Controlling Mechanisms on the Preservation of Organic Matter in the Late Cretaceous Eagle Ford Group, South Texas
The Eagle Ford Group consists of multilayered, heterogenic siliciclastic/carbonate sedimentary deposits, stratigraphically located between the Buda Limestone and the Austin Chalk Groups. The Eagle Ford Group has become a widely popularized topic of research for the past decade, due to the increased oil and gas demand that stimulated exploration efforts. In the Eagle Ford Group, the identification of specific target intervals has become critical to improve the efficiency of exploration throughout the extent of this oil and gas play. Organic Matter (OM) is one of the parameters that helps define target intervals rich for hydrocarbons. To develop models to predict the location of potential petroleum plays, geochemical analysis has become an accepted analytical tool for determining proper environmental parameters that existed at the time of deposition and that favored the enrichment and preservation of OM. The depositional environment of the Eagle Ford Group in the study area has been previously researched (e.g. Eldrett et al., 2015; Frébourg et al., 2016; Eldrett et al., 2016; Minsini et al., 2018; Lehrmann et al., 2019), as well as in the subsurface. Even if both surface and subsurface data are available, there is a lack of research tying the two datasets together, as well as a lack of research regarding the mechanisms that stimulated the primary productivity and the controlling factors for preserving OM in the Upper Eagle Ford. This study will focus on the Langtry member and the equivalent Upper Eagle Ford formation in the subsurface of South Texas. Samples and spectral gamma ray data from the Sycamore Bluffs outcrop have been collected and donated by Dr. David Ferrill, and his team at Southwest Research Institute. Geochemical data are collected using x-ray fluorescence (XRF) analysis in order to test the hypothesis that bottom water anoxia at the time of deposition favored the preservation of organic matter in the Upper Eagle Ford. Subsurface XRF and gamma-ray data for two wells have been provided by a private source. The correlation between surface and subsurface datasets is based on the stratigraphic evolution of gamma-ray values, and permits the comparison of geochemical data between study locations. Using proxies for paleoenvironmental conditions, trends in the geochemical data show an enrichment in redox-sensitive trace metal and in nutrient supply at the time of deposition in the Upper Eagle Ford, while detrital input decreased. From these trends identified in all three data sets, it is inferred that original controls for the preservation of organic matter in the upper section of the Eagle Ford Group are related to the installation of upwelling currents coming from the Tethys Ocean into the ancestral Gulf of Mexico.