbons," Geochimica et Cosmochimica
Acta, 64:1265-1277, 2000).
[0006] The above-described experiments are highly relevant to the
generation of natural gas in sedimentary basins. Transition metals
are common in sedimentary rocks (Boggs, S., Jr., Principles of
Sedimentology and Stratigraphy, 2.sup.nd ed., Prentice-Hall, Inc.,
NJ, pages 165 & 195, 1995), and could become catalytically
active (i.e., reduced to zero-valencies) given the reducing
conditions of petroleum habitats (Mango, "The light hydrocarbons in
petroleum: a critical review," Organic Geochemistry, 26:417-440,
1997; Mango, "The origin of light hydrocarbons," Geochimica et
Cosmochimica Acta, 64:1265-1277, 2000; Medina et al., "Low
temperature iron- and nickel-catalyzed reactions leading to coalbed
gas formation," Geochimica et Cosmochimica Acta, 64:643-649, 2000;
Seewald, "Organic-inorganic interactions in petroleum-producing
sedimentary basins," Nature, 426:327-333, 2003). All requisites are
in place: transition metal oxides in sufficient amounts to promote
the reaction and enough hydrogen to activate them to zero-valencies
and drive the reaction at subsurface temperatures (Mango, "The
origin of light hydrocarbons," Geochimica et Cosmochimica Acta,
64:1265-1277, 2000).
[0007] Catalysis may be the source of the huge gas deposits in the
Gulf Coast geosyncline of south Louisiana (Paine et al., "Geology
of natural gas in South Louisiana," American Association of
Petroleum Geologists, Memoir 9, Volume 1, Natural Gases of North
America, Beebe, B. W., Editor, 376-581, 1968). Oil is generally
found at depths above 10,000 feet and gas is generally found below
such depths, consistent with the thermal model. However, gas
probabilities are also a strong function of reservoir composition:
low in pure sandstone and high in sandstones interbedded with
outer-neritic shales that are often enriched in transition metals
(Mann and Stein, "Organic facies variations, source rock potential,
and sea level changes in Cretaceous black shales of the Quebrada
Ocal, Upper Magdalena Valley, Colombia," American Association of
Petroleum Geologests Bulletin, 81:556-576, 1997; Cruickshank and
Rowland, "Mineral deposits at the shelfbreak," SEPM Special
Publication No. 33, 429-436, 1983).
[0008] Given high enough temperatures and hydrogen partial
pressures at depth, transition metals in outer-neritic shales could
attain zero-valencies. Thus activated, in-reservoir catalytic
oil-to-gas would commence. In this instance, the important factor
for predicting oil or gas in reservoir rocks is the presence of
ZVTM in sufficient concentrations to promote catalytic oil-to-gas.
A rock assay specific to ZVTM in outcrop rocks, cuttings, or core
samples would thus be a powerful exploration tool for reservoirs
that either preserve oil (no ZVTM) or convert it to gas (with
ZVTM).
[0009] Other than commonly assigned co-pending U.S. patent
application Ser. No. 10/830,266, Applicant is unaware of any
practical tests for trace amounts (i.e., ppb or less) of ZVTM in
sedimentary rocks. Most rock methods use spectroscopic techniques,
such as atomic absorption (AA) spectroscopy or inductively-coupled
plasma atomic emission spectroscopy ICP-AES), that do not
differentiate between oxidation states. Nickel valency speciation
has been achieved by X-ray absorption fine-structure spectroscopy
using the National Synchrotron Light Source at Brookhaven National
Laboratory (NY) and with anodic stripping voltammetry (Galbreath et
al., "Chemical speciation of Nickel in residual oil ash," Energy
& Fuels, 12:818-822, 1998), but the complexities of these
methods preclude their use in routine rock analysis.
[0010] In addition to the above, a convenient assay for the direct
determination of intrinsic paleocatalytic activity within
sedimentary rock, for the purpose of making predictions in oil and
gas exploration, particularly during the course of well logging,
would also be highly desirable. Applicant is unaware of any assays
that measure the intrinsic catalytic activity of rocks to convert
oil to gas under subsurface conditions.
BRIEF DESCRIPTION OF THE INVENTION
[0011] Embodiments of the present invention are generally directed
to novel assays for measuring intrinsic paleocatalytic activities
(k) of sedimentary rocks for converting oil to gas and projecting
the activities to the subsurface based on the measured linear
relationship between ln(k) and temperature (T), and to well logging
protocols employing such assays. Sedimentary rocks sufficiently
catalytic to convert 90+% of their contained oil to gas at
temperature T for oil residence time t are designated "gas
habitats." Sedimentary rocks that cannot convert 90% of their oil
to gas in time t are designated "oil habitats." Some embodiments of
the present invention include approximating the intrinsic
paleocatalytic activity k(T) of an un-drilled reservoir at
temperature T from the linear relationship between ln k and T for a
drilled reservoir rock that is genetically similar to the
un-drilled reservoir rock. Some embodiments of the present
invention enable the prediction of oil or gas in an un-drilled
reservoir at temperature T for residence time t based on an
approximation of its intrinsic paleocatalytic activity k(T) taken
from the ln k vs T curve for a genetically related reservoir distal
from the undrilled reservoir. Some embodiments of the present
invention provide oil-gas habitat maps of stratigraphic rock units
contouring the interface between oil and gas habitats based on the
intrinsic paleocatalytic activities k(T) or their approximations at
various locations in a basin. Some embodiments of the present
invention enable prediction of the distribution of oil and gas in
various reservoirs within a stratigraphic rock unit based on the
oil-gas habitat map of that rock unit. Some embodiments of the
present invention enable prediction of the distribution of oil and
gas in various reservoirs in a stratigraphic rock unit proximal to
a stratigraphic source rock unit within which oil and gas is
generated and expelled into reservoirs within the proximal rock
unit based on the oil-gas habitat map of the source rock unit. Some
embodiments of the present invention enable the prediction of the
conversion of oil to gas within a conduit rock along an oil
migration pathway based on its intrinsic paleocatalytic activity
k(T) or an approximation thereof at temperature T and the residence
time t that migrating oil remains in the conduit.
[0012] Assays of the present invention typically comprise the
following general steps: 1) processing rock sample potentially
comprising zero-valent transition metal (ZVTM) so as to provide
freshly exposed surface under conditions that preserve intrinsic
catalytic activity; 2) exposing the rock sample to a mixture of
hydrogen gas and hydrocarbon material under appropriate conditions
such that the hydrocarbon material undergoes catalytic
decomposition yielding catalytically-generated methane (CGM) if
ZVTM is present; and 3) detecting the presence of any CGM. The
presence of CGM confirms intrinsic catalytic activity imparted by
ZVTM present in the sample. Generally, such rock samples are
sedimentary rock samples, and processing (e.g., grinding) to
provide freshly exposed surface is generally carried out in an
inert, non-oxidizing atmosphere. Similarly, the exposure of such
rock samples to hydrogen and hydrocarbon reactants is typically
carried out in an inert, non-oxidizing atmosphere.
[0013] Depending on the embodiment, such above-described methane
detection can provide qualitative and/or quantitative analysis of
the sample. In some embodiments, when assaying a rock as described
above, the qualitative analysis of catalytically-generated methane
is sufficient to make predictive assessments as to the content
(i.e., primarily oil or primarily gas) of the reservoir from where
the analyzed sample was extracted (source reservoir) or of any
other reservoir that is genetically similar to the source
reservoir. Two reservoirs are genetically similar if their overall
organic and inorganic compositions are similar and if their genetic
depositional environments are similar. In the Louisiana gas fields
cited above, for example (Paine et al., "Geology of natural gas in
South Louisiana," American Association of Petroleum Geologists,
Memoir 9, Volume 1, Natural Gases of North America, Beebe, B. W.,
Editor, 376-581, 1968), the various reservoirs comprising
interbedded sandstone and outer-neritic shales are genetically
related, as defined here, for they share a common outer-neritic
depositional environment. Outer-neritic environments include
deposition along shelf breaks that are often highly reducing,
organic rich sediments with high concentrations of transition
metals (Cruickshank, M. J., and Roland, T. J. Jr., "Mineral
deposits at the Shelfbreak," SEPM Special Publication No. 33,
429-436, 1983). Sandstone reservoirs interbedded with outer-neritic
shale are not genetically similar to sandstone reservoirs
interbedded with inner-neritic shale because outer-neritic and
inner-neritic depositional environments are dissimilar,
particularly with respect to metal concentrations and, therefore,
their respective catalytic activities. In some or other
embodiments, a more quantitative analysis provides greater insight
into the content of such a source reservoir.
[0014] In some embodiments, upon quantitatively and/or
qualitatively analyzing the catalytically-generated methane, the
rock sample is ascribed an intrinsic catalytic activity. Such an
intrinsic catalytic activity can then be projected onto the
reservoir from where the sample was extracted (i.e., the source
reservoir) or any other genetically similar reservoirs, to
determine whether the intrinsic catalytic activity is sufficient to
enable significant oil-to-gas conversion over geologic timescales
and under environmental conditions within the reservoir. By
processing the rock samples under inert conditions and exposing the
processed rock samples to reactants under non-oxidizing conditions,
the intrinsic catalytic activity so determined is equatable with
the native catalytic activity in the reservoir.
