Characterization of Deep-Water Carbonate Turbidites and Mass-Transport Deposits (MTDs) Utilizing High-Resolution Electrical Borehole Image Logs: Late Leonardian (E. Permian) Upper Bone Spring Limestone, Delaware Basin, Southeast New Mexico and West Texas
Late Leonardian (Early Permian) deep-water carbonates of the Upper Bone Spring Limestone member are comprised of cyclically-stacked, sub-reservoir scale (millimeter-centimeter thick) turbidites and mass-transport deposits (MTDs) separated by planar-laminated pelagic and hemipelagic siltstone and mudstone within the Delaware Basin, southeast New Mexico and west Texas. Previous subsurface investigations emphasize conventional wire-line log and seismic data sets to characterize these deposits in a proximal foreslope setting; consequently, the limited resolution offered by these data sets do not allow for accurate characterization of sub-reservoir scale architectural and compositional variations internally exhibited by these deposits. The present investigation integrates high-resolution (centimeter scale) electrical borehole image logs with conventional subsurface data sets as a means to 1) enhance recognition and stratigraphic cyclicity of turbidites and MTDs, and 2) provide detailed analysis of architectural attributes associated with different turbidite and MTD facies.
Methods utilized include high-resolution whole core and thin-section petrographic analysis for two cored wells to determine compositional and physical properties of individual carbonate flow deposits, and the transport mechanism likely responsible for sediment transport and deposition. Similar observations were then determined by visual analysis of electrical borehole image logs. These observations were then compared in order to establish degrees of continuity and variance with conventional wire-line log data sets.
Preliminary results indicate that 1) >50 % of strata from the two cored wells are dominated by turbidity current and MTD sedimentation processes, 2) application of MTD classification schemes allowed for subdividing observed MTDs into liquefied flows, grain flows, debris flows, and slump deposits based on compositional and architectural attributes, 3) >90 % of turbidites and MTDs observed in core are easily identified in image logs, 4) all observed turbidites and MTDs exhibit high-moderate resistivity due to increased limestone content, and are easily discerned from highly conductive pelagic and hemipelagic mudstones, and 5) decreasing gamma ray and increasing resistivity responses from conventional logs are directly correlative to turbidites and MTDs observed in whole core and electrical image logs. Preliminary results suggest integrating electrical image logs with conventional data significantly enhances recognition and characterization of sub-reservoir scale strata. Application of this methodology in characterizing unconventional reservoirs may significantly enhance exploration and production efforts by targeting highly-conductive intervals (shales or mudstones) with minimal high-resistivity zones (non-reservoir turbidites or MTDs) based on image log analysis.
AAPG Search and Discovery Article #90154©2012 AAPG Eastern Section Meeting, Cleveland, Ohio, 22-26 September 2012