Reservoir pressure is a very important component of hydrocarbon exploration and exploitation for unconventional resources. Within the realm of unconventional resources, shale oil and gas resources have received a great deal of attention, however, unconventional gas-bearing sandstone reservoirs, have recently received little attention, especially underpressured or subnormally pressured reservoirs. Overpressured hydrocarbon-bearing sandstone reservoirs are easily identified during drilling, however, it is not uncommon to drill through a subnormally pressured, low-porosity, low-permeability hydrocarbon-bearing sandstone with normal-weight drilling fluids and conclude the interval to be barren. As a consequence of this interpretation, a potentially productive sub-normally pressured reservoir may be incorrectly evaluated and subsequently overlooked.

In some areas, this uncertainty may be resolved through the use of present-day subsurface temperatures and thermal maturity. Well-documented abnormally pressured, low-permeability, hydrocarbon-bearing sandstones in some basins in the United States reveal the presence of two distinctly identifiable pressure domains. Hydrocarbon-bearing sandstones with equal or nearly equal levels of thermal maturity may be distinguished between overpressured and underpressured reservoirs by a comparison of subsurface temperatures; relatively high present-day subsurface temperatures are indicative of overpressured reservoirs, whereas low temperatures are in some instances, indicative of underpressured reservoirs. In the San Juan, Raton, and Appalachian Basins, burial history modeling of hydrocarbon-bearing, underpressured sandstone intervals reveal that during the earlier stages of burial, subsurface temperatures were higher than presently observed and reservoir pressures were probably higher. The early stages of overpressure are interpreted to be due to hydrocarbon-charging and the development of high pressures. The high pressures were needed to exceed the capillary pressure of the water-wet pore system. During subsequent burial history, temperatures were reduced with a partial loss of hydrocarbons and the development of underpressure. The interpretations offered here are of a preliminary nature and only applicable to low-permeability (unconventional) hydrocarbon-bearing sandstone reservoirs with thermal maturity values greater than 0.7% vitrinite reflectance. However, this method does not appear to be consistent in the Wattenberg Field of the Denver Basin.

URTeC 1575538

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