[0015] Accordingly, the present invention, and the knowledge of
catalytic activity gained thereby, is useful in predicting whether
a particular reservoir will be likely to contain predominantly oil
or predominantly gas.
[0016] In some embodiments, an analysis of the amount of methane
produced under a given set of conditions and in a given timeframe
generates a rate constant, k, for such a reaction at a particular
reaction temperature. If such rate constants are determined for two
or more such reaction temperatures, a linear plot of ln k versus T
(ln k vs. T plot) can be generated. Such ln k vs. T plots can be
extrapolated to yield a rate constant for the source reservoir that
is indicative of its paleocatalytic activity. With such a source
reservoir rate constant, it is possible to determine the extent and
significance of oil-to-gas processes within said reservoir under
sub-surface conditions over geologic timescales by integrating k dt
over the temperature interval in the subsiding basin.
[0017] The foregoing has outlined rather broadly the features of
the present invention in order that the detailed description of the
invention that follows may be better understood. Additional
features and advantages of the invention will be described
hereinafter which form the subject of the claims of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] For a more complete understanding of the present invention,
and the advantages thereof, reference is now made to the following
description taken in conjunction with the accompanying drawings, in
which:
[0019] FIG. 1 is a flow diagram depicting the steps involved in a
rock assay in accordance with embodiments of the present
invention;
[0020] FIG. 2 depicts an exposure chamber, operable for both static
and flow exposures, in accordance with some embodiments of the
present invention;
[0021] FIG. 3 depicts a reaction system in accordance with some
embodiments of the present invention;
[0022] FIG. 4 depicts another reaction system in accordance with
some embodiments of the present invention;
[0023] FIG. 5 is a ln(k) vs. T curve generated in accordance with
some embodiments of the present invention;
[0024] FIG. 6 is a plot of an application in which gas is correctly
predicted in one basin based on a rock assay from a genetically
similar reservoir rock from a different basin in accordance with
some embodiments of the present invention;
[0025] FIG. 7 is a plot of the differences between thermal cracking
and the current invention in their capacities to explain oil-to-gas
as seen in sedimentary basins;
[0026] FIG. 8 illustrates a scenario wherein oil converts to gas
while migrating from source rock to reservoir rock through conduits
constituting gas habitats;
[0027] FIG. 9 shows, graphically, data for the samples in Table 1,
wherein rate acceleration index A (log (.alpha.)) for the 10 rock
samples and the synthetic sample representing a sandstone
associated with 10% outer-neritic Monterey shale (% ONS), is
provided;
[0028] FIG. 10 depicts hydrocarbon composition as a function of
depth within the various reservoirs shown in Table 1; and
[0029] FIG. 11 depicts nickel concentrations in various sandstones
from North America.
DETAILED DESCRIPTION OF THE INVENTION
[0030] In the following description, specific details are set forth
such as specific quantities, sizes, etc. so as to provide a
thorough understanding of embodiments of the present invention.
However, it will be apparent to those skilled in the art that the
present invention may be practiced without such specific details.
In many cases, details concerning such considerations and the like
have been omitted inasmuch as such details are not necessary to
obtain a complete understanding of the present invention and are
within the skills of persons of ordinary skill in the relevant
art.
[0031] Referring to the drawings in general, it will be understood
that the illustrations are for the purpose of describing a
particular embodiment of the invention and are not intended to
limit the invention thereto.
[0032] The present invention concerns paleocatalysis, a new field
concerning catalysis proceeding over geologic time. Because these
reactions are orders of magnitude slower than traditional catalytic
reactions, they present formidable analytical challenges. A typical
sedimentary rock can have as little as parts-per-billion (ppb)
levels of zero-valent transition metal (ZVTM) and be effective
paleocatalysts in the subsurface. In contrast, typical industrial
catalyst have metal concentrations in the parts-per-hundred range.
Industrial catalytic reactions will take minutes where
paleocatalytic reactions will take millions of years. The
analytical challenge is this: a reservoir rock that converts oil to
gas in two million years at 150.degree. C., will generate only
.about.7.times.10.sup.-7 g gas/(g rock hr) at 280.degree. C. In one
embodiment of the invention disclosed herein, this problem is
addressed by sending 100% of the product (catalytic methane)
directly into the analytical detector, typically a flame ionization
detector (FID), to maximize accuracy and sensitivity. Using such
techniques, it is possible to accurately measure paleoactivities as
low as 10.sup.-9 g gas/(g rock hr) at reasonable laboratory
temperatures. In another embodiment of the invention, unusually
stable light hydrocarbons (ethane and propane, for example) are
used as reactants so that high-temperature assays can be employed
to boost product yield without contaminating the product with
thermal cracking products. In still other embodiments, the present
invention is directed at using such techniques in concert with well
logging. The Applicant is unaware of other analytical procedures
for determining paleocatalytic activities with this degree of
sensitivity and accuracy. The Applicant is also unaware of other
analytical procedures for determining intrinsic catalytic
activities of reservoir rocks, activities that realistically
project to subsurface activities under natural conditions. The
Applicant is also unaware of methods for predicting oil or gas in
various reservoirs based on their intrinsic catalytic
activities.
[0033] While most of the terms used herein will be recognizable to
those of skill in the art, the following definitions are
nevertheless put forth to aid in the understanding of the present
invention.
[0034] "Sedimentary rock," as defined herein, refers generally to
rock formed by the accumulation and cementation of mineral grains
transported by wind, water, or ice to the site of deposition or
chemically precipitated at the depositional site. The sedimentary
rocks specific to this invention include reservoir rocks, source
rocks, and conduit rocks. Reservoir rocks are rocks that trap and
sequester migrating fluids. "Source rocks" are rocks within which
petroleum is generated and either expelled or retained. "Conduit
rocks", as defined herein, are rocks through which petroleum
migrates from its source to its final destination. A "sedimentary
basin," as defined herein, is an accumulation of a large thickness
of sediment, as in sedimentary rock. "Outcrop rocks," as defined
herein, generally refers to segments of bedrock exposed to the
atmosphere.
[0035] A "target reservoir", as defined herein, refers to a
drilling prospect in a sedimentary basin comprising a sedimentary
rock believed to be a reservoir containing economic quantities of
oil or gas.
[0036] A "gas habitat," as defined herein, refers to a "sedimentary
rock" within a sedimentary basin that is sufficiently catalytic to
convert 90% or more of its contained oil to gas over the specified
time-temperature residence interval, typically 10 million years
(Ma) for basins where subsidence rates place reservoirs at
temperature T.sub.r (.+-.10.degree. C.) for 10 Ma. The rate of
oil-to-gas conversion at T, k(T), is determined from the linear ln
k vs. T equation, like that shown in the plot of FIG. 5 determined
for a rock from multiple rock assays at different temperatures.
Alternatively, k(T) can be approximated from a single rock assay
from the Arrhenius equation in Mango, "Transition metal catalysis
in the generation of natural gas," Organic Geochemistry,
24:977-984, 1996, assuming Ni is equivalent to ZVTM. A temperature
vs. time t plot, T vs log(Ma), like the "Ni Equivalent" curves in
FIGS. 6 & 7, define the gas habitat field to the right of the
temperature-time curve. These curves, or curves for genetically
similar reservoir rocks, define where in time-temperature space a
given rock will have a high probability of containing gas.
[0037] An "oil habitat," as defined herein, refers to a
"sedimentary rock" within a sedimentary basin that is not
sufficiently catalytic at a specified basin temperature T to
convert 90% or more of its contained oil to gas over a specified
period of geologic time t. The temperature-time curve for a
specified rock defines the oil habitat field for that reservoir
rock to the left of the curve. Thus, there is a high probability of
finding gas in a reservoir targeted for drilling designated a "gas
habitat" and oil in a reservoir targeted for drilling designated an
"oil habitat." See commonly assigned co-pending U.S. patent
application Ser. No. 10/830,266. Conversely, such ZVTM content can
be inferred indirectly through a determination of intrinsic
catalytic activity.
[0038] "Oil-to-gas," as defined herein, refers to geological
processes in which crude oil (higher molecular weight hydrocarbons)
converts into natural gas (lower molecular weight hydrocarbons). In
the "thermal model," as defined herein, which is the generally
accepted but imperfect model, oil-to-gas proceeds through thermal
cracking and is thus a function of reservoir temperature and
geologic time. Oil-to-gas in the "catalytic model," as defined
herein, refers to a newer, but experimentally confirmed process,
whereby oil is catalytically converted to gas with the aid of ZVTM.
See Mango et al., "Role of transition-metal catalysis in the
formation of natural gas," Nature, 368:536-538, 1994. The reservoir
rock in the catalytic model is an active agent in oil-to-gas and a
passive agent in the thermal model. Concentrations of ZVTM control
oil-to-gas rates in the catalytic model and the kinetic parameters
associated with thermal cracking control oil-to-gas rates in the
thermal model. The two models have profoundly different predictive
powers in oil and gas exploration.
[0039] An "active reservoir," as defined herein and in accordance
with the oil-to-gas catalytic model, refers to a reservoir in which
the surrounding sedimentary rock comprises at least a critical
concentration of ZVTM as defined above. An "inactive reservoir," as
defined herein and in accordance with the oil-to-gas catalytic
model, refers to a reservoir with a less than critical
concentration of ZVTM.
[0040] "Transition metal," as defined herein, refers to metals
comprised of elements of the "d-block" of the Periodic Table.
