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Previous work in the Green River basin of Wyoming indicates that overpressuring is the result of gas generation in low-permeability rock sequences. We concur, but suggest that an equally important aspect of overpressuring is the development of an effective seal.
Most porosity in these tight reservoirs results from dissolution of mineral grains and cements. The effectiveness of this porosity-enhancing process is dependent, in part, on the ability of pore fluids to transport dissolved products away from the sites of dissolution. We suggest that, in low-permeability rocks, at depths beginning at subsurface temperatures of 190°-200°F (88°-93°C), rates of thermogenic gas generation exceed gas loss, causing fluid pressure to increase. In the larger pores, free water is forced upward into zones of lower pressure. As a result, a water block is formed, with water-bearing reservoirs updip and gas-bearing reservoirs downdip. In the active gas-generating zone, the remaining water is irreducible. This water is immobile and incapable of removing dissolution products. Thus, while other porosity-reducing processes continue, porosity-enhancing processes become ineffective, resulting in a pore network with very low porosity and permeability.
The initial stages of our model take place in a subsiding basin where subsurface temperatures are at equilibrium with organic matter metamorphism. However, most basins currently are not at equilibrium, and the relationships of organic maturation, temperature, and overpressuring are obscured due to local or regional uplift and temporal variations of paleotemperature. Despite these modifications, the seal, as proposed here, is retained. We suggest that the San Juan basin of New Mexico and Colorado is a postequilibrium example of our model. This basin has progressed through the overpressured equilibrium stage and is now abnormally low pressured due to cooling and gas-volume contraction accompanying regional uplift. However, the gas seal developed during the overpressured stage is present as a low-permeability updip water block.
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