Underpressured Reservoirs Mapped in the United States

Abstract

The spatial distribution of underpressured reservoirs across the United States is worthy of attention for a variety of reasons. Abnormally low-pressured reservoirs may not be detected during exploration and may cause problems in reservoir management during production. Underpressured reservoirs can have properties favorable for carbon dioxide storage. Deep underpressured strata are attractive for wastewater injection because injection pressures are low. In some cases, ensembles of underpressured reservoirs are best understood in terms of regional subnormal pressure. Pressure gradients, or pressure–depth ratios, can be calculated from reservoir records of depths and initial pressure measurements in the Nehring Associates, Inc., 2012 version of the Significant Oil and Gas Fields of the United States. Of over 25,000 reservoirs in this database, more than 3,000 screened reservoirs show pressure gradients less than hydrostatic pore pressure of 0.433 pounds per square inch per foot (psi/ft). These are conventional reservoirs either onshore or under State waters that provide reliable readings of initial formation pressures and depths. The Nehring database omits conventional reservoirs in the Appalachian Basin. There are almost 2,000 reservoirs with pressure gradients less than 0.4 psi/ft, and of these, just over 500 are less than 0.3 psi/ft. The locations of such reservoirs on a map of oil and gas province boundaries show that underpressured reservoirs are scattered across the major oil-bearing provinces of the United States but are concentrated in specific basins. A band of significantly underpressured reservoirs (pressure gradients less than 0.3 psi/ft) runs from the Arkoma Basin through the Midcontinent region's Anadarko Basin and Las Animas Arch to the Denver Basin where more than 200 reservoirs have pressure gradients less than 0.3 psi/ft. There are also clusters of significantly underpressured reservoirs in the Permian Basin, Bend Arch–Fort Worth Basin, Powder River Basin, and several basins in California, near the Four Corners area of New Mexico and Utah, and in the North-Central Montana province. A map showing their spatial distribution lays the groundwork for understanding the geologic context of underpressured reservoirs.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015