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uAbstract
uFigure
captions
uIntroduction
u Overpressure
uCapillary
sealing
uGas-bearing
layers
uPore
throat size
uConclusions
uAcknowledgments
uReferences
uAbstract
uFigure
captions
uIntroduction
uOverpressure
uCapillary
sealing
uGas-bearing
layers
uPore
throat size
uConclusions
uAcknowledgments
uReferences
uAbstract
uFigure
captions
uIntroduction
uOverpressure
uCapillary
sealing
uGas-bearing
layers
uPore
throat size
uConclusions
uAcknowledgments
uReferences
uAbstract
uFigure
captions
uIntroduction
uOverpressure
uCapillary
sealing
uGas-bearing
layers
uPore
throat size
uConclusions
uAcknowledgments
uReferences
uAbstract
uFigure
captions
uIntroduction
uOverpressure
uCapillary
sealing
uGas-bearing
layers
uPore
throat size
uConclusions
uAcknowledgments
uReferences
uAbstract
uFigure
captions
uIntroduction
uOverpressure
uCapillary
sealing
uGas-bearing
layers
uPore
throat size
uConclusions
uAcknowledgments
uReferences
uAbstract
uFigure
captions
uIntroduction
uOverpressure
uCapillary
sealing
uGas-bearing
layers
uPore
throat size
uConclusions
uAcknowledgments
uReferences
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Figure and Table Captions
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Figure 1. Location map for the Anadarko
Basin showing the locations of 21 wells sampled in this study.
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|
 |
Figure 2. Well logs from 3420 m depth to
3465 m depth from well Cobb 2-27 in the Anadarko Basin (T 15 N,
R 23 W, S 27, latitude 37.740oN, longitude 99.635oW).
Well logs provided courtesy of Apache Oil Corporation, Tulsa,
Oklahoma. Section shown is in the Prue Sand, which is a part of
the Lower Desmoinesian Cherokee Group (Middle Pennsylvanian)
(from Deming et al., 2002). More details and examples are found
in Cranganu (2005). |
|
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Figure 3. Mercury pressure versus relative
mercury saturation for a Pennsylvanian age shale from the
Thompson 1 well in the Anadarko Basin (see
Table 1). The initial increase in
pressure associated with saturations below ~40% is produced by
surface effects. To estimate the pore throat size, we
extrapolate the plateau of the curve until it reaches the zero
percent saturation and then note the associated pressure; in
this example, Pc is 50.5 x 106 Pa. |
|
 |
Table 1. Capillary Pressures (Pc) and
Pore Throat Radius for Shale Samples from the Anadarko Basin.
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Many
sedimentary basins throughout the world experience fluid pressures above
or below the normal (i.e., hydrostatic) pressure (Fertl, 1976; Hunt,
1990; Bigelow, 1994). The common name for these situations is “abnormal
high- or low-pressure”, “geopressure”, “overpressure” (above the
hydrostatic) or “underpressure” (below the hydrostatic pressure value).
More
common than the “underpressure”, the “overpressure” situation occurs in
a sedimentary basin due to one or more of the following causes (Osborne
and Swarbrick, 1997) (1) increase of compressive stress, (2) change in
the volume of the pore fluid or rock matrix, and (3) fluid movement or
buoyancy. A fast subsidence process can generate considerable
overpressure due to increase loading during sediment burial, especially
when sediments have low permeability and fluids cannot be expelled from
buried sediments at the same rate as the subsidence rate. Gas generation
could possibly produce overpressure, depending upon the type or organic
matter (kerogen) implied in the process, temperature history of the
sedimentary basin, abundance of organic matter, etc.
Overpressure in the Anadarko Basin, Oklahoma
The
Anadarko Basin in southwestern Oklahoma (Figure 1)
is known to contain today areas of extensive overpressure (Hunt, 1990;
Jorgensen, 1989; Jorgensen et al., 1993; Al-Shaieb et al., 1992; Al-
Shaieb et al., 1994, 1994b). Al-Shaieb et al. 1992, 1994a, 1994b, stated
that fluid pressures exceeding hydrostatic pressure generally start at
~2.3 to 3.0 km depth, but return to near-hydrostatic pressure value
below the Woodford Shale. They also hypothesized about the presence,
within overpressure zone, of three different levels of
compartmentalization, such that overpressurized fluids are not
ubiquitous present in the basin, but confined to distinct compartments.
