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Capillary Sealing as an Overpressure Mechanism in the Anadarko Basin*
By
Constantin Cranganu1 and Maria A. Villa1
Search and Discovery Article #40187 (2006)
Posted February 28, 2006
*Modified from extended abstract prepared for presentation at AAPG Annual Convention, Calgary, Alberta, June 19-22, 2005
1Brooklyn College, Brooklyn, NY ([email protected])
Abstract
The Anadarko Basin
in southwestern Oklahoma is known to contain today areas of extensive
overpressures (pressures higher than hydrostatic pressure). Explaining the
origin and maintenance of overpressured pore-fluids in the basin over long
periods of time cannot be achieved by invoking classical, common causes, such as
compaction disequilibrium or gas generation. We propose a capillary sealing
mechanism that is responsible for both generating and maintaining almost all
overpressure observed today in the Anadarko Basin. 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 and estimating
the magnitude of their pressure sealing implies two main aspects: (1) measuring
the pore throat radius of coarse- and fined-grained clastic rocks, and (2)
detecting the presence of gas-bearing layers using geophysical logs and other
data. Measurements by injecting mercury into rock pores allow 
estimation
of the
pore throat radii controlling the capillary sealing. 21 fine-grained rock
samples from the Anadarko Basin were thus measured and the average pore throat
radius was found to be 2.5 x 10-8
m. The proposed model also requires the presence of gas-bearing layers
interbedded into shale layers. Using a suite of geophysical logs from more than
100 wells, we were able to identify such gas-saturated layers in more than 50
wells. Further calculation indicates that a capillary sealing mechanism in the
overpressured area of the Anadarko Basin may produce ~40 MPa of pressure, or
~80% of the maximum observed overpressure in the basin.
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IntroductionMany 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, OklahomaThe 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 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.
Capillary Sealing in the Anadarko BasinFollowing 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.
Detecting Gas-bearing Layers in the Anadarko BasinIdentifying 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.
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