Characteristics of Late Pennsylvanian Reef/Mound Complexes, Eastern Shelf, West-Central Texas

WAITE, LOWELL E., Mobil Exploration and Producing Services Inc., Dallas, TX

The Eastern shelf of the Midland basin contains oil and gas reserves (3-75 MMBOE/field) located in shallow (5000-7000 ft subsea) Upper Pennsylvanian carbonate reef/mound complexes. Discoveries define a 250-mi, north-south oriented, producing trend. The regional setting and geological and geophysical characteristics of known reef/mound complexes provides insight into reef development.

Reef growth was initiated during the early Strawn on an extensive, structurally stable, carbonate ramp. Subsidence throughout the late Paleozoic, combined with long-term relative sea level rise during the Pennsylvanian, allowed construction of large back-stepping carbonate bank systems. Reefing was established along the seaward margins of the low-energy bank systems, and persisted in isolated locations during continued sea level rise. During development, the complexes were subject to glacially driven, short-term relative sea level changes, resulting in both intermittent exposure/erosion and in development of solution-enhanced porosity.

Individual reef/mounds occur in clusters, delineating barrier- and atoll-like associations. Furthermore, known reefs can be categorized by (1) reef pattern, (2) age, (3) total thickness, (4) position relative to the early Strawn ramp margin, and (5) seismic expression. Mound complexes show similarities in shape, physical orientation, and facies, but have varying seismic expression depending on overall reef size and complexity of the overlying geology.

Characteristics of the Eastern Shelf reefs are similar to time-equivalent reefs of the nearby Horseshoe atoll. Regional geologic mapping suggests a possible connection of the Horseshoe atoll and the Eastern Shelf bank system during the early Strawn.

AAPG Search and Discovery Article #91011©1991 AAPG Southwest Section Meeting, Abilene, Texas, February 9-12, 1991 (2009)