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More than 5,000 m (16,404 ft) of Upper Cretaceous and Tertiary nonmarine rocks have accumulated in the northern Green River basin. At depths below 3,000 m (9,842 ft), they contain large reserves of natural gas in low-permeability, overpressured sandstones and siltstones. Isotopic, petrographic, and mineralogic studies of cores from seven wells reveal that an intricate sequence of diagenetic events has acted upon mineralogically immature sediments to produce the observed low permeabilities. In large portions of the basin, this low permeability impedes the leakage of pore fluids, including gas. Gas accumulates in sandstones because it is generated from humic matter at a rate that exceeds its ability to escape. Gas entrapment due to low permeability is demonstrated by overpr ssuring. The overpressuring results from a combination of overburden removal and generation of fluids by organic matter maturation.
In the central part of the basin, normal hydrostatic pressures exist down to about 2,500 m (8,200 ft). Sandstone porosities in this zone range from 10 to 15% and permeabilities usually exceed 10 md. Below this depth, sandstones have greatly reduced porosities and permeabilities and become increasingly overpressured. At depths of about 3,500 m (11,483 ft), overpressuring and gas accumulation are associated with sandstones that have average porosities of about 7% and in-situ permeabilities of approximately 0.005 md. This transition is not marked by a depositional boundary.
Porosity reduction, which is assumed to be paralleled by permeability loss, proceeds by some combination of three principal processes: (1) precipitation of calcite or silica cements early in the burial history; (2) porosity loss through grain deformation and compaction; and (3) the filling and coating of residual and secondary pores by illite, chlorite, microcrystalline quartz, or ferroan carbonates.
A wide range of porosities is present in each depth interval, but maximum sandstone porosity follows a relentless course of destruction with depth. Only locally has the magnitude of grain and cement dissolution been great enough to reverse the porosity-depth trend. Zones of conventional reservoir porosity and permeability have not been recognized in areas of overpressuring and gas accumulation, nor are they to be expected. Because the gas is diagenetically entrapped, the search for economic accumulations should, paradoxically, be limited to sandstones of low porosity and permeability.
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