Abstract

The Eagle Ford Formation in South Texas has been an established hydrocarbon play since 2008. This study uses geohistory and thermal modeling analysis to map the boundary between the oil window and immature areas in the Maverick Basin. The updip limit of the oil window of the Eagle Ford Formation is present as far north as the uppermost parts of Maverick, Zavala, and Frio counties based on estimated vitrinite reflectance (%Ro). This limit correlates to a subsea structure depth of ~650 ft (198 m) in Maverick county, ~2900 ft (884 m) in Zavala county and ~3650 ft (1113 m) in Frio county. The change in depth in relation to maturity reflects the amount of burial and subsequent uplift and erosion along the Chittim Anticline. %Ro estimated from geohistory and thermal modeling are consistent with published studies of cores and cuttings and correlate well with published %Ro maps. Total organic carbon (TOC) was calculated using a well log overlay analysis technique. TOC varies laterally and stratigraphically throughout the study area. TOC is higher in the lower Eagle Ford compared to the upper Eagle Ford. An area of high TOC (>10%) is noted in the lower Eagle Ford near the center of the Maverick Basin. The log overlay analysis technique for calculating TOC shows reasonable results when compared to actual measurements from cores and cuttings. These techniques can provide a quick look to define the petroleum potential in frontier areas where data are limited.