The Role of Geology in Characterizing Low-Permeability Sandstones, North Appleby Field, East Texas Basin

Abstract

Abstract Geological knowledge of low-permeability (permeability <0.1 md) hydrocarbon-bearing sandstone reservoirs enhances exploration and exploitation strategies. Structural, stratigraphic, and petrographic analyses are required to interpret trap style, reservoir quality, and porosity and permeability distribution. Information compiled in such studies facilitates (1) the estimation of reservoir volume and drainage area, (2) the design of drilling and completion programs, (3) the location of development wells, and (4) the calibration of logs. At North Appleby field, data from cored wells in the lower Travis Peak Formation have established a foundation for predictions about reservoir geometry, occurrence of natural fractures, and the degree and type of diagenesis that the reservoir has undergone. Only after integrating structural, stratigraphic, and petrographic data on the evolution of these low-permeability sandstones can relationships among sand-body geometry and orientation, diagenesis, and natural fractures be discerned. Broad, tabular reservoirs were deposited by braided- to meandering-fluvial systems. Reservoir seals and internal fluid-flow barriers consist of mudstones and porosity-occluding quartz cement. Pore types range from micropores to primary and secondary macropores. Vertical natural fractures are macropores that are oriented at approximately right angles to sandstone trends. Natural fractures are abundant in sandstone with very extensive quartz cement (>17%); in these rocks natural fractures should be considered in reservoir evaluations and hydraulic-fracture treatment design. Geometry and quality of Travis Peak reservoirs vary with depositional environment and degree of diagenesis. Reservoirs at the top of the formation are thin and separated by mudstones; few natural fractures occur in these sandstones. Productive sandstones at the base of the formation, however, are thicker and more sand rich; these contain a greater percentage of quartz cement and are naturally fractured.