SALLER, ARTHUR H.
Unocal Corporation, Sugar Land, Texas

Abstract: Sequence Stratigraphy and Phylloid Mound Development in Pennsylvanian and Lower Permian Limestones, Midland Basin, West Texas, USA

Phylloid algal mounds are widely scattered in the Pennsylvanian and Lower Permian of the Permian basin. Phylloid algal mounds are important reservoir rocks in the Southwest Andrews Area on the western flank of the Midland basin and BC (Canyon) Field in the middle of the Midland basin. In both areas, phylloid mounds occur in relatively thick cycles deposited during long-term transgressions. The Southwest Andrews Area contains approximately 87 middle Pennsylvanian to lower Permian cycles, but phylloid algal boundstones (mounds) were only observed in four of those cycles. The phylloid mounds are most widespread four cycles below a major drowning and backstepping of the carbonate shelf margin in the middle Wolfcamp. In that cycle, phylloid mounds are most common near the shelf margin, but also occur at least 14 km onto the shelf. The phylloid boundstones (mounds) are approximately 4-6 m thick, and present in some wells, but not adjacent wells approximately 250 m away.

In BC (Canyon) Field, phylloid boundstone mounds occur in three cycles immediately below a maximum flooding surface when the shallow marine carbonate platform drowned and stepped back to the Horseshoe Atoll. The three intervals of phylloid boundstones are each 3-6 m thick and were stacked, apparently due to growth on depositional highs. Three-dimensional seismic data suggest that the mound complexes are approximately 150-300 m across and are scattered in an area at least 10 km by 10 km at the southern margin of the precursor to the Horseshoe Atoll platform.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas