Pressure seals: their diagenetic banding patterns

Abstract

Pressure seals are economically significant geological phenomena since they play an important role in deep natural gas entrapment. Identified in basins worldwide, they offer a new frontier for exploration of natural gas reservoirs below 10,000 ft. (3000 m). Pressure seals are low-permeability envelopes which enclose abnormally pressured reservoirs. There are three different types of seals-basal, lateral and top. Basal seals, which define the bottom of abnormal pressure compartments, usually follow a stratigraphic horizon. Lateral seals are generally associated with faults. Top planar seals may parallel or cut across time-stratigraphic boundaries; they may be developed in any lithology. The southeastern portion of the Anadarko Basin displays a layered sequence of abnormally pressured fluid compartments at depths ranging from 3,000 to 13,500 ft. (900 to 4150 m). These compartments are separated from each other by pressure seals. In McClain County, a top pressure seal separating two abnormally pressured compartments is located between 10,000 ft. (3000 m) to 12,000 ft. (3650 m) within the Simpson Group (Ordovician). The seal zone is characterized by alternating cemented and porous intervals. Silica and carbonate cements comprise a diagenetic laminated and banded pattern. The compositional and textural heterogeneity within this structure evolved through diagenetic processes active during the seal evolution.