Pressure Seals--Implications for Deep Gas Exploration in Anadarko Basin

Vanessa A. Tigert, Zuhair Al-Shaieb

Pressure seals are economically significant geological phenomena because they play an important role in deep natural gas entrapment. Pressure seals identified in basins worldwide may offer a new frontier for exploring natural gas reservoirs below -10,000 ft. Pressure seals are low-permeability envelopes that enclose abnormally pressured internal reservoirs. There are three different types of seals: basal, lateral, and top planar. Basal seals define the bottom of abnormal pressure compartments and usually follow a stratigraphic horizon. Lateral seals are usually associated with fault patterns. Top planar seals may cut across time-stratigraphic boundaries, different lithologies, and structures, and are by far the most significant type.

The southeastern portion of the Anadarko basin in the Mill Creek graben area displays a layered sequence of abnormally pressured fluid compartments between -3,000 and -16,000 ft. These compartments are separated from each other as well as from overlying and underlying normal pressure zones by pressure seals. In McClain County, a top planar pressure seal separating two abnormal pressured compartments is located between -11,000 and -12,000 ft within the Simpson Group. Diagenetic signatures identified in the seal zone are characterized by carbonate and/or silica cemented intervals alternating with more porous and permeable units generating distinctive banded and/or laminated structures, coined "zebra structures." The resultant compositional and textural heterogeneity of the Simpson sands ones may be related to diagenetic modification that occurred during the seal evolution.

AAPG Search and Discovery Article #91025©1989 AAPG Midcontinent, Sept. 24-26, 1989, Oklahoma City, Oklahoma.