Abstract: Clay Mineralogy and Reservoir Implications Within the Pennsylvanian Cross Cut Sandstone: TWP Field, Runnels County, Texas

Steven K. Henderson

Clay minerals in sandstone hydrocarbon reservoirs may have profound effects on the producibilities of those reservoirs. Among the factors controlling reservoir porosity are the type, abundance, and morphologies of clays. Initial reservoir porosity and permeability may be reduced because of the pore-lining and/or pore-filling nature of the clays. Further reduction in permeability may result from migration of fine-grained particles, water sensitivity, and precipitation of insoluble compounds caused by completion and production techniques. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) generally form the foundation of descriptive studies designed to circumvent such problems.

The Pennsylvanian Cross Cut Sandstone of TWP Field, Runnels County, Texas, contains a clay assemblage of kaolinite, illite, and chlorite. These clays are irregularly distributed throughout the reservoir and have the potential of seriously degrading producibility. Kaolinite is the most abundant clay component and occurs as vermicular booklets in intergranular pores. Because of kaolinite's abundance and tendency to migrate, regulation of flow-rates and salinity during completion is necessary to prevent dispersion of the clays and potential "brush-piling" at pore throats as well as precipitation of insoluble compounds. Minor amounts of illite are present in the analyzed samples and occur as thin, discontinuous coatings on epitaxial quartz overgrowths, but pose little threat to reservoir uality. Chlorite is of such low abundance that is effects on reservoir quality are negligible; however, unless exact chlorite chemistry can be established, iron sequestering agents should be added to completion fluids in order to avoid possible precipitation of insoluble compounds.

AAPG Search and Discovery Article #90960©1995 AAPG Southwest Section Meeting, Dallas, Texas