Abstract

The Mid-Max Field, Andrews County, Texas, provides an example for the exploration and development of subtle stratigraphic targets in the Permian Basin. The integration of all available data, including seismic, log, core and engineering data, is essential in this mature area as the search for hydrocarbons turns toward subtle structural and stratigraphic prospects. The Mid-Max Field, on the Central Basin Platform, is a Silurian reef. Seismic data support the geologic interpretation of Woodford Shale thinning over a Silurian high, a Silurian to Fusselman thick, and the presence of a relatively flat Fusselman reflector beneath the Silurian high. Whole cores and rotary sidewall cores from the field contain stromatoporoids, corals, algal stromatolites, crinoidal grainstones and other reef elements. The reservoir is predominantly dolomite with porosity and permeability controlled by the presence of lime mud, fracturing and diagenesis. The 26° to 29° API gravity sour crude produced at Mid-Max Field is atypical of Siluro-Devonian oils, which are commonly sweet crudes of 35° to 45° API gravity. This atypical oil is explained by a biodegraded mixture of two types of Woodford-sourced oils.

Tectonism appears to have played a minor role in the formation of the Mid-Max Field, unlike the conventional faulted-anticline prospects in the Siluro-Devonian of the basin. Although modern CDP seismic data and a dry Silurian test existed over the Mid-Max prospect prior to discovery, the prospective reef had been misinterpreted or overlooked. This would suggest that there might be similar subtle Silurian prospects elsewhere in the Permian Basin.