Specifically, these include elements 21-29 (scandium through
copper), 39-47 (yttrium through silver), 57-79 (lanthanum through
gold), and all known elements from 89 (actinium) on. Iron (Fe),
cobalt (Co), and nickel (Ni) all have special relevance, however,
due to their established catalytic activity. See Mango and
Hightower, "The catalytic decomposition of petroleum into natural
gas," Geochimica et Cosmochimica Acta, 61:5347-5350, 1997.
[0041] "Zero-valent transition metal(s)," as used herein, are
transition metals in their zero-oxidation (i.e., neutral) state.
This includes metal compounds in which a fraction of the metal in
the compound retains valence electrons. For example, nickel does
not retain valence electrons in NiO and NiS by this definition
because it surrenders both of its valence electrons to oxygen and
sulfur, respectively, but compounds like Ni.sub.xO and Ni.sub.yS,
in which x and y are >1, are potentially catalytic because a
fraction of Ni (i.e., (x-1) and (y-1), respectively) retains
valence electrons and thus a fraction of the Ni in these compounds
is zero-valent. The transition metals in the minerals pentlandite
((Fe,Ni).sub.9S.sub.8), skutterodite ((Co,Ni)As.sub.3), and
ullmannite (NiSbS) are viewed as containing zero-valent metals by
this definition and are, therefore, potentially catalytic.
[0042] "Quantitative analysis," as defined herein, generally refers
to the determination of species quantity and/or concentration with
a high level of precision. In contrast, "qualitative analysis"
generally describes a lower level of precision, but still at a
level capable of being used for predictive determinations.
[0043] An "assay," according to the present invention, generally
refers to a quantitative or qualitative analysis (i.e., evaluation)
of a sample. To assay a sample is to subject it to quantitative or
qualitative analysis.
[0044] "Catalytically-generated methane," abbreviated "CGM" and as
used herein, refers to methane generated via the catalytic
decomposition of hydrocarbon material. Such catalytic
decomposition, in the assays of the present invention, is induced
via the catalytic activity of rock samples comprising ZVTM, and in
accordance with the catalytic oil-to-gas model. Without such ZVTM
present in the rock sample being assayed, no CGM will be
produced.
[0045] "Catalytic activity," as defined herein, refers to the
propensity of a catalyst to catalyze the catalytic decomposition of
hydrocarbons to form CGM. "Intrinsic catalytic activity" refers to
an unadulterated catalytic activity (i.e., such activity has not
been compromised by exposure to oxygen) of a rock sample (source
rock)) that is equitable to the native catalytic activity of the
reservoir or reservoiric region from where the sample was
extracted.
[0046] "Genetically similar," as defined herein, refers to rocks
that are similar in overall organic and inorganic composition and
which were deposited under similar depositional environments.
Genetically similar rocks can be expected to contain similar
concentrations of ZVTM and have similar levels of catalytic
activity.
[0047] "Paleocatalysis," as defined herein, refers to catalytic
reactions that proceed over geologic time. The paleocatalytic
reaction specific to this invention is the conversion of oil to
natural gas catalyzed by ZVTM. Like conventional commercial
catalysts, paleocatalysts will express different levels of
catalytic activity depending on how they were synthesized. Thus,
one rock possessing ZVTM need not be similar in catalytic
properties to another rock possessing similar concentrations of
ZVTM. Genetically similar rocks, on the other hand, having been
naturally synthesized under similar circumstances, should be
similarly catalytic.
[0048] "Habitat maps," as defined herein, refers to maps of
stratigraphic rock units showing the lines of intersection between
oil and gas habitats as defined herein.
[0049] A "well log," as defined herein, is a record of the measured
or computed physical characteristics of the rock section
encountered in a well, typically as a function of depth. See
Dictionary of Geological Terms, 3.sup.rd edition, R. L. Bates and
J. A. Jackson (eds.), Anchor Books, New York: 1984. "Well logging,"
as defined herein, is the process of acquiring such well
log(s).
[0050] Embodiments of the present invention are generally directed
to novel assays for ZVTM, and to methods of assaying rock samples
potentially comprising ZVTM for intrinsic catalytic activity. The
novel assays of the present invention are generally methods or
processes for quantitatively and/or qualitatively evaluating the
catalytic activity of, and the presence of ZVTM in, rock samples.
Furthermore, application of such assays to oil and gas exploration
provides for revolutionary advances in the predictability of oil
and gas deposits based upon observed catalytic activity and/or the
levels of ZVTM present in the surrounding sedimentary rock.
[0051] Referring to FIG. 1, assays of the present invention
generally comprise the following steps: (Step 1001) processing rock
sample potentially comprising ZVTM so as to provide freshly exposed
surface; (Step 1002) exposing the rock sample to a mixture of
hydrogen gas and hydrocarbon material (reaction mixture) under
appropriate conditions such that the hydrocarbon material undergoes
decomposition yielding catalytically-generated methane if ZVTM is
present; and (Step 1003) detecting the presence of any
catalytically-generated methane (CGM). The presence of CGM confirms
intrinsic activity imparted by ZVTM in the sample. Generally, such
rock samples are sedimentary rock samples. Typically, a heat
extraction step is carried out between Steps 1001 and 1002 to
extract from the sample any non-catalytically produced hydrocarbons
(including methane) that could contaminate the final
catalytically-generated methane (CGM) product. In some embodiments,
a separating step is carried out between Steps 1002 and 1003,
wherein CGM is separated from other hydrocarbons. Additionally, in
some embodiments, the amount of CGM generated in Step 1002 is
measured and quantified.
[0052] Typically, the rock sample is obtained from a reservoir of
interest such that information about said rock, acquired through
the assays of the present invention, is equatable to the reservoir
itself (the source reservoir) and any other genetically similar
reservoir. There is great flexibility in the quantity of rock
sample used in the assays of the present invention. Generally, the
amount of rock sample used is between about 0.1 g and about 20 g,
typically between about 0.5 g and about 10 g, and more typically
between about 0.5 g and about 5 g. In some embodiments, "side wall"
rock samples are selected for the assay because such samples are
less likely to be contaminated by oxygen. In some embodiments,
outcrop rock are selected for the assay because such samples, if
sufficiently large (a few mm in diameter), retain an inner core
uncontaminated by oxidation.
[0053] Processing rock sample potentially comprising ZVTM so as to
provide freshly exposed surface generally comprises a grinding
technique, wherein the rock sample is ground. Such grinding can be
accomplished with mortar and pestle by hand or mechanically milling
by placing the rock samples in a closed brass cylinder containing a
brass ball and shaking the cylinder with a mechanical `paint
shaker` for a short period of time, typically 15 minutes.
Mechanical rock crushing in brass prevents sample contamination by
transition metals in steel cylinders and balls. Because mechanical
rock crushing can generate heat, and thus promote the oxidation of
ZVTM if mechanical crushing is carried out in air, it is best to
seal the cylinder in an inert atmosphere free of oxygen.
[0054] There can be considerable variability in the mesh size and
surface area of the particles of which the ground rock sample is
comprised. In some embodiments, the ground sample is sieved to
include or exclude particles of a particular size or range of
sizes.
[0055] Generally, the above-described processing to provide freshly
exposed surface is generally carried out in an inert, non-oxidizing
atmosphere. Suitable inert, non-oxidizing atmospheres include, but
are not limited to, inert gases like Ar, He, N.sub.2, Kr, and
combinations thereof. In some embodiments, the inert gases are
scrubbed of oxygen (O.sub.2) by passage through a special filter.
Such filters typically comprise metals which reacts with the
O.sub.2.
[0056] Care is generally taken to ensure that the processed samples
do not contact O.sub.2 until after they have been exposed to
hydrogen/hydrocarbon in the assay reaction. If such processed
samples do come into contact with O.sub.2, any zero-valent metals
potentially present in such samples will be at least partially
oxidized, and any catalytic activity that the rock might possess
will be reduced below the native catalytic activity. This is
because the catalytic oil-to-gas process is highly specific to
zero-valent metals and the active sites on the surfaces of
zero-valent metals are extremely sensitive to destruction by
oxygen. Such oxidation will lead to catalytic activity
determinations for the rock sample that are below that for the
source reservoir. Consequently, any projection of such determined
activity onto the source reservoir will be underestimated.
[0057] The step of exposing can be carried out in either a static
or flow system. Referring to FIG. 2, in an exemplary static system,
a rock sample 201 is placed in a reaction chamber 200 capable of
being heated with a heating element 202. Hydrogen reactant gas is
introduced through inlet 203 with valve 204 open and valve 205
open. Hydrocarbon reactant is introduced through inlet 206 with
valve 207 open. With sufficient reactants in the reaction chamber
200, the system is closed by closing valves 204, 205 and 207. The
closed chamber is then heated to the desired level (determined via
thermocouple 208). After sufficient time, valves 204 and 205 are
opened allowing hydrogen gas to pass through the reaction chamber
200 carrying the product gases, potentially comprising CGM, out of
the reaction chamber through exit 209 to a separator that removes
CGM from all higher hydrocarbons and sends it to a detector for
analysis. Alternatively, such a system could be run in a flow mode,
wherein valves 204, 205, and 207 are kept open. Such flow
scenarios, however, generally require detection techniques with
greater sensitivity than that required for the static systems.
