Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
2004 Southwest Section AAPG Meeting, El Paso, Texas, March 6-9, 2004
Regional Stratigraphy and Reservoir Development in Middle Paleozoic (Silurian and Devonian) Carbonate Successions, West Texas: A Review
Stephen C. Ruppel
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.
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.