ABSTRACT: Ramp Buildups in the Lower Strawn Limestone (Pennsylvanian): Controls on Stratigraphic Reservoir Variability

David C. Harris

Algal bioherms of Pennsylvanian (Desmoinesian) age form stratigraphic hydrocarbon reservoirs in the lower Strawn limestone of the Tatum basin around Lovington, Lea County, New Mexico. Strawn bioherms developed in a low-energy, middle to outer ramp setting, as dip-parallel elongate buildups on the northern flank of the Central Basin platform. Lithofacies identified include (1) coralgal-foraminiferal lime pack/wackestones, (2) argillaceous lime mudstones, (3) echinoderm grainstones, and (4) siliceous black shales. The coralgal bioherm lithofacies is composed of phylloid algal plates and encrusting tubular forams with minor corals (Chaetetes). Lime mudstones are dark, argillaceous, carbonaceous, and pyritic. Mudstones encase algal mounds, and are interpreted as a deeper, off mound ramp facies. Bioherms developed in several cycles during regressive ramp aggradation. Cherty echinoderm grainstones disconformably cap the lower Strawn carbonate interval in more distal parts of the mound trend. Grainstones grade upward into basinal black shales, and are interpreted as a transgressive facies deposited after subaerial exposure of the underlying interval.

Porosity within mounds averages 8-10% and consists of primary shelter and intergranular pores, and secondary moldic, vuggy, and fracture (breccia) porosity. In updip areas, porous bioherms are sealed by nonporous intermound mudstones. In downdip areas, mounds occur at the top of the interval, and are directly overlain by echinoderm grainstones. These mounds are typically nonproductive either due to low porosity/permeability or high water saturations. Stratigraphic juxtaposition with grainstones may have resulted in increased calcite cementation of underlying bioherms or a lack of top seal, and defines the basinward play extent. Algal mounds overlain by intermound mudstones have higher porosities and contain trapped oil accumulations. Mudstones not only acted as hydrocarbon seals, but elped to preserve porosity in bioherms during burial.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990