[0058] Exposure duration, i.e., the time in which a reaction
mixture is in contact with a processed rock sample, can vary
considerably. Generally, such exposure duration is between about 1
minute and about 30 days, typically between about 1 minute and
about 24 hours, and more typically between about 1 minute and about
1 hour.
[0059] Exposure conditions include variables such as temperature
and pressure. The temperature at which the step of exposing is
carried out is generally between about 150.degree. C. and about
450.degree. C., typically between about 200.degree. C. and about
350.degree. C., and more typically between about 220.degree. C. and
about 300.degree. C. The hydrogen gas partial pressure at which the
step of exposing is carried out is generally between about 1 torr
and about 100 torr, typically between about 1 torr and about 50
torr, and more typically between about 1 torr and about 5 torr.
These temperatures are generally above that typically found in
source reservoirs.
[0060] Typically, when exposing a rock sample to a mixture of
hydrogen gas and hydrocarbon material, the hydrocarbon to hydrogen
gas ratio can be between about 1:1000 and about 1000:1, as
determinable by their partial pressures. Because this catalytic
reaction is zero-order, its rate is independent of reactant
concentrations beyond sufficient concentrations to saturate the
active sites dispersed over the rock surface. Reactant
concentrations (partial pressures of hydrogen and hydrocarbon) are,
therefore, critical only below saturation concentrations. To obtain
accurate assays, it is essential to maintain hydrogen and
hydrocarbon concentrations above saturation. In some embodiments,
one or both of the hydrocarbon material and hydrogen gas are
optionally scrubbed of oxygen prior to being introduced into the
reaction chamber. In some embodiments, the hydrocarbon material and
hydrogen gas are pre-mixed prior to being introduced into the
reaction chamber through inlet 207, while in other embodiments they
are mixed within the reaction chamber by mixing the rock sample
with hydrocarbon prior to placing the sample into the reaction
chamber 200 or by injecting hydrogen through inlet 203 and
hydrocarbon through inlet 206 separately.
[0061] The hydrocarbon material typically comprises one or more
gaseous hydrocarbon species, but may also comprise liquid
hydrocarbon material. In some embodiments, a quantity of a single
hydrocarbon material is used, but mixtures of hydrocarbon species
can also be employed. Typically, the hydrocarbon material comprises
hydrocarbon species having between two and eighteen carbon atoms.
Such hydrocarbon species can be aliphatic and/or aromatic and may
contain one or more heteroatoms (e.g., O, N, S).
[0062] As mentioned above, in some embodiments, between the steps
of exposing and detecting, a separating step is employed. Such
separating steps can be used to separate any catalytically
generated methane, potentially produced in the exposing step, from
other hydrocarbon species. In some embodiments, this separation
involves a cold trap (e.g., a liquid nitrogen trap) that condenses
all other hydrocarbons, but allows methane to pass through and on
to the detector/analyzer. In other embodiments, a chromatographic
separation is employed. In such latter embodiments, a gas
chromatographic column is usually employed, the column comprising
any one of a number of suitable stationary phases suitable for the
separation of methane from heavier hydrocarbons.
[0063] In some embodiments, detecting the presence of CGM in the
above-described assay involves a detection device selected from the
group consisting of a flame ionization detector (FID), a
mass-selective detector, a spectroscopic detector, an electron
capture detector, a thermal conductivity detector, a residual gas
analyzer, and combinations thereof.
[0064] Depending on the embodiment, such above-described methane
detection can provide qualitative and/or quantitative analysis. In
some embodiments, when assaying a rock as described above, the
qualitative analysis of catalytically-generated methane is
sufficient to make predictive assessments as to the content (i.e.,
primarily oil or primarily gas) of a reservoir from where the
analyzed rock was extracted. In some or other embodiments, a more
quantitative analysis provides greater insight into the content of
the source reservoir.
[0065] In some embodiments, oil or gas predictions can be made on a
reservoir other than the source reservoir if the two reservoirs
share a common depositional environment and thus can be expected to
be similar in overall composition and ZVTM content. Such reservoirs
are referred to here as "genetically similar" reservoirs. This
application is particularly powerful because it can potentially
predict oil or gas in an un-drilled reservoir based on analysis of
rocks taken from drilled genetically similar reservoirs distal from
the un-drilled reservoir. In other embodiments, stratigraphic units
can be mapped for catalytic activity by assaying representative
rock samples covering the various depositional environments
throughout the stratigraphic units. From the paleocatalytic
activities at depth and residence times, habitat maps can be
constructed showing where in these units oil will convert to gas
and where it should not, thus where in the basin the probability
for oil is high (oil habitats) and where it is low (gas habitats).
Habitat maps could be particularly useful in mapping sedimentary
rocks that are particularly rich in transition metals such as the
outer-neritic shales (Cruickshank, M. J., and Roland, T. J. Jr.,
"Mineral deposits at the Shelfbreak," SEPM Special Publication No.
33, 429-436, 1983; Mann, U., and Stein, R. "Organic facies
variations, source rock potential, and sea level changes in
Cretaceous black shales of the Quebrada Ocal, Upper Magdalena
Valley, Colombia," American Association of Petroleum Geologests,
Bulletin 81:556-576, 1997) and the so-called black shales (Rimmer,
S. M., "Geochemical paleoredox indicators in Devonian-Mississippian
black shales, Central Appalachian Basin (USA)," Chemical Geology
206:373-391, 2004.).
[0066] In some embodiments, upon quantitatively and/or
qualitatively analyzing the catalytically-generated methane, the
rock sample is ascribed an intrinsic catalytic activity. Such an
intrinsic catalytic activity can then be projected onto the
reservoir (from where the rock was extracted (i.e., the source
reservoir), or a genetically similar reservoir, so as to determine
whether the intrinsic catalytic activity is sufficient to enable
significant oil-to-gas conversion over geologic timescales (e.g.,
eons) and under environmental conditions (temperatures and
pressures) within the reservoir. By processing the rock samples
under inert conditions, thereby precluding oxidation of the active
sites in any ZVTM potentially present, the intrinsic catalytic
activity so determined is equatable to the native activity within
the reservoir.
[0067] The usefulness of many such above-described embodiments lies
in using the knowledge of catalytic activity to predict whether a
particular reservoir will be likely to contain predominantly oil or
predominantly gas, based upon the catalytic activity of the
reservoir, as determined from analyzing a rock sample obtained from
said reservoir or a genetically similar reservoir, with an assay of
the present invention. Such assays can permit the designation of a
reservoir as being a gas habitat or an oil habitat, in accordance
with the oil-to-gas model, with a direct measurement of the
catalytic activity of source rock from said reservoir or from a
genetically similar reservoir.
[0068] In some embodiments, an analysis of the amount of methane
produced under a given set of conditions and a given timeframe
permits the generation of a rate constant, k, for such a reaction
for a particular reaction temperature. If such rate constants are
determined for two or more such reaction temperatures, a plot of ln
k versus T (ln k vs. T plot) can be generated. Such ln k vs. T
plots can be extrapolated to yield a rate constant for the source
reservoir or genetically similar reservoir. With such a source
reservoir rate constant, it is possible to determine the extent and
significance of oil-to-gas processes within said reservoir over
geologic timescales.
[0069] Predictive determinations of oil or gas in a reservoir are
based upon the required presence of ZVTM for catalytic conversion
of heavier hydrocarbons to natural gas (Mango, "The origin of light
hydrocarbon," Geochimica et Cosmochimica Acta, 64:1265-1277, 2000).
For example, outer-neritic shales (black shales) are one of the
richest sources of transition metals in sedimentary rocks, and
reservoirs comprising such shales are much more likely to be active
reservoirs, i.e., gas habitats as opposed to oil habitats. The
present invention permits such predictive determinations to be made
via direct evaluation of the intrinsic catalytic activity of source
rock-as opposed to determining whether such rock has a threshold
concentration of ZVTM.
[0070] While not intending to be bound by theory, it is believed
that interaction of the reaction mixture is primarily a surface
phenomenon. In such cases, the levels of potential catalytic
activity, relative to the surface area and/or surface area per unit
mass of the sample, can be quantified. An exemplary method of
determining surface area is by Brunauer, Emmet, and Teller (BET)
analysis.
[0071] In some embodiments, one or more of the above-described
processes may comprise one or more contamination control measures,
wherein such contamination control measures are employed when
handling samples prior to or during the assay process.
[0072] The ZVTM of significance with respect to the methods and
processes of the present invention include all ZVTM that suitably
catalyze the decomposition of hydrocarbons to yield
catalytically-generated methane in accordance with the methods and
processes of the present invention. For the purposes of oil and gas
exploration, these include, but are not limited to, iron (Fe),
cobalt (Co), and nickel (Ni).
[0073] While the discussions herein have focused primarily on
catalytic activity afforded by ZVTM, the present invention is
generally directed toward ascertaining intrinsic catalytic activity
of rock samples for the purpose of ascertaining catalytic
oil-to-gas conversion within reservoirs. As such, Applicant does
not preclude the possibility that zero-valent metals other than
transition metals may provide some catalytic activity. Thus, it is
possible that rare earth metals, in their zero-valent state, can
contribute to the oil-to-gas conversion, even if such contribution
is small by virtue of their presence in trace amounts.
