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Preliminary Results on Depositional Facies, Sediment Origin and Diageneses of Late Paleozoic Shale Horizons From the Midland Basin, West Texas

Abstract

The Midland Basin is a long-standing significant hydrocarbon basin within the larger Permian Basin of southern United States. Recently unconventional shale reservoirs have become of interest due to new technology and reservoir development techniques, yet little research has emphasized provenance, mineralogy, and diagenetic effects in these new reservoir types, especially within the margins of the Midland Basin. Core shale sections from the northern Midland Basin were used to collect mineralogical and geochemical data to better understand the dynamics of paleoenvironment controls on deposition and sediment provenance. Sampling strategy included shale acquisition from intervals ranging from -4948 to -6474 ft. subsea. Intervals include Mississippian (-6278 to -6474 ft.), Pennsylvanian (-5478 to -5681 ft.), and Permian (-4948 to -5073 ft.) strata. In addition to detailed macroscopic and microscopic core inspection, this research relied on several analytical techniques that included XRD on global and clay fraction as well as electron microbeam investigations in thin-sections (QEMSCAN and SEM-EDS) to reveal clay and heavy mineral morphology and phase chemistry. Shale strata is primarily interbedded with carbonates and, only rarely, with sandstones. Lithofacies of shale range from laminated to homogenous massive carbonate-rich mudstones. Preliminary geochemical data point to stable sediment sources throughout the Late Paleozoic considering the consistency of normalized trace and rare-earth element concentrations. Normalized elemental abundances tend to be characteristic of felsic to intermediate igneous sources, thus providing the most immediate information on the origin of detrital component of shales. Mineralogy of analyzed shale is dominated by quartz, clay minerals, and calcite. The clay minerals are mostly 10Å phases accompanied by 10Å/15Å interstratifications (i.e. illite-smectite) with sporadic occurrences of chlorite and kaolinite. These preliminary compositional data suggest a shallow carbonate platform setting influenced by siliciclastic sedimentation from multiple crystalline source areas that eventually gave rise to the interplay of carbonate and clay dominant lithologies. Further analyses will help create a robust post-sedimentation model for the northern Midland Basin which will utilize comparative maturation studies based on organic matter and clay mineral indices.