Abstract

On the outer part of the drowned, deeper water, South Texas Comanche Platform, the Del Rio–Buda section is not well documented because of the lack of rock data. A core that recovered the Buda and Del Rio formations on the outer platform area in LaSalle County is used to characterize the strati-graphic section. The Del Rio Formation is comprised of marly chalk to a chalky marl. It is highly bioturbated and contains calcispheres, planktic foraminifers, saccocomids, ostracods, echinoderms, benthic foraminifers, and thin bivalves in a coccolith-rich to siliciclastic-rich matrix. Total organic carbon (TOC) is very low (below source-rock quality). The Buda Formation is a highly burrowed, relatively clean chalk to marly chalk (i.e., minor siliciclastic content). It contains similar biota as the underlying Del Rio Formation and has low TOC.

On the basis of abundant bioturbation and low TOC, the depositional environments of both the Del Rio and Buda strata are interpreted as oxygenated to slightly stressed. The lack of wave-produced ripples or cross stratification and predominantly open-marine planktic fauna suggest deposition below storm-wave base. Both stratigraphic sections contain a pore network of nano- to micropores. This micropore network suggests that the Del Rio-Buda section on the outer platform may have reservoir potential as tight-carbonate gas reservoirs. The relative shallow-depth of burial (<10,000 ft [<3050 m]) locates it at an economical drilling depth.