Explaining the origin of overpressure regime and its maintenance over
long periods of time in the basin is not a simple task if one tries to
apply the classical concepts (disequilibrium compaction or gas
generation). The Anadarko Basin has not experienced significant
sedimentation for more than 200 million years, and has undergone uplift
and erosion for ~100 million years (Gilbert, 1992). Consequently, one
cannot invoke compaction disequilibrium as a possible mechanism for
overpressure regime in the Anadarko Basin, unless a pressure seal,
capable of maintaining overpressure for millions of years, is invoked.
Analyzing this hypothesis, Lee and Deming (2002) considered that the
presentday overpressures in the Anadarko Basin were remnant of Paleozoic
compaction disequilibrium preserved for about 250 million years. They
found out that the process of preserving overpressures for such period
of time would require a pressure seal 100 m thick with a permeability
lower than 10-27 m2. This permeability is many
orders of magnitude lower than the lowest rock permeabilities ever
measured (Neuzil, 1994, 1995).
Another
hypothesis to explain the generation and maintenance of fluid
overpressures in the Anadarko Basin was gas production from a source
rock (Lee and Deming, 2002). The Anadarko Basin is rich in gas: through
1985, petroleum reservoirs in the basin produced 82.2 trillion cubic
feet (2.33 x 1012 m3) of gas (Davis and Northcutt,
1989), and the ultimate recovery of natural gas from the Anadarko Basin
in 1980 was estimated to be 3.1 x 1012 m3 (Rice et
al., 1989; Dyman et al., 1997).
Even
though the Anadarko Basin possesses huge amount of gas and despite the
fact that gas generation could be a likely source of overpressuring
observed in the basin today, Lee and Deming (2002) dismissed this
hypothesis because the apatite fission track data indicate unambiguously
that the Anadarko Basin was uplifted and cooled starting ~40 – 50
million years ago. The amount of uplift was in the range of 1 – 3 km. As
a result, the temperature in the basin dropped by 20oC and
stopped the gas generation. It follows that the initial overpressure
produced when the gas generation was active, should have been preserved
for the last several tens of millions of years. But Lee and Deming
(2002) pointed out that the containment of overpressure by layers
thinner than ~100 m would require permeabilities lower than 10-25 m2,
which are 2 orders of magnitude lower than the lowest shale
permeabilities ever measured or inferred (Neuzil, 1994).
If the
previous discussed hypotheses, involving two widespread mechanisms,
cannot fully account for production and preservation of overpressures in
the Anadarko Basin over long periods of time, it is necessary to look
for another model.
Following the seminal papers published by Larry Cathles and his
co-workers from Cornell University (Revil et al., 1998; Cathles, 2001;
Shosa and Cathles, 2001), Deming et al. (2002) and Cranganu (2004)
tested another hypothesis, which can be termed “capillary sealing”.
Capillary sealing occurs in a sedimentary basin when capillary forces
act at gas-water interfaces between coarse- and fine-grained clastic
rocks. Detecting capillary seals implies two main aspects: (1) detecting
the presence of gas layers using geophysical logs or other data, and (2)
estimating the pore throat radius of coarse- and fine-grained clastic
rocks by using mercury injection (porosimetry) measurements.
Identifying gas-bearing layers in a sedimentary basin implies a thorough
and complex interpretation of information contained in well logs. We
searched over 100 logs from oil and gas wells in the deep Anadarko
Basin. In about half of these logs we identified multiple thin layers of
gas. The procedures followed and the precautions taken to avoid any
possible mistake are described in detail in Cranganu (2005).
The
detection of gas-bearing layers with open-hole logs is tied primarily to
the use of porosity type logs. These are the only logs generally run in
open hole that are really influenced by presence of gas versus the
presence of oil or water. The gas detected is in the invaded zone close
to the borehole wall, or sometimes in the virgin formation if there is
little to no invasion. The response of these porosity devices must be
understood to fully appreciate the attitudes assumed in setting up gas
detection systems. The following example (Figure 2)
is representative for our technique of detecting thin gas-bearing layers
in the Anadarko Basin.
