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Refining XRF Lithofacies with SEM Analysis for the Permian Wolfcamp Shale from the Delaware Basin, Texas


Cores from two wells in the Permian Wolfcamp Shale from Reeves Co. in the Delaware Basin of west Texas were examined to determine the lithologies present. Cores cover much of the Wolfcamp A and B sections and represent >800 ft of depth. Initial lithofacies picks were guided by handheld X-ray fluorescence (XRF) analysis at a 2-inch vertical spacing. These picks were subsequently refined using a limited number of X-ray diffraction analyses. Samples from all the lithofacies were also taken for petrographic thin sections and for Ar-ion-milling and examination in the SEM using EDS mapping to delineate mineralogy and texture. Twelve lithofacies were defined in the cores of which siliceous and argillaceous mudstone facies are dominant. Other volumetrically significant lithofacies include carbonate clast floatstone (debrite) and very fine sandy mudstone. Two other carbonate-rich lithofacies make up minor amounts of the core. Very fine sandstone is found only in one core, in the Wolfcamp A section, and makes up < 1% of that core. The minor lithofacies are typically too thin to be defined on wireline logs. SEM examination shows that XRF alone has difficulty recognizing dolomite-rich and Mg-chlorite-rich lithofacies. The siliceous wackestone/packstone lithofacies, which was defined by combined XRF, optical, and megascopic description, is shown in SEM to be particularly heterogeneous, varying from a siliceous dolomitic siltstone to a siliceous calcareous mudstone. The laminated quartz mudstone lithofacies is a heterogeneous interlayering of quartz siltstone and argillaceous seams that was homogenized by the XRF analysis. The calcareous mudstone lithofacies samples are shown in SEM to be calcareous due to calcitized radiolarians and this may be true for the entire lithofacies. Differences in the matrix grain size between beds were apparent in the SEM samples of the carbonate clast floatstone but not in core. SEM observations reveal important information on lithology as well as grain size and type, that is not readily apparent from other methods and scales of observation. Lithofacies are thus better defined with the integration of petrographic thin sections and SEM samples.