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Facies Variability Within a Single, Deep-Water Basin-Floor, Mixed Carbonate-Siliciclastic Fan (Upper Wolfcamp Fm., Permian, Delaware Basin, New Mexico)


Rarely are sea-floor fans containing a significant volume of carbonate detritus documented or discussed. Such fans are common within the lower Permian Wolfcamp Fm. in the Delaware Basin in SE New Mexico and west Texas, U.S.A. Three cores retrieved as part of an unconventional oil/gas exploration and development program in SE New Mexico Wolfcamp preserve interlayered wackestone, packstone, and mixed siliciclastic-carbonate mudstones. Core combined with regional subsurface studies show that the sediments are organized into an approximately 350 ft. thick mixed carbonate-siliciclastic deep-water fan. Carbonate debrites are concentrated in more axial positions and siliciclastic mudstones in more distal areas. Cores collected represent the frontal to distal fringe, off-axis, and lateral fringe portions of the fan. The fan prograded SW. The carbonate dominated portion of the fan trends at least 35 mi. in a NE-SW direction and 11 mi. NW-SE across. It is partially bounded to the east by a fault. Lobe complexes can be recognized which are bounded by regionally correlative horizons (A, B, C, and D, from older to younger). An overall upward fining across B and C horizons records a progressive back-stepping of the fan through time. Unlike siliciclastic fans where axial facies are dominated more by turbulent flow deposits (turbidites), the axis, off-axis, and lateral fringe facies in the Wolfcamp are dominated by laminar flow deposits such as coarse carbonate debrites and mass transport deposits (MTDs). Mixed carbonate siliciclastic hybrid event beds (HEBs) and finer grained background sediments form a minor component in these areas. Coarse carbonate deposition decreases towards the frontal fringe areas where facies are dominated by mixed carbonate-siliciclastic mud-rich HEBs 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 often 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?) ungraded debrites and MTDs. Facies changes from axis to frontal fringe are gradual but facies changes from axis to lateral fringe are rapid and may change significantly over a 2 mi. horizontal well.