Paleoenvironmental analysis and reservoir characterization of the late cretaceous eagle ford formation in Frio county, Texas, USA

The objective of this research is to document the paleoenvironment and reservoir characteristics of the Eagle Ford Formation in Frio County, TX, which lies in the updip, north-central region of the productive fairway. This was achieved through analysis and interpretation of two full-diameter cores, inorganic and organic geochemical data, and two hundred well logs. Subsurface mapping revealed a homoclinal ramp structure dipping 2º to the southeast. The Eagle Ford has approximately 6,000 ft of structural relief across the county. Thickness mapping showed increasing subsidence (thickness) both towards the Maverick Basin and the Gulf of Mexico Basin. The thickness decreases updip to the north where the Eagle Ford eventually crops out or pinches out. Core analysis revealed that there were two main lithofacies: limestone-dominated and marl-dominated. An additional two depositional facies were found within the marl-dominated units: inoceramid-rich marl and organic-rich marl. The limestone-dominated units are characterized by traction current sedimentary structures and the marl-dominated units have with suspension depositional features. Inorganic geochemical analysis, bulk mineralogy and elemental composition, were used to confirm core descriptions. Increases in the content of reducing authigenic trace elements were used as a proxy for benthic redox conditions at the time of deposition. Other elements were normalized to serve as proxies for grain size, paleoenvironment precipitation level, silica enrichment and sediment provenance. Organic geochemical analysis showed that the organic material is a mixture of type I and type II kerogen. The average organic content in the lower Eagle Ford across the productive fairway in Frio County ranges between 3 and 5 wt%. Infrequently, discrete units measured up to 14 wt% in the lower Eagle Ford. Total organic carbon was found to have a strong relationship with bulk density and density porosity in both of the cored wells. That relationship was applied to density porosity logs across Frio County and an average TOC contour map was created. The greatest organic content is in the updip and thermally immature portions where the kerogen has not been cracked into hydrocarbons. Stratigraphic modified lorenz plots identified three major flow units that are stratigraphically bound to lithofacies and correlate from well to well. It also showed flow unit two, the inoceramid-rich marl facies, has the best reservoir process speed and that the limestone-dominated units in the lower Eagle Ford are major permeability barriers. The most geologically prospective area in the county for hydrocarbon charge appears to be in the southeast corner, where the Eagle Ford is deeply buried, distal from shoreline, thick, organic carbon rich, and most thermally mature. Flow unit analysis suggests that the inoceramid-rich marl facies, not the organic-rich facies, may be the best target in the Eagle Ford for horizontal completions. In the updip part of the county, the Eagle Ford is not a possible target because it is immature and has not generated hydrocarbons.