The Sonora Canyon is one of several "Canyon Sands" (Virgilian-Wolfcampian) intervals that exist in the Val Verde Basin of southwest Texas. This paper describes the presence and attributes of natural fractures in Sonora Canyon sandstones in Sutton County, Texas. Data obtained from three cored wells through a cooperative research program conducted by the Gas Research Institute and industry show that natural fractures are locally abundant. At least three distinct natural fracture classes coexist that have contrasting distributions, characteristic sizes, and/or mineral fills. The most abundant fracture class consists of clay- or clay- and carbonate-filled fractures existing only in siderite-cemented zones in sandstones. Owing to their clay content, these fractures locally may be barriers to fluid flow and their presence could cause reservoir heterogeneity and anisotropy. Calcite- and quartz-cemented fractures are less common fracture classes. These fractures are larger than those in siderite layers and are partly open as a result of propping by diagenetic minerals. Our results show that fractures in Sonora Canyon sandstones should be considered in completion and stimulation design. For example, high treatment pressures observed in some Canyon Sandstone stimulations may be due to natural fractures promoting propagation of multiple fracture strands near the wellbore. Inconsistencies between hydraulic fracture strike and maximum horizontal stress may be due to natural fractures guiding hydraulic fracture growth.

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