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Sedimentological and Geochemical Record of Oceanic Anoxic Events (OAE's) in the Aptian-Albian Pearsall Formation of South Texas


Using shallow water secular dC13 isotope curves, it is possible identify oceanic anoxic events (OAEs) and predict the potential existence of a more distal shale-gas system, as was documented in the Pearsall Shale. The Lower Cretaceous Pearsall Formation was deposited in Aptian and earliest Albian time. The Pearsall Formation contains three OAE's when organic carbon was favorably preserved. Two of these, the OAE 1-A and OAE 1-B, are thought to be global, while there is also a regional event in the section. The OAEs in the Pearsall Formation, particularly the regional event which corresponds to the Lower Bexar Shale Member, may be of significance for hydrocarbon production. Furthermore, these events are analogous to the OAE 2 which corresponds to the Eagleford Formation, which is productive in this area. South Texas cores were analyzed for carbon isotopes, sedimentology, and total organic carbon. The data set consisted of cores in both proximal and distal ramp environments in Maverick, La Salle, and Medina Counties. Detailed stratigraphic correlations were made using the dC13 curves as well as SP and resistivity logs. These correlations were found to be consistent between wells across the study area. The excursions in the dC13 curve were also found to be constant with changes in sedimentology within cores in the distal part of the section but not necessarily to changes in sedimentology the well oxygenated waters up dip. These changes also corresponded to increases in TOC preservation in the distal cores as well. Data from these cores was compared to data sets previously developed by Bralower in Santa Rosa Canyon in Northern Mexico and Phelps on the San Marcos Arch. Correlations could be made from the study area to these outlying areas. Also it is shown that dC13 curves from shallow environments could be potentially useful in predicting the presence of accumulations of TOC and thus hydrocarbons in distal shales.