[0074] In some embodiments, the optional step of separating is
coupled with the detection/analysis step. This is particularly well
suited for embodiments employing chromatographic separation, and to
gas chromatographic separations in particular. Suitable gas
chromatographic (GC) methods, coupled with a detection/analysis
technique, include, but are not limited to, gas chromatography-mass
spectrometry (GC-MS), gas chromataography-electron capture
detection (GC-ECD), gas chromatography-pulsed flame photometric
detection (GC-PFPC), gas chromatography-Fourier transform infrared
spectroscopy detection (GC-FTIR), and combinations thereof.
[0075] In some embodiments, any or all of the above-mentioned
methods and assays can be used during the course of, or in concert
with, well logging. Accordingly, such well logging can provide well
logs with intrinsic catalytic activity of sedimentary rock as a
function of well depth.
[0076] Most generally, the present invention is directed to methods
of making predictive determinations whether a reservoir is active
or inactive, in accordance with the oil-to-gas catalytic model, by
assaying the surrounding sedimentary rock for catalytic activity,
and by consequence, ZVTM.
[0077] In economic terms, if a reservoir is sufficiently removed
from natural gas markets, then the economic incentives for drilling
in an oil habitat greatly outweigh those for drilling in a gas
habitat. The present invention permits such determinations to be
made inexpensively with a relatively high level of accuracy,
helping to avoid significant and costly exploration processes in
order to ascertain the reservoiric content.
[0078] The following Examples are provided to demonstrate
particular embodiments of the present invention. It should be
appreciated by those of skill in the art that the methods disclosed
in the Examples which follows merely represent exemplary
embodiments of the present invention. However, those of skill in
the art should, in light of the present disclosure, appreciate that
many changes can be made in the specific embodiments described and
still obtain a like or similar result without departing from the
spirit and scope of the present invention.
EXAMPLE 1
[0079] This Example serves to illustrate an embodiment by which the
present invention can be used to assay sedimentary rock for
oil-to-gas catalytic activity via the detection of
catalytically-generated methane produced when a gaseous hydrocarbon
species is contacted with said rock in the presence of hydrogen and
under static exposure conditions.
[0080] A one gram sample of source rock (Miocene Monterey
formation, California, taken from an outcrop on Venice Beach) was
ground to 60 mesh and mixed with 5 cubic centimeters (cc) of sand
under argon, and then heat-extracted at 300.degree. C. for 30
minutes in flowing purified H.sub.2. Referring to reaction system
300 in FIG. 3, this mixture was sealed in a 10 cc brass reactor
301, then pressure vented five times at room temperature with a gas
mixture of 97% H.sub.2 and 3% propane (C3), the gases purified by
passage through commercial O.sub.2 scrubbers 302 (BOT-2 purchased
from Agilent Technologies, Willington, Del.). Each pressure vent
involved opening 3-way valve 303 to vent. With valve 304 closed,
the reactor was then pressurized to 50 psig with the gas mixture
(valve 305 open), then closed by closing valve 306. With valve 306
closed, valve 304 was opened slowly to allow the reactor to vent to
atmospheric pressure. This was repeated five times to remove all
oxygen from the sample particles, the reactor 301, and all
associated tubing. Inside the closed reactor at 50 psig gas mixture
(valves 306 & 307 closed), a mixture of rock sample, hydrogen,
and propane, the hydrogen and propane above catalyst saturation
partial pressures, was then heated to 260.degree. C. for 30
minutes, then brought back to room temperature. With the reactor at
200.degree. C., it was then opened (valves 306 & 307 open) to
flowing H.sub.2 gas (valve 305 closed and valve 308 opened) through
a liquid nitrogen (LN.sub.2) trap 309 and directly into a FID
detector 310 (3-way valve 303 open to FID) at a flow rate of
.about.0.2 cc/min (adjusted via needle valve 311), the same rate
used to calibrate the detector using a 3% propane/hydrogen mixture
as a standard for calculating g CH.sub.4/pA see (pA=pico amperes).
An integrator attached to the FID detector integrated the eluting
methane signal which indicated a rock activity of
.about.3.times.10.sup.-5 g CH.sub.4/(g rock hr).
EXAMPLE 2
[0081] This Example serves to illustrate an embodiment wherein the
hydrocarbon material is a liquid and illustrates activity
suppression by high concentrations of hydrocarbon and also
illustrates the catalytic nature of the reaction.
[0082] A sample of Monterey rock (0.88 g) like that in Example 1,
heat-extracted at 350.degree. C. for 30 minutes in purified
H.sub.2, was saturated with 100 micro liters of n-nonane (C9) and
placed in Reactor 301. After five pressure-vents, as described in
Example 1 using ultra pure H.sub.2 purified through oxygen scrubber
302 at room temperature, the reactor was closed (valves 306 and 307
closed) and heated to 240.degree. C. for 30 minutes. The product,
vented to FID (310) at 100.degree. C. over about 30 minutes,
indicated a catalytic activity of .about.3.times.10.sup.-6 g
CH.sub.4/(g rock hr). The reaction was repeated without adding
additional n-nonane. The second product was .about.70 times that of
the first with an activity of .about.2.times.10.sup.-4 g
CH.sub.4/(g rock hr). These results illustrate activity suppression
in the first reaction by excess liquid hydrocarbon suppressing
hydrogen diffusion to the active sites. Similar suppressions were
observed for pure nickel powder when a film of wax was dispersed
over its surface. Because excess hydrocarbon was removed between
the first and second reaction on venting to FID (310), hydrogen
access to the active sites was unimpeded in the second reaction.
The dramatic increase in methane yield between the first and second
reactions also illustrates the catalytic nature of the reaction. If
thermal cracking were the source of methane, the yield in the first
reaction would be greater than that in the second, not less.
EXAMPLE 3
[0083] This Example serves to illustrate an embodiment wherein the
products are separated by gas chromatography. It also illustrates
1) that sedimentary rocks are catalytic in their natural state
without added hydrogen or hydrocarbon, 2) that catalytic activity
increases by a factor of 20 with H.sub.2 addition, 3) that
catalytic activity is destroyed with the addition of
oxygen-contaminated 1% pentane/hydrogen, and 4) that hydrocarbons
in a natural source rock (and pentane) undergo insignificant
thermal decomposition to thermal methane under reaction conditions.
These results illustrate that sedimentary rocks are naturally
catalytic in the conversion of hydrocarbons to natural gas.
[0084] Referring to FIG. 4, a Monterey source rock similar to those
used above (0.25 g), except that it was not heat-extracted, was
placed in reactor 401 and flushed with purified He (through oxygen
scrubber 402) to vent (valves 403, 404, 405, and 406 open) as the
reactor temperature was increased from room temperature to
280.degree. C. Reactor 401 was then closed (valves 404 and 405
closed) for 23 hours. Opened to He flow (280.degree. C.) (valves
404 and 405 opened), 100 micro liter aliquots were taken at time
intervals selected to capture maximum product from the effluent
stream and sent to the GC unit 407 through sample loop 408 for
product separation and analysis. A typical product was 69% methane
(wt % C1-C4) with a methane GC peak intensity of 43 pA sec. The
above reaction was repeated (0.3 g rock) using purified H.sub.2
(through scrubber 402) in place of He. The product extracted from
the effluent stream and analyzed under the same conditions showed
significantly higher concentrations of methane: 350 pA sec. The
composition of the product gas, corrected for olefins (thermal
products), was 97% methane (C1-C4). A third reaction on the
Monterey source rock under the same conditions except that the
purified H.sub.2 was replaced by a mixture of unpurified pentane
(1%) in hydrogen (99%) (valve 409 open, valve 403 closed) showed
only trace amounts of methane. This third reaction serves as a
blank experiment in which the catalytic activity of the Monterey
rock was destroyed by the unpurified gas. The experiment
demonstrates that hydrocarbons (in the Monterey source rock and
pentane) undergo minimal thermal decomposition to methane under
these reaction conditions. Thus, the methane produced under the
same conditions using purified gases was catalytic methane.
EXAMPLE 4
[0085] This Example serves to illustrate how a ln k vs. T plot can
be generated and how such a plot can be extrapolated to yield k for
reservoiric conditions.
[0086] Steady-state flow reactions were carried out using
ultra-pure nickel powder (.about.1 g) obtained from Sigma-Aldrich,
203904-25 (99.99% Ni), 100 mesh. 1% n-Pentane in H.sub.2 was passed
through the oxygen scrubber 302 in FIG. 3 at a flow rate of 0.4
cc/sec directly to FID detector 310 (valves 305, 306, and 307 open
and 3-way valve 303 directed to detector 310). The methane signal
(pA) climbed smoothly from 240 to 270.degree. C. The plot in FIG. 5
shows a perfect linearity between ln k vs. T, where k was
calculated from a prior calibration using 3% C3/H.sub.2
(k=(pA).times.4.077.times.10.sup.-11). The linearity in ln k vs. T
and the independence between k and hydrogen and hydrocarbon
concentrations (demonstrated in separate experiments) is consistent
with zero-order kinetics.
EXAMPLE 5
[0087] This Example serves to illustrate an embodiment wherein the
hydrocarbon material is a liquid dispersed on sand. It further
illustrates an embodiment wherein the analysis of a rock from one
basin correctly predicts gas in a genetically similar rock in an
adjacent basin. A thermal cracking model incorrectly predicts
oil.