Capillary effects can block two-phase (water and gas) flow perpendicular
to alternating layers of fine- and coarse-grained clastic sediments
(e.g., Berg, 1975; Surdam et al., 1997). The capillary pressure
Pc
in the sedimentary formations is given by the Young- Laplace
equation (Revil et al., 1998):
P
c =
(2/r)g
K
(1)
where
g is the interfacial
tension of the gas-water interface (approximately 72 x 10-3 N
m-1 at 25oC), r is the effective pore
throat radius, and K is the “wetting coefficient” usually taken equal to
unity. Free gas accumulates in the coarse-grained sediments. The flow of
both water and gas is blocked and a gas capillary seal is formed when
the saturation of gas reaches a level at which the gas phase becomes
interconnected in micro “gas caps” and the pressure drop
DPc
across the water-gas interface reaches the value (Revil et
al., 1998):
DPc
=
2g (1/rfine
-
1/rcoarse)
(2)
where
DPc
(Pa) is the
pressure drop across a gas-water interface,
rfine
(m) is the pore throat radius of the fine-grained layer, and
rcoarse
(m) is the pore throat radius of the coarsegrained layer. In
the case of alternating layers of sandstones and shales, the sandstone
pores are usually larger by at least a factor of 10 than the shale
pores. Therefore, Equation (2) can be approximated as:
DPc
» 2g
/
rfine
(3)
To
estimate the pore throat radius of shales in the Anadarko Basin, we made
21 mercury injection measurements (Figure 3)
using a procedure described in Cranganu (2004). The data obtained are
listed in Table 1. The average pore throat
radius of the 21 samples is 2.5 x 10-8 m. The result
coincides with the one obtained by Krushin (1997). With the interfacial
tension g for a gas-water
interface at in situ conditions equal to 2.5 x 10-2 kg s-2
(Schowalter, 1979), the pressure drop
DPc
across each gas-water interface could be as great
as
DPc
= 2x106Pa
(4)
If the
shales are not hydrodynamically connected in three dimensions, the
capillary pressure drops across each individual interfaces are additive
(Shosa and Cathles, 2002). In the example from
Figure 3, where we have 10 gas-saturated sands interlayered with
water-saturated shales, we have 20 total interfaces
producing a maximum possible pressure change of 40 x 106
Pa. In the western Roger Mills County the maximum overpressure reaches ~
50 x 106 Pa (Al-Shaieb et al., 1994b). Thus capillary sealing
due to gas-water interfaces could potentially be responsible for ~80% of
the maximum overpressure observed nowadays in the Anadarko Basin.
This
article tries to explain the generation and maintenance of abnormal
fluid pressures in the Anadarko Basin, Oklahoma, by invoking a capillary
sealing mechanism, in which a pressure drop across each gas-water
interface is required in order to push gas from a coarse-grained layer
(usually, sandstone) into and through an overlying fine-grained layer
(usually, shale). The model proposed requires the presence of thin gas
layers interbedded into shale layers. Use of a suite of geophysical logs
ensures the detection of these gas layers. Measurements by injecting
mercury into rock pores allow estimation of the shale pore throat
radius.
Acknowledgment is made to the Donors of the American Chemical Society
Petroleum Research Fund (PRF Grant # 39372-AC9) for partial support of
this research.
Al-Shaieb, Z., J. Puckette, P.
Ely, and V. Tiger, 1992, Pressure compartments and seals in the Anadarko
basin, in K.S. Johnson and B.J. Cardott, eds., Source rocks in
the southern midcontinent, 1990 symposium: Oklahoma Geological Survey
Circular 93, p. 210-228.
Al-Shaieb, Z., J.O. Puckette, A.A.
Abdalla, and P.B. Ely, 1994a, Megacompartment complex in the Anadarko
basin: A completely sealed overpressured phenomenon, in P.J.
Ortoleva, ed., Basin compartments and seals: AAPG Memoir 61, p. 55-68.
Al-Shaieb, Z., J.O. Puckette, A.A.
Abdalla, V. Tigert, and P.J. Ortoleva, 1994b, The banded character of
pressure seals, in P.J. Ortoleva, ed., Basin compartments and
seals: AAPG Memoir 61, p. 351-367.
Jorgensen, D. G., 1989,
Paleohydrology of the Anadarko basin, central United States, in
K.S. Johnson, ed., Anadarko basin symposium, 1988: Oklahoma Geological
Survey Circular 90, p. 176-193.
Jorgensen, D.G., J.O. Helgesen,
and J.L. Imes, 1993, Regional aquifers in Kansas, Nebraska, and part of
Arkansas, Colorado, Missouri, New Mexico, Oklahoma, South Dakota, Texas,
and Wyoming--geohydrologic framework: U.S. Geological Survey
Professional Paper 1414-B, 72p.
Lee, Youngmin, and David Deming, 2002,
Overpressures in the Anadarko basin, southwestern Oklahoma: Static or
dynamic?: AAPG Bulletin, v. 86, p. 145-160.
Neuzil, C.E., 1994, How permeable
are clays and shales?: Water Resources Research, v. 30, p. 145-150.
Neuzil, C.E., 1995, Abnormal
pressures as hydrodynamic phenomena: American Journal of Science, v.
295, p. 742-786.
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