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The free energies of formation and fugacities of the fluid components of an accumulating sedimentary pile are acquired largely during geothermal heating of initially cool constituents of highly porous sediment. Thermal maturation of organic matter, with accompanying release of potentially migrant products (CO2, H2O, CH4, and various hydrocarbons), consists of a complex of specific burial diagenetic and low temperature (< 250°C) metamorphic reactions activated and promoted by heat moving upward through the pile. Likewise, the progressive dewatering of a sedimentary pile, manifested in the net reduction of intergranular porosity with increased burial depth (temperature), reflects another, only partly interrelated, complex of thermoch mical reactions. Fluid migration is thus a dynamic response to induced gradients of temperature, fluid pressure, and concentration as determined by routing of heat through lithologically controlled nonisotropic arrangements of thermal conductivities. Rapid fluid movements along permeable pathways may locally influence shallow subsurface temperature distributions in dynamic and possibly transient ways through convective heat transfer.
For establishing practical limits to investigations of such burial changes, paired values of virgin reservoir temperature and fluid pressure (T and Pf) were compiled from the international literature > 700 commercial oil and gas reservoirs. Higher-than-average geothermal gradients were deliberately sought. The limits of the T-Pf field defined by these points are well constrained, and only eight reservoirs were recorded that produced commercially at temperatures 175°C. Deep-ocean petroleum prospects appear to offer advantageous characteristics in terms of the T-Pf field because of the low T and high Pf at the deep sea floor.
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