[0088] n-Nonane (C9) was dispersed on pure quartz sand by
evaporating to dryness a slurry of 100 cc sand and 50 ml pentane
containing 1 g n-nonane. A sidewall core sample of Barnett shale
(Mississippian) from the Hardeman basin, Texas was ground to 60
mesh under Ar and heat-extracted at 350.degree. C. in flowing
purified H.sub.2 for 30 minutes. The product (0.84 g) was mixed
with 5 cc of the nonane-impregnated sand and the mixture placed in
Reactor 301 (FIG. 3). The reactor was pressure-vented (50 psig)
with ultra pure H.sub.2 as described above. With the reactor closed
at room temperature and 50 psig H.sub.2 (valves 306 & 307
closed), the reactor was heated to 280.degree. C. for 1 hour, then
cooled to 200.degree. C., then opened to FID unit 310 through valve
303 by opening valves 306 and 307. The integrated methane product
indicated a rate constant k=.about.1.times.10.sup.-4 g CH.sub.4/(g
rock hr).
EXAMPLE 6
[0089] This Example serves to illustrate an embodiment wherein the
reaction is carried out under static conditions from steady-state
flow at constant temperature. It further illustrates the embodiment
wherein unusually stable light hydrocarbons are employed as
reactants so that high-temperature assays (350+.degree. C.) can be
employed to boost reaction rates without contaminating the
catalytic methane with thermal cracking methane. Propane, used in
EXAMPLE 6, has a half-life at 200.degree. C. of 800 million years
(Laidler, K. J., Sagert, N. H., and Wojciechowske, B. W. "Kinetics
and Mechanisms of the thermal decomposition of propane,"
Proceedings of the Royal Society A270, 242-253, 1962) while ethane
has a half-life of 50 billion years at the same temperature
(Laidler, K. J., and Wojciechowske, B. W. "Kinetics and Mechanisms
of the thermal decomposition of ethane," Proceedings of the Royal
Society A260, 91-102, 1961). Cycloalkanes are also unusually stable
and can be employed for high-temperature assays without
contaminating the catalytic methane product with thermal methane
(Mango, F. D., "The origin of light cycloalkanes in petroleum,"
Geochim. Cosmochim. Acta 54, 23-27, 1990). This further illustrates
an embodiment wherein a rock from one basin correctly predicts gas
in a genetically similar rock in the same basin.
[0090] A sample (4.6 gm) of Barnett shale core (Ft Worth basin,
Sims-2 well) ground to 60 mesh under Ar was placed in reactor 301
(FIG. 3), pressure-vented 5 times with 50 psig pure H.sub.2
(oxygen-scrubbed through 302), then heat-extracted under purified
H.sub.2 flow (0.2 cc/sec) for 30 minutes at 350.degree. C. The
inlet gas was then switched to 3% propane in hydrogen (purified
through 302) by closing valve 308 and opening valve 305. Gas flow
to FID (0.2 cc/sec) was then continued at 350.degree. C. until the
FID signal was constant, whereupon the reactor was closed (valves
306 and 307 closed) for 5 minutes, then opened to FID. A catalytic
methane peak emerged after about 10 minutes: A=1.03.times.10.sup.5
pA see, corresponding to a catalytic activity of k(350.degree.
C.)=1.1.times.10.sup.-5 g C1/(g rock hr). This rock would have a
nickel-equivalent activity of 2.46.times.10.sup.-13 g C1/(g rock
hr) at 160.degree. C. and would convert oil to gas in 6 Ma at this
temperature (3% porosity filled with oil). Since the Barnett shale
was at 160.degree. C. for 20 Ma, genetically similar facies of
Barnett shale would be designated gas habitats. After destroying
all catalytic activity by injecting .about.1 cc air with gas flow
at temperature, a repeat of the reaction with 3% propane,
350.degree. C., five minutes closed, showed no detectable amounts
of methane, demonstrating the feasibility of high-temperature assay
uncontaminated by thermal cracking.
[0091] FIG. 6 is a plot of temperature vs. residence time (in
million of years, Ma) for the Barnett shale analyzed above (Example
5) showing where in time-temperature space this reservoir will
contain oil (to the left of the Barnett curve, Curve A) and where
it will contain gas (to the right of Curve A). The x axis is the
log time, in millions of years (Ma), for 100% conversion of oil to
gas in a rock with 3% porosity filled with oil (.about.0.013 g
oil/g rock) at the indicated temperatures. Curve A, the Barnett
curve (the `Ni Equivalent Curve`), was constructed from the kinetic
equation published by Mango (Mango, "Transition metal catalysis in
the generation of natural gas," Org. Geochem. 24:977-984, 1996) for
zero-valent nickel. A rate constant for Barnett at each temperature
T (k.sub.T) was calculated from the following equation, where k' is
the rate constant for Ni published by Mango and k.sub.T is the
`nickel-quivalent` rate constant for Barnett:
k.sub.T=(k'.sub.T/k'.sub.280).times.1E-04. This curve is only an
approximation of the true Barnett T vs time curve which is best
constructed from multiple assays at multiple temperatures to obtain
an adequate linear relationship between ln k and T. It should be
stressed that any `true` Barnett curve thus obtained would need
calibration to natural conditions where lower hydrogen partial
pressures and retained hydrocarbons would serve to suppress the
intrinsic activities measured in assays. Curve B, the thermal
cracking curve, was constructed from the oil cracking kinetic data
published by Waples for the same 3% porosity rock (Waples, D. W.,
"The kinetics of in-reservoir oil destruction and gas formation:
constraints from experimental and empirical data, and from
thermodynamics," Org. Geochem. 31:553-575, 2000). The data point
represents the gas deposits in the Mississippian Barnett shale in
the Ft Worth basin, Texas with an estimated residence time of 20 Ma
at a temperature of .about.160.degree. C. A thermal cracking model
based on the Waples curve will incorrectly predict oil in the Ft
Worth basin while the Barnett curve, from a rock assay of Barnett
shale in a genetically similar reservoir in an adjacent basin
(Hardeman basin), correctly predicts gas.
[0092] The box in FIG. 7 encloses the time-temperature region where
most oil-to-gas occurs in sedimentary basins according Hunt (Hunt,
Petroleum Geochemistry and Geology, 2.sup.nd ed., W. H. Freeman,
New York, Chapter 7, 1996). Thus, any method for predicting gas
must be effective in this time-temperature region. The thermal
cracking model published by Waples (Waples, "The kinetics of
in-reservoir oil destruction and gas formation: constraints from
experimental and empirical data, and from thermodynamics," Organic
Geochemistry, 31:553-575, 2000), which is typical of most such
models, can explain only .about.30% of the observed cases. The
catalytic model, as reflected in rock assays on Barnett and
Monterey rocks described herein, will predict gas throughout the
critical zone.
[0093] FIG. 8 illustrates another application of the invention
where oil converts to gas while migrating from source rock to
reservoir rock through conduits constituting gas habitats.
Consider, for reference, the examples of oil-to-gas reported by
Paine (Paine et al., "Geology of natural gas in South Louisiana,"
American Association of Petroleum Geologists, Memoir 9, Volume 1,
Natural Gases of North America, Beebe, B. W., Editor, 376-581,
1968) in the giant gas fields in southern Louisiana occurring in
sandstone reservoirs interbedded with outer-neritic shales at
depths usually greater than 10,000 ft. (temperatures
>140.degree. C.). Outer-neritic shales tend to be rich in
transition metals like the Monterey source rock analyzed herein.
The Monterey rock exhibits robust activity in assay which projects
to very high paleoactivities at the temperatures indicated in the
Paine publication (see FIG. 7). At 160.degree. C., for example, a
sandstone reservoir interbedded with 1% Monterey shale would be a
gas habitat at all residence times greater than 5,000 years--a tiny
slice of geologic time. Migrating oil with a residence time greater
than 5,000 years, at temperatures .about.160.degree. C., would be
converted to gas as indicated in FIG. 8. This concept, the
conversion of oil to gas in migration, is new to oil and gas
exploration. It provides a potentially powerful explanation for how
oil from one reservoir (an oil habitat) becomes gas in a shallower
reservoir, also an oil habitat. Gas habitats along migration
pathways constitute gas conduits in an otherwise all oil plumbing
system.
[0094] Thus, in light of the foregoing, the present invention
provides extremely sensitive assays for determining the catalytic
activity of sedimentary rocks at levels as low as 0.01 .mu.g
CH.sub.4/(g rock hr) based upon any ZVTM-induced catalytic
decomposition of hydrocarbon material to generate methane. The fact
that this activity is destroyed by oxygen points to a low
valent-specific catalyst. That the catalytic action of pure
zero-valent nickel is similarly destroyed by oxygen indicates that
low-valent metals dispersed on the rock's surface are the active
agents in sedimentary rocks. Furthermore, such
catalytically-generated methane, if present, is indicative of the
intrinsic catalytic activity of the rock sample. Assays at
different temperatures yield a linear activity curve (ln k vs T)
that is useful in predicting activities k at subsurface
temperatures. Because subsurface conditions are different from
laboratory conditions (hydrogen partial pressures, hydrocarbon
concentrations and other unanticipated factors that might alter
reaction rates) the activity curve should be calibrated on
reservoirs for which residence time t, temperature T, and % oil
conversion to gas are known, thus giving subsurface activity at
temperature T, k.sub.s(T). A correction factor .alpha.
