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Applications of Whole Rock Bulk Geochemistry for Evaluating a CO2 Injection Well in Late Triassic Sandstones


The South Georgia Rift Basin (SGRB) is comprised of thick successions of Late Triassic red beds that have been intruded by diabase sills and dikes of an approximate Early Jurassic age. The SGRB covers intermittent parts of South Carolina, Georgia, Alabama, and western Florida and is buried by coastal plain sediments. The Rizer #1 test borehole, located in Colleton County, South Carolina, was drilled to evaluate these Late Triassic red beds for potential CO2 sequestration. Petrographic analysis concluded that extensive compaction and diagenesis destroyed reservoir quality in these arkosic litharenites to lithic arkoses. To better understand the diagenesis, provenance, and weathering processes, elemental data was gathered by energy dispersive x-ray fluorescence (ED-XRF) to supplement the overall petrologic data. A total of 340 samples from the 2600 to 6200 ft interval were analyzed for 32 elements, and consist of 299 cuttings, 31 rotary sidewalls, and 10 core samples. A majority of samples were analyzed specifically for characterizing relatively unknown lithologies not examined by core description or rotary sidewalls thin sections. Results were compared to in-house mineralogy acquired by x-ray diffraction (XRD), available point count data, and 57 feet of described core. Six major chemostratigraphic variations (packages) were primarily defined by changes between mafic (MgO, Fe2O3, MnO, TiO2) versus felsic (Al2O3, K2O, NaO2) associated elements, which were further subdivided into 21 units delineated by discrete geochemical variants within the overall package. Four sharp spikes in mafic elements are identified to be diabase intrusions, verified by petrographic analysis. Elevated trace element data (Y, Zr, Nb, Hf) in cooperation with heavy minerals (zircon, sphene, epidote) suggest a metamorphic siliciclastic source. Tentative interpretations suggest minimally weathered sediments with approximately three distinctive provenances, including distribution of feldspars that is exhibited by alternating dominance between plagioclase (Na2O, CaO) and orthoclase (K2O, Al2O3). Diagenetic indicators show well-defined, package-level calcareous cementation trends are differentiated from low-calcium plagioclase trends. Additional diagenetic trends observed are associated with minor metasomatic fluid flow from diabase intrusions into deep phreatic zones that migrated through multiple fractures and faults confirmed by borehole imaging logs.