Cycle Stratigraphy and Porosity in Pennsylvanian and Lower Permian Shelf Limestones, Eastern Central Basin Platform, Texas

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 200250 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 118-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 se 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 1045% of the identified subaerial exposure surfaces. Where pres-ent, reservoir-grade porosity occurs in the upper part of cycles, 0.35 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 (1.54 m) grainstones, which are concentrated near the shelf margin. Thin (