(.alpha.=k.sub.s(T)/k(T)) thus converts the assay activity curve to
the sub-surface curve: ln k.sub.s vs. T. Such curves give the rate
constants for source reservoirs and all genetically similar
non-source reservoirs at all sub-surface temperatures. The time for
90+% oil conversion to gas can then be calculated for all
sub-surface temperatures. This yields a curve like the Ni
equivalent curve in FIG. 6 that divides temperature-time space into
oil and gas habitats, regions where the subject reservoirs have a
high probability of containing oil or gas, respectively.
EXAMPLE 7
[0095] This Example serves to illustrate the analysis of various
deltaic reservoir rocks from the Bastian Bay and Lake Raccourci
fields, and how such analysis can be useful in well logging
procedures, in accordance with embodiments of the present
invention. All rocks were found to be catalytic, accelerating rates
of gas generation by factors ranging from 2000 to well over 50000.
Rocks with interbedded shales were on average five times more
active than rocks without interbedded shale. Based on their
measured activities, reservoirs would convert oil to gas at depths
between 8,000 and 11,000 ft, remarkably close to Paine's
observation that reservoir rocks with interbedded shales usually
contain gas at depths greater than 10,000 ft (Paine et al.,
"Geology of natural gas in South Louisiana," American Association
of Petroleum Geologists, Memoir 9, Volume 1, Natural Gases of North
America, Beebe, B. W., Editor, 376-581, 1968). Activities were
surprisingly high and pervasive, covering essentially all of the
deltaic rocks so analyzed. The results presented here explain, for
the first time, the vast amounts of gas in reservoir rocks that
have never reached thermal cracking temperatures. Pure sandstone
reservoir rocks are clearly catalytic and this property is likely
responsible for the erratic and unpredictable distribution of gas
in sandstone reservoir rocks. Nickel, one of the more active
transition metals in sedimentary rocks, varies substantially in
sandstones, from 5 ppm to 230 ppm in the 9 sandstones cited in
Boggs (Principles of Sedimentology and Stratigraphy, 2.sup.nd ed,
Prentice Hall, Upper Saddle River, N.J., 1995, p. 165). Thus, the
catalytic activities of different sandstone strata should similarly
vary. At a given temperature, high-activity reservoir rocks should
thus contain gas while low-activity reservoir rocks should contain
oil. A well log that includes catalytic activity would thus be
useful in correlating stratigraphic units across a basin and
uniquely powerful for predicting hydrocarbon composition (% gas)
within different strata at various depths throughout a basin.
[0096] Bastian Bay and Lake Raccourci fields are 2 of 18 fields
cited in Paine et al. (1968) as being representative of over 670
fields in south Louisiana producing oil and/or gas from that
portion of the giant Gulf Coast geosyncline as of 1962. Bastian Bay
is located about 55 miles southeast of New Orleans in Plaquemines
Parish. There are 133 Bastian Bay wells produced from Upper Miocene
Textularia articulate sandstone. Bastian Bay was the largest
gas-producing field in south Louisiana in 1962 yielding about 7.2
thousand bbl crude oil, 3.3 million bbl condensate, 1.5 billion scf
solution and associated gas, and 100 billion scf of nonassociated
gas, or about 90% gas (oil equivalent). Lake Raccourci field is
located in Lafourche and Terrebonne Parishes, approximately 42
miles south-southwest of New Orleans. Forty-eight wells produce
from the Upper Miocene Eponides (Buccella) mansfieldi
sandstones.
[0097] A Pan American Petroleum Corporation Bastian Bay core
(13,298 to 13,606 ft.; API 1707501617) from well # A-17 (L. L.
& E) was obtained from the Bureau of Economic Geology, Houston
Research Center (11611 West Little York, Tex. 77041). It was mostly
clean sandstone with intervals of sandstone with thin laminae of
interbedded shale. A side-wall core from Lake Raccourci field
(16,128 ft., Amco Production S/L 4599 #10, API 17-057-22201) was
also obtained from the Bureau of Economic Geology.
[0098] Ten samples constitute the database described here: eight
from Bastian Bay, one from Lake Raccourci, and the last sample a
synthetic rock constructed to reflect genuine outer-neritic shale
with 90% pure sandstone and 10% outer-neritic shale from the
Monterey formation (Miocene) (Piper, D. Z., and Isaacs, C. M.
(2001) "The Monterey Formation: Bottom-water redox conditions and
photic-zone primary productivity," From Rocks to Molecules, Eds. C.
M. Isaacs and J. Rullkotter, Columiia University Press, New
York.2001; Mann and Stein, "Organic facies variations, source rock
potential, and sea level changes in Cretaceous black shales of the
Quebrada Ocal, Upper Magdalena Valley, Colombia," AAPG, Bulletin
81:556-576, 1997; Cruickshank and Rowland, "Mineral deposits at the
Shelfbreak," SEPM Special Publication No. 33:429-436, 1983)
obtained from outcrop on Venice Beach, Calif. (10%ONS). Rocks were
analyzed for catalytic activity by a procedures described herein.
The kinetic experiments with pure nickel to obtain
nickel-equivalent curves (ln(k) vs T) are given in Mango (1996).
Pure nickel powder was obtained from Sigma-Aldrich (sub-micron
sized nickel powder, 99.999% Ni).
[0099] In a typical experiment, rock chips taken from the core were
ground to a powder, sieved to 60 mesh, and placed in a reactor
similar to that shown in FIG. 3 (rock sample=6.54 gm) in an
atmosphere of argon. The hydrocarbon feed gas (3% Propane in
hydrogen) was purified to remove oxygen by passing the gas through
one or more oxygen scrubbers 302 (OR-10 Pd/Alumina oxygen removing
purifiers, Johnson Matthey Gas Purification Technology). The
Ultra-Pure hydrogen gas was also purified by passing it through an
OR-100 Oxygen Removing Purifier fitted before the oxygen scrubber
302. The system was purged of residual air by a series of five
pressure-vents. Referring to FIG. 3, reactor 301 was then heated to
reaction temperature (250.degree. C.) under hydrogen flow and the
3-way valve 303 opened to FID detector 310. With liquid nitrogen in
trap 309, hydrocarbon feed was then injected into the reactor by
closing valve 308 and opening valve 305 (3% C3/H.sub.2 gas at 50
psi) for 1 minute. Valve 305 was then closed, valve 308 opened and
valves 307 and 306 closed. After 30 minutes standing at 250.degree.
C., the reactor was opened to the FID detector 310 by opening
valves 306 and 307. Thus, the catalytic methane generated in the
reactor over 30 minutes was swept into the detector 310 with
flowing hydrogen and the peak integrated (A=1.02 .times.10.sup.+7
.mu.V sec). From the calibration coefficient (using 3% propane in
hydrogen to calibrate) 1.7.times.10.sup.-12 g methane/ .mu.V sec,
the yield was 1.73.times.10.sup.-5 g methane (k=5.3.times.10.sup.-6
g C1/(g rock hr)).
[0100] Catalytic activity is expressed here as a ratio, the rate
acceleration ratio .alpha., defined as: .alpha.=(t.sub.(pure
quartz)/t.sub.(rock)), where t is the time required for 99%
conversion of oil to gas at 200.degree. C. Since pure quartz has no
catalytic activity, t.sub.(pure quartz) represents thermal cracking
calculated from Waples (Waples, D. W., "The kinetics of
in-reservoir oil destruction and gas formation: constraints from
experimental and empirical data, and from thermodynamics," Org.
Geochem. 31:553-575, 2000). Because .alpha. spans 7 orders of
magnitude for common rocks, catalytic activity is also expressed
logarithmically in the acceleration index A: A =log (.alpha.).
Table 1 shows the acceleration indices for the 11 samples analyzed
here including one containing 1% pure zero-valent nickel for
comparison. FIG. 9 shows the data graphically, wherein rate
acceleration index A (log (.alpha.)) for the 10 rock samples in
Table 1 and the synthetic sample representing a sandstone
associated with 10% outer-neritic Monterey shale (% ONS). Table 1
also lists rate constants for converting oil to gas at 200.degree.
C. in units g CH4/(g rock hr) determined experimentally
(k(200.degree. C.) was determined by nickel-equivalent curves). The
rate acceleration ratio .alpha.=(t.sub.(pure quartz)/t.sub.(rock)),
where t is the time required for 99% oil conversion to gas at
200.degree. C. Since pure quartz has no catalytic activity,
t.sub.(pure quartz)=1.36 Ma for oil thermally cracking to gas
(Waples, 2000). For the present example, it was assumed that the
rock had a 3% porosity filled with oil (0.013 g oil/g rock),
thermally cracking to gas under first order kinetics and
catalytically cracking to gas under zero-order kinetics. All of the
rock samples designated D- are from Bastian Bay field except for
D-48, a sidewall core sample from Lake Raccourci field. Samples
designated SS are sandstone rocks with no visible signs of
interbedded shale while SS/S rocks are sandstone rocks with
interbedded shales clearly visible. Rock labled (A) are adjacent SS
and SS/S rocks as are the rocks labeled (B). TABLE-US-00001 TABLE 1
Sample Composition Depth k (200.degree. C.) .alpha. .times.
