ABSTRACT

The Pennsylvanian Tonkawa sand of the Douglas Group in the Virgilian Series was studied in southern Ellis County and northern Roger Mills County, Oklahoma. No previous work has included the South Peek field. The South Peek field is a viable location to study the Tonkawa sand because of available core. Techniques employed include analysis of cores, identification of thin section microscopy, mapping of structure, mapping of net sandstone thickness, and preparing cross sections.

The sandstone interval is composed of very fine, well-sorted sublitharenite. Monocrystalline and polycrystalline quartz, rock fragments, feldspar, and muscovite are the main constituents. Diagenesis affected the porosity and permeability of the Tonkawa sand. Silica, dolomite, calcite, and siderite cement are some of the diagenetic components that have destroyed porosity and permeability. Locally, these cements dominate the fabric of the sandstone. Chlorite is the most dominant authigenic clay mineral in the Tonkawa sand. Other minerals found in local abundance are kaolinite, and minor amounts of illite. Minor primary porosity is in the form of intergranular porosity. Secondary porosity occurs in the form of moldic, elongate, and microporosity. These have been caused by dissolution of feldspars, fossil fragments, and carbonate cements.

The sandstone zone is present as elongated and coalescing lobes in a deltaic environment. The various facies were deposited in an environment of overall regression. Delta front, crevasse splays, and channel-fill sandstones are examples of reservoir facies that make the Tonkawa sands important for oil and gas