Regional Stratigraphy and Reservoir Development in Middle Paleozoic (Silurian and Devonian) Carbonate Successions, West Texas: A Review

RUPPEL, STEPHEN C., Bureau of Economic Geology Jackson School of Geosciences
The University of Texas at Austin, Austin, TX

    The middle Paleozoic of the Permian Basin is rich in hydrocarbon resources. Original oil in place in this carbonate-dominated succession has been calculated at 7.8 billion barrels. Of this, only about 2.1 billion barrels has been produced indicating that these rocks contain a sizeable remaining resource.

    The most widespread of the middle Paleozoic successions is the Fusselman Formation (Upper Ordovician and Lower Silurian). These rocks comprise basal ooid grainstones and overlying pelmatozoan packstones deposited on a broad, low-gradient, shallow-water platform. Ooid grainstones contain primary interparticle porosity whereas reservoir development in pelmatozoan packstones is dominantly the result of dissolution and local overprinting karst.

    Wristen Group (Middle-Upper Silurian) carbonates include outer ramp/slope to basin mudstones (Wink and Frame Formations) and complex platform-margin and inner platform shallow water carbonates (Fasken Formation). Reservoirs are limited to platform margin, coral/stromatoporoid-buildup successions and cyclic, laterally variable, inner platform facies of the Fasken Formation. Porosity in inner platform rocks is enhanced by diagenesis and karst associated with periodic sea level fall. Because of their depositional and diagenetic complexity, these rocks are the least well understood and, not surprisingly, contain the greatest remaining resource.

    The Thirtyone Formation (Lower Devonian) comprises a ramp succession of updip skeletal grainstones and downdip spiculitic cherts and siliceous limestones. Reservoirs are best developed in outermost ramp cherts which display tabular (proximal) and channel form (distal) geometries. Porosity in these rocks is typically bimodal consisting of spicule molds and microcrystalline pores. Siliceous limestones (dominantly fine grained skeletal packstones) occupy a medial ramp position and contain intercrystalline and microcrystalline pores.