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Book Chapter

Controls on Overpressure in Rapidly Subsiding Basins and Implications for Failure of Top Seal

By
D.R. Converse
D.R. Converse
ExxonMobil Upstream Research Company Houston, Texas, U.S.A.
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P.H. Nicholson
P.H. Nicholson
ExxonMobil International Ltd. London, U.K.
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R.J. Pottorf
R.J. Pottorf
ExxonMobil Upstream Research Company Houston, Texas, U.S.A.
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T.W. Miller
T.W. Miller
ExxonMobil Upstream Research Company Houston, Texas, U.S.A.
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Published:
January 01, 2000

Abstract

Understanding the principal mechanisms responsible for overpressure development in sedimentary basins is crucial for exploration and safe drilling in overpressured basins. Two mechanisms of overpressure generation, compaction–disequilibrium and stress unloading, have significant impacts on subsurface pressure distributions over geologic time. The contribution of different overpressure mechanisms can be analyzed using stress distribution and velocity data. In wells from both the North Sea and the Far East, mistaking the overpressure mechanism can lead to errors in pressure prediction. Further controls on overpressure are faults or stratigraphic changes that act as impediments to subsurface flow to create pressure cells. In the North Sea, faults are capable of supporting large pressure differences. Graben-bounding faults can also be important in transmitting deep fluids to shallower sandstones along the graben margin. In one example, the overpressure distribution away from a fault is controlled by the stratigraphic distribution of sandstones and thin interconnecting siltstones. Accurate pressure prediction from seismic velocities can help delineate pressure cell boundaries and aid in exploration. A key point is that a pressure difference across a pressure cell boundary indicates slow flow on a geologic time scale, not necessarily a lack of flow. Another level of control on subsurface pressure distribution is from leakage via top seal failure or fault leakage. Hydraulic seal failure is an important failure mechanism in many overpressured basins. Careful estimation of the total minimum stress is required to predict hydraulic top seal failure and is done using an empirical leakoff pressure versus depth trend. Combining both pressure and failure stress predictions, a simple method predicts the maximum hydrocarbon column that a top seal or fault can support. Often, a sizable hydrocarbon column can be trapped beneath a leaking top seal.

Subsurface overpressures in sedimentary basins can vary greatly over geologic time. Estimation of the stress variation is critical to evaluation of seal integrity and possible migration pathways. Evolution of overpressure can result in changes in migration pathways via the development of new leak points. We estimated the basic stress variation using a proprietary 1-D model of burial and stress evolution. Most models of pressure evolution are uncalibrated, as there are few paleobarometers in typical hydrocarbon-bearing sections. To address the question of controls on overpressure through time, we applied a useful paleobarometer derived from a comparison of coeval aqueous and hydrocarbon fluid inclusions trapped in the same cement. The difference in physical behavior of the inclusions allowed calculation of the pore pressure when the fluid inclusions were trapped. Combination of these results with a model of pressure history permitted estimation of the time of overpressure development and possibly hydrocarbon entrapment. The results of these calculations can be important, especially if overpressure development plays a major role in controlling hydrocarbon migration pathways and integrity of the seal.

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Contents

AAPG Memoir

Petroleum Systems of South Atlantic Margins

Marcio Rocha Mello
Marcio Rocha Mello
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Barry J. Katz
Barry J. Katz
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American Association of Petroleum Geologists
Volume
73
ISBN electronic:
9781629810706
Publication date:
January 01, 2000

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