Anomalously Pressured Gas Accumulations in the Laramide Basins, North America

Zung S. Jiao and Ronald C. Surdam

Establishing the boundary between normally pressured (0.43-0.46 psi/ft) and anomalously pressured (<0.43 or <0.46 psi/ft) fluid-flow regimes is essential to the successful exploration of hydrocarbons, especially gas, in the deep basin centers in the Laramide basins of North America. A new technique--3-D modeling of the pressure regime--has been developed to examine this boundary in detail. Utilizing computerized pressure, production, temperature, and this 3-D modeling provides us with a better understanding of the relationship of the pressure regime and gas accumulation.

Sets of plots for the major gas reservoirs in the Powder River, Bighorn, Wind River, Washakie, and Denver basins have been generated using a computer database. These plots include 2-D and 3-D pressure/gas saturation regime modeling, pressure versus depth, pressure gradient versus depth, reservoir temperature versus depth, reservoir temperature versus pressure, cumulative gas production versus depth, pressure gradient versus cumulative production, and the distance of gas reservoir to the top of anomalous pressure versus cumulative gas production. The top of the anomalous pressure is determined from decompacted sonic logs.

This study reveals that hydrocarbons tend to accumulate primarily near the top (within 1500 ft of the boundary) of regional anomalously pressured compartments regardless of depth, temperature, or lithology in Laramide basins. Nearly 75% of the gas production is from this section. Gas accumulation decreases consistently with increasing pressure and temperature below this point.

This research was supported by the Gas Research Institute contract #5089-260-1894.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California