2012 Bill Hailey Memorial Short Course

 

 

Understanding carbonate reservoirs of different ages is complicated by evolutionary trends of organisms (especially as punctuated by major extinctions events) and by changes in climate and tectonic regimes. The Upper Paleozoic of Texas and New Mexico is a fine example of such interpretive complexities. The Late Devonian extinction event decimated reef-building faunas, so Mississippian strata have no barrier reefs and hardly any reefal deposits of any sort. In the absence of barriers, open shelf and slope deposits predominate and range from crinoidal grainstones to organic-rich mudstones that are interspersed with small, muddy bryozoan/microbe buildups. By Pennsylvanian time, phylloid algae and fusulinid foraminfers had evolved and combined with corals, red algae and other organisms to produce widespread mounds and buildups. These, however, were generally small in overall size and thus rarely formed restrictions to open shelf water circulation. Active tectonics and repeated glacioeustatic sea-level changes in the Late Pennsylvanian led to extensive meteoric alteration of these rocks, with locally extensive secondary porosity development . By Mid-Permian time, continued faunal evolution led to the development of major reef complexes, including the sponge/algal/microbial/cement barriers of the Permian Basin. Coupled with waning southern hemisphere glaciations and cessation of collisional tectonics, this led to widespread restricted shelves with thick evaporates, associated dolomites and spectacular reservoirs.

 

Modern analogs are typically used to gain an understanding of the processes and environments of carbonate sediment formation. But these three “different worlds” of Paleozoic sedimentation require judicious selection and “blending” of modern analogs from a wide range of locations, coupled with outcrop studies of Mississippian to Permian strata, to make real sense of the depositional and diagenetic patterns found in the diverse Late Paleozoic reservoirs of the southwest. No single analog can suffice to model these complex strata, and there can be no simple model for understanding the porosity distribution in those reservoirs. But modern and ancient analog studies at least allow an understanding of the processes involved in both deposition and diagenesis and that knowledge can significantly improve exploration modeling.

 

---

---