Regional Analysis of Stratigraphy, Reservoir Characteristics, and Fluid Phases in the Eagle Ford Shale, South Texas

Abstract The Eagle Ford Shale, a rapidly developing hydrocarbon play, occurs in an approximately 50-mi wide belt extending from the Mexican border northeastward to the Texas-Louisiana border. Exploration and production are focused in South Texas, where Eagle Ford wells produce oil, condensate gas, or dry gas. Although it has been developed intensely since 2008, factors controlling the success of the Eagle Ford Shale wells are unclear. As many as 1088 well logs were analyzed and correlated to map Eagle Ford Shale structure and thickness. From outcrop, the Eagle Ford Shale dips southeastward, exceeding a depth of 14,500 ft at the Sligo Shelf Margin. Based on gamma ray and resistivity responses, the Eagle Ford Shale was divided into lower and upper units. The upper Eagle Ford Shale was further subdivided into two units. The lower Eagle Ford Shale is organic rich (high gamma ray and high resistivity) and is present throughout the basin. Thickness ranges from less than 50 ft in the northeast to more than 200 ft in the Maverick Basin depocenter. The carbonate-rich upper Eagle Ford Shale is confined to the Maverick Basin depocenter, where its thickness exceeds 400 ft. The shale volume (Vshale) of the Eagle Ford Shale (from gamma ray log analysis) was mapped for each stratigraphic unit. Vshale assessment suggests stratigraphic and regional variations in Eagle Ford Shale properties. Single-well analyses of well logs from Maverick and LaSalle counties indicate upward decrease of total organic carbon and increase in carbonate content of the Eagle Ford Shale. Vshale maps document both regional and stratigraphic lithologic variability that most likely impact reservoir character of the Eagle Ford Shale. A gas/oil ratio map shows the hydrocarbon fluids evolve from black oil to dry gas with increasing depth. Future work, following well log normalization, will clarify regional variations of total organic carbon (TOC) and other rock properties and will investigate controls on oil and gas production.