Evaluation of organic matter, subsurface temperature and pressure with regard to gas generation in low-permeability upper cretaceous and lower tertiary sandstones in Pacific Creek area, Sublette and Sweetwater counties, Wyoming

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Abstract
Investigations of a sequence of Upper Cretaceous and lower Tertiary rocks in the Pacific Creek area of the northeastern Green River Basin show that studies of organic matter content, type, and maturity in conjunction with subsurface temperature and reservoir pressure, will help define prospective gas-saturated intervals and delineate areas of maximum gas-resource potential. The onset of overpressuring occurs at about 11,600 ft (3,500 m) , near the base of the Upper Cretaceous Lance Formation. Drill stem test data indicate that at about 12,800 ft (3,900 m) the pressure gradient is as high as 0.94 psi/ft (19.0 kPa/m). The development of overpressuring may involve several processes; however, interpretation of the available information indicates that active generation of large amounts of wet gas is one of the more important processes. Nearly coincident with the top of overpressuring is a reversal of the spontaneous potential (SP) curve that we believe is caused by a reduction of formation water salinity. We Interpret that the very small amounts of water produced during thermochemical decomposition of organic matter and the dehydration of clays during clay transformation may provide enough low-salinity water to effectively dilute the original formation water to a degree that the formation water resistivity greater than the mud filtrate resistivity. Microscopic and geochemical evaluation of organic matter shows that the analyzed samples contain cominantly humic-type kerogen. Total organic carbon contents of 26 samples range from 0.25 to 7.84 weight percent. Most samples exceed 0.5 percent organic carbon and the average is 1.38 percent. A vertial profile of organic maturation, determined by vitrinite reflectance and color alteration of organic material, shows that the top of overpressuring and beginning of important wet-qas generation occur at vitrinite reflectance values of 0.74 to 9.25 and color alteration values of about 2.8. The present minimum temperature at the top of overpressuring is at least 190°F (88°C) as determined from uncorrected bottom-hole temperatures recorded during logging of wells drilled in the area. The preservation of abnormally high pressures is probably due to presently active generation of gas in a thick interval of discontinuous, very low-permeability shales, siltstones, and sandstones.

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