Facies variability within a mixed carbonate-siliciclastic sea-floor fan (upper Wolfcamp Formation, Permian, Delaware Basin, New Mexico)

Sea-floor fans containing both carbonate and siliciclastic detritus are rarely recognized in the rock record but are common within the Permian upper Wolfcamp Formation in the Delaware Basin in southeast New Mexico and west Texas. One such compositionally mixed fan has been studied in detail as part of an unconventional oil and gas exploration and development program in southeast New Mexico. Three cores were collected through an approximately 100-m ( approximately 350-ft)-thick interval that defines the structurally controlled fan. Cores collected represent the frontal to distal fringe, off-axis, and lateral fringe parts of the fan. Unlike siliciclastic fans where axial facies are characterized more by turbidites, the axis, off-axis, and lateral fringe cores in the Wolfcamp are dominated by carbonate debrites. Coarse carbonate deposition decreases toward the frontal fringe areas where core facies are composed of mixed carbonate-siliciclastic mud-rich hybrid event beds and background sedimentation. The core through the lateral fringe differs from the off-axis core in that the debrites in the lateral fringe are thinner and commonly rheologically stratified with finer grained debrites sitting directly on top of coarse-grained debrites, suggesting a genetic link in their formation. The axial facies appear to be dominated by thick amalgamated debrites. Changes in carbonate content from axis to frontal fringe appear to be gradual but rapid from axis to lateral fringe over a 3-km (2-mi) distance. The carbonate-rich part of the fan trends at least 55 km (34 mi) in a northeast-southwest direction and extends 18 km (11 mi) northwest-southeast across.