Sedimentology and reservoir characterization of the Upper Pennsylvanian Cline Shale, Midland Basin, Texas
The Cline Shale, an organic rich mudrock comprising the Canyon and Cisco Groups in the Midland Basin, has recently become an exploration target and production interval. The Cline is a basin-restricted facies specific to the Midland Basin, and is interpreted to have been deposited in a deep water environment by hemipelagic suspension and mass transport debris flow and turbidity flow. Based on core description, thin section observation, and bulk compositional XRD data carried out on seven cores, seven lithofacies have been identified including various types of clay-rich mudstone, carbonate and sandstone. Regional stratigraphic sections show that the Cline structurally dips towards the Central Basin Platform and ranges from 117 ft to 530 ft in thickness.
Gamma-ray log patterns can indicate vertical changes in lithology by responding to clay content and organic matter. Two types of cycles in the Midland Basin are identified by upward-increasing and upward-decreasing gamma ray patterns. Cycles in the Midland Basin are correlated to those on the shelf, and are interpreted to correspond to sea level changes on the shelf. Fifteen stratigraphic cycles have been distinguished from a typical basin center core, where eight are in the Lower Cline and seven are in the Upper Cline. These cycles are laterally continuous across the Midland Basin. More stratigraphic cycles can be recognized near the toe of slope, because not all the depositions occurred within a cycle extend across the basin floor. It is inferred from correlation of gamma ray patterns that high-frequency sea-level fluctuation affected depositional processes on the platform and controlled sediment deposition in the basin.
Wireline logs are a crucial tool for providing a quantitative evaluation of reservoir quality. The Cline pay zone is defined based on (1) Gamma ray>110 API, (2) Density porosity (DPHI)>3.4%, (3) TOC>2.0 wt.%. The possible exploration target is located in northern Glasscock, western Howard and southern Martin counties. The pay zone thickness varies from 50 ft to 100 ft. The average TOC of pay zones is greater than 2.5 wt.% and the average dry helium porosity is greater than 5.0%.