Geopressured environments in young deltaic basins, such as the Gulf of Mexico, can give explorers clues to how oil and gas move through - and get trapped in - thick sediment sections. Recent US Dep-artment of Energy-supported research points to dynamic pressure transition zones (seals) as a primary control on how fast and in what direction fluids move through these basins. These are called dynamic seals because they are maintained by a slow, ongoing flow of formation fluids containing dissolved gases migrating out of the deep basin. When these gases come out of solution in a shale section, further fluid flow is restricted and pressure builds up below the resulting seal. This research indicates that such pressure seals can also dictate how oil and gas are trapped in many commercial accumulations. Key concepts include:
- An improved understanding of geopressure seals and how they interact with geologic structure and subsurface fluid flow helps identify which deep structures are likely to hold gas reserves.
- An improved understanding of seismic velocity-fields is important to the making of prospect maps in the geopressured section. Such maps require reliable seismic velocity-fields because seismic velocity varies rapidly over geopressure anomalies. Many major fields in young basins form around areas of anomalous geopressure, adding substantial new reserves in and near these fields requires geophysicists to deal with velocity issues.
- Imaging of geopressure seals within a Gulf Coast gas field provides the best indication yet that dynamic seals help control geopressure formation in this basin.
In laboratory experiments, gas coming out of solution forms a dynamic barrier to vertical flow. This "vapor-lock" pressure seal is indistinguishable from the more conventional lithologic seal except when it crosses stratigraphic boundaries or by direct, visual observation. As pressure continues to build below the seal, the seal is occasionally breached and allows fluid flow for some period of time.
Lab work, modeling
The first work on dynamic pressure seals started with a lab model. A tank with water moving upward through a sediment section developed an overpressured zone under a seal that formed when a dissolved gas came out of solution part way up the column. This model was later generalized to more realistic basin conditions with computer-based equation-of-state modeling. These numerical simulations showed that under subsurface conditions, gas solubility minima would result in gas exsolution (where gas comes out of solution) along isotherms (zones of equal temperature).
That study used a gas mixture of 75% methane and 25% carbon dioxide, and showed that vapor-lock pressure seals would be expected along two isotherms: 165° F (73° C) and 270° F (132° C). Different gas compositions would lead to slightly different temperatures where gas comes out of solution (and therefore different depths to geopressure). Continued gas generation from underlying source rock, transport, and subsequent exsolution maintain the vapor-lock seal. In an unpublished study, we also observed that termination of gas generation leads to a slow decay of the pressure seal (fossil geopressure).
The modeling work was expanded to evaluate the effects of sand bodies on seal formation, and to evaluate expected geophysical responses of vapor-lock seals. Briefly, Benzing et al. (1996) showed a relationship between pressure seal location and hydrocarbon accumulation. Sands in contact with the zone of exsolution become charged with hydrocarbons; those not in contact with the pressure seal may or may not be overpressured (depending on their location relative to the seal), but do not contain hydrocarbons.
Shook et al. (1998) further showed that sand body/pressure seal aspect ratio largely dictates the nature of hydrocarbon accumulations near pressure seals. A sand body completely embedded within the pressure seal develops significant overpressure that cannot bleed off into normally pressured sediments. The degree of overpressure developed is such that breaching of the seal itself is possible, leading to charging of shallower sands. Shook et al. also observed geochemical halos surrounding these smaller sands that were caused by flow diversion around localized overpressure. Either of these phenomena - seal breaching or flow diversion - would lead to charged sands up-column.
Because the shape of isotherms in the subsurface controls the seal's geometry, a local high in the seal is generated above a local thermal high. Excess pore pressure in the seal high generates a local minimum in effective properties directly below the high. Petrophysical properties sensitive to effective pressure, such as sound velocity, also display a minimum.
Thus, in the case of single or parallel seals, the presence of high-pressure, low-velocity sediment beneath a high in the pressure seal can generate a false time-syncline of up to one second in two-way-travel time below the seal. In cases where seals diverge, for example when an upper seal is breached and releases fluids, the reduced pressure in the deeper section will result in a velocity pull-up of deep reflections. This happens because of the slightly higher velocities within the pressured section under the seal-breach, when compared to the surrounding more highly geopressured sediments.
Geomechanical modeling and experiments on "vapor-lock" pressure seals point to a feedback relationship between seal formation and sediment deformation. Because of the dynamic nature of the seal, an additional load is applied to the sediment just above the seal while the load gradient is reduced just below the seal. Consequently, relative sediment compression and expansion occur above and below the seal respectively, and structural closure can be created during seal formation.
The transition between compression and dilation forces creates a minimum in effective stress along the flank of the pressure seal, creating potential for the formation of growth faults along the seal. This feedback relationship may help explain why deeper geopressured pay often occurs in a "halo" surrounding some larger Gulf Coast fields.
To summarize, the formation of dynamic pressure seals can lead to the following physical conditions:
- Pressure seals associated with temperature regimes, independent of lithology
- Hydrocarbon accumulations associated with pressure transition zones (pressure seals)
- Geochemical halos surrounding highly overpressured sands
- Charging of shallower sands from either flow diversion or seal breach
- Velocity warping of structures imaged with seismic data
- Formation of structures and faults around geopressure anomalies
- Changes in seismic reflection attributes associated with dynamic seals.
The presence of such multiple seals is confirmed in Gulf of Mexico sediments by pressure readings from repeat formation testers, seismic velocity imaging, and 3D temperature distribution mapped from bottom-hole-temperature (BHT) measurements.
Acknowledgement
Support for this work was provided by the US Department of Energy. This research was performed by the Idaho National Engineering & Environmental Laboratory and EarthView Associates, Inc. Conoco Inc. supplied seismic and well data from West Cameron 66 Field for the purpose of evaluating some of the methods described in these reports (Supply of this seismic data should not be construed as endorsement of the validity or effectiveness of these results and proposed methods.) Alan Huffman of Conoco, Tim Green of INEEL, Hal Adams and Mike McCardle of Adams & Starr, Geophysicists, and Reddy Ravula of AlphaGeo, Inc. provided valuable insight.
References
Benzing, W., 1994, The 'vapor-lock' pressure seal: poster abstract in AAPG Hedberg Conference, Denver, Colorado, June 8-10, 1994.
Benzing, W., 1998, Shale response to the Formation of "Vapor-Lock" Pressure Seals: A Dynamic Geomechanical Model. In: Pressure Regimes in Sedimentary Basins and Their Prediction - The American Association of Drilling Engineers Industry Forum.
Benzing, W., Shook, G., 1996, Study advances view of geopressure seals: Oil and Gas Journal, v. 94, no. 21, May 20.
Benzing, W., Shook, G., and LeRoy, S., 1996, The formation and behavior of 'vapor lock' pressure seals and associated hydrocarbon accumulations in geologically young basins: Gulf Coast Association of Geological Societies Transactions, v. 46.
Kan, Tze-Kong, Kilsdonk, Bill, West, C., 1999, 3-D geopressure analysis in the deepwater Gulf of Mexico: The Leading Edge, April 1999.
Shook, G., LeRoy, S., Benzing, W., 1998, Reservoir and Geophysical Properties of Vapor-Lock Pressure Seals In: Pressure Regimes in Sedimentary Basins and Their Prediction - The American Association of Drilling Engineers Industry Forum.
