The Eagle Ford play extends along strike from the San Marcos arch in the northeast into the Maverick basin along the international border with Mexico. The highest initial production is in a strike-parallel belt between the Karnes trough and the Cretaceous shelf margin. Three lithologies comprise the bulk of the Eagle Ford Shale in this area: argillaceous mudrock (shale), calcareous mudrock (marl) and limestone. The marls consist mainly of coccoliths and contain more total organic carbon (TOC) than the other lithologies. The sand- and silt-sized grains in the marls and limestones consist predominantly of planktonic foraminifera, radiolarians and calcispheres with lesser amounts of inoceramid fragments and other carbonate grains. The limestones may be partly to entirely recrystallized. The strength and rigidity of the rocks increase with calcite content—the limestones are stronger and more rigid than the marls. Argillaceous mudrocks (shale) comprise only a small portion of the Eagle Ford between the San Marcos arch and the Maverick basin, but are more common in the lower part of the formation and along strike to the northeast. Seven stratigraphic intervals can be recognized and mapped within the Eagle Ford Shale between the San Marcos arch and the Maverick basin. Changes in major, minor and trace element composition mark the boundaries of these intervals. The boundary between the Upper and Lower Eagle Ford as commonly picked on well logs is below the Cenomanian-Turonian boundary. Typically, the Upper Eagle Ford contains less vanadium, molybdenum, uranium and TOC than the Lower Eagle Ford, indicating bottom water oxygen levels were higher during deposition. Isopach maps show that the Eagle Ford as a whole and each of its major subdivisions thin across an area in southwestern Karnes County. The percentage of limestone within the Eagle Ford and each of its major subdivisions increases over this area. Southwestern Karnes County sits above a high on a time-structure map on the top of the Buda. Changes in thickness and facies within the Eagle Ford suggest the area above the high on the time-structure map was a topographic high on the sea floor and furthermore that changes in bathymetry influenced facies distribution and ultimately production from the Eagle Ford Shale.

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