ABSTRACT
Pennsylvanian and Lower Permian shelfal limestones were studied in core and wireline logs on the eastern side of the Central Basin platform in west Texas. Sixty-three (63) cycles were delineated in the study interval, which includes 200–250 m of Canyon (Missourian), Cisco (Virgilian), and Wolfcamp strata. Four general lithofacies are present: fossiliferous wackestones and packstones, grainstones, phylloid algal boundstones, and shales. These lithologies typically occur in 1–18-m-thick cycles bounded by subaerial exposure surfaces. Grainstones in the upper part of some cycles indicate a shallowing of environments prior to subaerial exposure. Many cycles have subaerial exposure surfaces developed on subtidal fossiliferous wackestones or packstones suggesting rapid falls in sea level. Long-term transgressive intervals (transgressive systems tracts or TST) are dominated by thick (>4 m) cycles, whereas long-term regressive intervals (highstand systems tracts or HST) are dominated by thinner cycles. Stable carbon isotope data suggest that thick cycles in TSTs were subjected to short periods of subaerial exposure, whereas thin cycles in the HSTs were subjected to much longer subaerial exposure.
Porosity is quite variable beneath subaerial exposure surfaces. At any given well, reservoir-grade porosity (>4%) is present below only 10–45% of the identified subaerial exposure surfaces. Where present, reservoir-grade porosity occurs in the upper part of cycles, 0.3–5 m below subaerial exposure surfaces. Porosity in thick cycles of the TST is relatively widespread and not facies-selective. Porosity is less abundant and facies-selective in the HST, with porosity occurring only in relatively thick (1.5–4 m) grainstones, which are concentrated near the shelf margin. Thin (<2 m thick) cycles in the shelf interior (HST) have no reservoir-grade porosity. Wackestones and packstones generally are not porous in the HST, even where they occur immediately below a subaerial exposure surface. Lower porosity is correlated with greater duration of subaerial exposure. Prolonged subaerial exposure apparently reduced matrix porosity by allowing more time for calcite precipitation, which was especially effective in reducing porosity in micritic strata below exposure surfaces.
