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Abstract

In the past, petroleum basins were traditionally largely viewed as either static or uniformly evolving entities. It is now more widely recognized that petroleum basins are subject to various intense geological processes, and hence substantial changes, during their evolutionary histories. One such geological process is exhumation, which is usually accompanied by significant erosion, decrease in both burial temperatures and fluid pressures, and often decrease in geothermal gradients. These changes can have profound consequences regarding hydrocarbon (HC) deposits. Besides all the obvious consequences (halting source-rock generation, damaging or destroying seals, expansion of gas with trap flushing, etc.), less obvious, but none the less equally meaningful, consequences also result.

For example, in basins with high geothermal gradients (e.g. Central Sumatra, Los Angeles), migration-accumulation processes result in most of the basin's reservoired oil being emplaced within the first 1-2 km of the surface. Strong erosion can largely destroy almost the entire oil resource of the basin, leaving a gas-only province (e.g. San Juan Basin, USA). Pore waters at high temperatures and pressures carry large amounts of both dissolved HC gases and inorganic mineral species (ions). Significant falls in burial temperatures from exhumation thus cause two results in the deeper regions of petroleum basins: (1) basin-centred gas deposits; (2) widespread destruction of deep-basin porosity, but especially permeability, from wholesale precipitation of dissolved mineral species as diagenetic minerals. In the case of basin-centred gas deposits, in going updip from the basin depocentre, eventually a location is reached where insufficient HC gas was dissolved in the

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