Controls on Overpressure in Rapidly Subsiding Basins and Implications for Failure of Top Seal
D.R. Converse, P.H. Nicholson, R.J. Pottorf, T.W. Miller, 2000. "Controls on Overpressure in Rapidly Subsiding Basins and Implications for Failure of Top Seal", Petroleum Systems of South Atlantic Margins, Marcio Rocha Mello, Barry J. Katz
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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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Petroleum Systems of South Atlantic Margins
“Giant oil and gas fields have been discovered through recent exploration in the deep and ultra-deep water regions of Angola, Brazil, and Nigeria. This volume includes 1. Descriptions of the petroleum geology of more than 12 basins along the two margins 2. Discussion of various components of the petroleum systems within these basins 3. Discussion on the use of the petroleum systems concept in the South Atlantic marginal basins to classify and characterize the diversity of the systems 4. Methodologies used in the assessment of petroleum system components 5. Overviews of the different components of the region's petroleum systems. Petroleum explorationists and researchers working in the South Atlantic will welcome this addition to their libraries.
The printed version of Memoir 73 was the Winner of the 2000 Dott Award for best AAPG Special Publication.”