10.sup.-3 D-45 SS(A) 13,507 2.49E-11 2.33 D-49 SS 13,593 6.26E-11
5.85 D-46 SS/S(A) 13,506 1.03E-10 9.66 D-40 SS(B) 13,481 1.09E-10
10.2 D-38 SS/S 13,445 1.52E-10 14.2 D-42 SS/S 13,319 1.72E-10 16.1
D-43 SS/S 13,319 1.78E-10 16.6 D-44 SS/S 13,447 2.3E-10 21.5 D-48
(Lake R) SS/S 16,128 3.3E-10 30.9 D-41 SS/S(A) 13,481 6.0E-10 56.1
10% ONS SS/S -- 3.55E-7 332 1% Ni -- -- 1.42E-3 1.33E+6
[0101] Rate constants were projected to subsurface temperatures
assuming nickel-equivalence (i.e., Ni is dominant active metal in
the rock). Oil conversion to gas was calculated from the geothermal
gradient in Paine (1968) and the burial history curve of the Lower
Miocene in South Padre Island (Huc and Hunt, "Generation and
migration of hydrocarbons in offshore south Texas Gulf Coast
dediments," Geochimica Cosmochimica Acta, 44:1081-1089, 1980). This
gives an approximation of the critical depth, the depth at which
the various reservoir rocks will convert 99% of their oil to gas.
Because of the uncertainties in the assumptions, these depths are
at best relative. While not intending to be bound by theory, there
is probably substantial error in the estimated activities. This is
because activity can only be lost in transportation and analysis,
not gained. Thus, the indicated rates of conversion are probably
below a rock's intrinsic rate of conversion--leading to an
underestimation of activity. They are, nevertheless, qualitatively
reliable. These rocks should be significantly catalytic at the
basin depths indicated and rocks with interbedded shales should be
significantly more active than those without shales.
[0102] The calculated depth for the most active natural rock (D-41,
SS/S) was 8,300 ft and for the least active rock (D-45, SS),
10,700. A synthetic rock with 10% outer-neritic shale (10% ONS)
would convert its oil to gas at about 7,000 ft, well above the
critical depth of 10,000 ft. cited by Paine. Critical depths for
all of the rocks analyzed in this study are shown in FIG. 10 along
with an approximation of the critical depth for pure sandstone
(pure quartz) which should retain oil to depths well below 20,000
ft. FIG. 10 also depicts hydrocarbon composition as a function of
depth within the various reservoirs listed in Table 1. The line
dividing oil and gas is the critical depth at which 99% of the oil
within these rocks would convert to gas. A geothermal gradient was
constructed from the data in Paine et al. (1968) and a
time-temperature burial history was approximated from the South
Padre Island burial curve in Huc and Hunt (1980). Each rock was
assigned a nickel-equivalent Ahhreneus equation (log (k) vs 1/T) by
fitting the measured rate constant for the rock to the Ahhreneus
equation for pure nickel (Mango, "Transition metal catalysis in the
generation of natural gas," Org. Geochem. 24:977-984, 1996). Two
assumptions were made to justify this: (1) k is directly
proportional to metal concentration, and (2) Ni makes the major
catalytic contribution to the rock's activity. The 99% conversion
depth was obtained by integration over the time-temperature burial
curve using nickel-equivalent rate constants. The data arbitrarily
truncates at 22,000 ft.
[0103] The catalytic activities for relatively pure sandstone rocks
(D-45, D-99, & D-40), although significantly lower than those
associated with shales, were nevertheless surprisingly high,
certainly high enough to impact the distribution of gas in these
basins. Unlike outer-neritic shales which are enriched in metals
through organic sedimentation, sandstone rocks enjoy no similar
input. But sandstones do contain transition metals over a broad
range of concentrations. Nickel, for example, varies from 5 to 230
ppm in the nine North American sandstones in FIG. 10. The
Rhinestreet sandstone with 230 ppm Ni matches the most catalytic
rock in nickel content, the outer-neritic Monterey shale with 250
ppm Ni. Metal concentrations, of course, do not necessarily equate
with catalytic activity. Nickel is active only in its metallic
form, all higher-valent nickel compounds are inactive. Because
catalytic activity is pervasive across all forms of sedimentary
rocks analyzed herein, this suggests facile metal reduction in the
subsurface. FIG. 11 depicts nickel concentrations in various
sandstones from North America taken from Boggs (p. 165, 1995): (a)
Shawangunk Formation (Silurian); (b) Millport Member of the
Rhinestreet Formation (Devonian); (c) Oneota Formation (L
Ordovician); (d) Cloridorme Formation (Ordovician); (e) Austin Glen
Member of the Normanskill Formation (M Ordovician); (f) Renessalaer
Member of the Nassau Formation (Late Proterozoic to Early Cambrian;
(g) Renessalaer Member, additional analyses; (h) Rio Culebrinas
Formation, Pureto Rico; (j) Turbidites from DSDP site 379A. Thus,
the high metal content shown in FIG. 11 could very well signal
significant activity in the subsurface. This would explain the
unexpected activities measured here.
[0104] The results clearly support the hypothesis that
outer-neritic shales impart sufficient catalytic activity to
sandstone reservoir rocks to explain Paine's observation that these
rocks contain predominantly gas at basin depths greater than 10,000
ft. The rocks from Bastian Ban and Raccourci fields are naturally
catalytic and should convert oil to gas well below thermal cracking
depths and certainly within the zones cited by Paine. All would
accelerate rate of gas generation by factors ranging from 1,000 to
well over 50,000. Similar catalytic activity can clearly be
expected in the subsurface. Rocks are naturally catalytic and
remain so over geologic time in subsurface environments. Potential
poisons like water, sulfides, carbonates and so on are passive
agents in the subsurface. If zero- or low-valent transition metals
(LVTM) were poisoned by them, their activities would be
irreversibly destroyed and remain so at the surface, which is
clearly not the case. The conclusion that LVTM remain active in the
subsurface is thus inescapable. This activity, however, does not
guarantee paleocatalysis. Sustained catalysis requires a sustained
supply of substrate, hydrocarbons and hydrogen--all of which are in
abundant supply in petroliferous environments. Hydrogen
concentrations are surprisingly high in gas habitats, on average
0.07% by volume of natural gas (Mango, "The origin of light
hydrocarbons," Geochim. Cosmochim. Acta 64:1265-1277, 2000). This
projects to a hydrogen partial pressure of 0.2 atm at 10,000 ft,
certainly high enough to sustain the catalytic generation of
gas.
[0105] All deltaic rocks analyzed here exhibit surprisingly high
levels of catalytic activity, accelerating rates of gas generation
by factors from 1,000 to well over 50,000. These activities are,
moreover, low estimates of natural activities. Activity can only be
lost in transportation from the subsurface and analysis, not
gained. The rocks are probably more active than the analyses of the
present invention might suggest. This could explain the most
striking features to Bastian Bay hydrocarbons, the fact that they
are essentially without oil (>95% gas and condensates). Because
condensates represent the final stages of oil's conversion to gas,
Bastian Bay hydrocarbons are approaching terminal maturity,
maturity not easily explained by thermal evolution alone. Most
Bastian Bay production is below 15,000 ft, at temperatures below
160.degree. C. (Paine et al., 1968). Oil should remain stable at
these temperatures for at least 100 Ma (Waples, 2000), and should
thus be thermally stable indefinitely in Miocene reservoirs.
In-reservoir thermal cracking simply cannot explain the extensive
amounts of gas and condensates in the Bastian Bay field. The
measured activities of Bastian Bay reservoir rocks place the oil
floor somewhere below 8,000 ft for the more active rocks, and below
11,000 ft. for the less active. This will explain most of the light
hydrocarbons in the basin. If catalytic gas generated in migration
is included, as oil converts to gas while passing through conduit
rocks of high activity, the nearly complete disappearance of oil
becomes understandable. Bastian Bay reservoir rocks are naturally
catalytic and their measured activities reasonably explain the
composition of hydrocarbons in Bastian Bay. Rocks with interbedded
shales are on average five times more active than pure sandstone
rocks and this explains Paine's general observation for all fields
in the basin that reservoirs associated with outer-neritic shales
have a high probability for gas at depths below 10,000 ft.
[0106] Thus, in light of the foregoing, the present invention
provides extremely sensitive assays for determining the catalytic
activity of rock samples based upon any ZVTM-induced (and possibly
LVTM-induced) catalytic decomposition of hydrocarbon material to
generate methane (e.g., in parts-per-billion quantities). Such
catalytically-generated methane is unequivocal evidence of
zero-valent metals dispersed on the rock's surface. Such
catalytically-generated methane, if present, is indicative of the
intrinsic catalytic activity of the rock sample, and via
projection, the source reservoir. Furthermore, when such assays are
coupled with well logging, they provide a well log that includes
catalytic activity that is useful in correlating stratigraphic
units across a basin and in predicting hydrocarbon composition (%
gas) within different strata at various depths throughout a
basin.
[0107] All patents and publications referenced herein are hereby
incorporated by reference. It will be understood that certain of
the above-described structures, functions, and operations of the
above-described embodiments are not necessary to practice the
present invention and are included in the description simply for
completeness of an exemplary embodiment or embodiments. In
addition, it will be understood that specific structures,
functions, and operations set forth in the above-described
referenced patents and publications can be practiced in conjunction
with the present invention, but they are not essential to its
practice. It is therefore to be understood that the invention may
be practiced otherwise than as specifically described without
actually departing from the spirit and scope of the present
invention as defined by the appended claims.
* * * * *