During development of the Eagle Ford unconventional resource near the San Marcos Arch, a non-productive mudstone associated with drilling issues was identified between the primary Eagle Ford producing zone and the underlying Buda Limestone. As the top of the Buda typically exhibits evidence of karsting but is unaltered when overlain by this mudstone, and the mudstone contains higher abundances of clay than the Eagle Ford, two questions were posed: (1) Does this mudstone represent a depositional system separate from the Eagle Ford and (2) does it act as a fracture barrier between the Eagle Ford and underlying water-bearing rocks?
The current study analyzed two cores from Lavaca and Fayette counties, which included petrographic, XRD, and geomechanical (point-load penetrometer and micro-rebound hammer) analyses to determine the mineralogy and geomechanical properties of the mudstone, the Eagle Ford, and the Buda. Logs from 345 wells within a six-county were used to correlate and map four horizons associated with the mudstone. These results were integrated with an earlier core study that included biostratigraphic, petrographic, XRD, and XRF analyses, and regional log correlations across the arch into the Brazos Basin.
The geomechanical tests found that the mudstone is significantly weaker than the overlying Eagle Ford, averaging 32% lower calculated unconfined compressive strength (UCS) values derived from the penetrometer and 36% lower using the micro-rebound hammer. Higher clay and lower calcite abundances within the mudstone are responsible for its lower strength; the XRD analyses found that the shale samples from the mudstone contained an average of 47% clay, whereas the Eagle Ford marls contained an average of 34% clay. The petrographic analyses found that the clay is concentrated in structureless layers that are interpreted to represent fluid-mud deposits associated with hypopycnal plumes.
The biostratigraphic study identified Early Cenomanian markers associated with the Maness Shale of East Texas which lies between the Woodbine and Buda, in agreement with the regional cross-sections which correlated the mudstone to the Maness. A hot gamma ray spike produced by a phosphatic lag at the top of the mudstone was key to the correlations. Thickness trends of the Maness differ considerably from the Eagle Ford; it has a distinct northeast-southwest trend and pinches out in southern Karnes County, suggesting that it was a depositional system unrelated to the Eagle Ford.
Comparison of Maness thicknesses with cumulative first year oil and water production data from over 2000 horizontal wells in the study area found a significant correlation between Maness thickness and water/oil ratios. In particular, there is a 50% decrease in water/oil ratios between Maness thicknesses of 5 to 10 ft, (1.5-3 m) suggesting that the Maness may be acting as a fracture barrier where it is >10 ft (3 m) thick.