Abstract

Our understanding of reservoirs in the Lower Paleozoic section in the Permian Basin has evolved over the past 30 years or so, attendant with the recognition of the different types of reservoirs and traps that actually are present in these rocks. Many of the reservoirs in the Ordovician (Ellenburger Group) and Silurian-Devonian units (Fusselman, Wristen, Thirtyone formations) resulted from karst-associated erosion and dissolution that overprinted primary depositional fabrics and prior-formed dolomites. In turn, these reservoirs were later overprinted by tectonic processes. Consequently, there commonly is a component to these reservoirs that is not always obvious through conventional subsurface or seismic mapping, or not seriously considered as a viable geologic mechanism that created reservoir traps. Many operators still drill short of all of the potential reservoir zones in these rocks once the first porosity zone beneath the Woodford Shale is encountered and tested. A wet test immediately beneath the Woodford, however, does not preclude the possibility of the presence of oil reservoirs farther down in the section, perhaps as deep as the Ellenburger.

Various types of sub-unconformity and intra-formational disconformity traps are present, with or without pronounced structural expression, in the Lower Paleozoic formations in the Permian Basin. For example, subunconformity truncation traps are present throughout the Ellenburger, in sections that are identifiable and mapable by insoluble residue analysis. There locally is a folded, porous and productive oolite-rich facies truncated by a karst unconformity in the lower part of the Fusselman. The contact of the Fusselman and overlying Silurian Wristen Formation also is karsted, and stratigraphically adjoining reservoirs within these formations may or may not be in communication. Likewise, the upper contact of the Devonian Thirtyone Formation with the Woodford Shale is a major unconformity, but intraformational disconformities also define multi-layered reservoirs that are not always at the base of the Woodford, or not reflected by mapping on the basal Woodford.

3D seismic and more sophisticated evaluation tools, along with core and cuttings sample descriptions and drill-stem tests, help to interpret the fundamental attributes of these reservoirs. Such an approach has proven to lead to discovery of new plays in these rocks, and help us more efficiently develop resources. A number of field examples are presented that show a variety of reservoir traps that were not recognized as such when these fields were first discovered and developed. Every Lower Paleozoic field in the Permian Basin has unique attributes that require the explorationist to keep an open mind to recognize and search for more subtly-expressed traps and seals. An understanding of the various reservoir scenarios would provide for cost-effective exploration and development of new fields.