New exploration concepts for the Edwards and Sligo margins, Cretaceous of onshore Texas

We examined the potential in onshore Texas of the most prolific reservoirs found to date in the Gulf of Mexico basin, Cretaceous carbonates, and in pa rticular the Edwards and Sligo formations. Two-dimensional (2-D) and three- dimensional (3-D) seismic data with lithologic and biostratigraphic informa tion led to a detailed sequence stratigraphic framework, This framework res ulted in a concentrated exploration effort in Lavaca County a redefinition of the Edwards shelf margin, and confirmation of a major sequence boundary in the Sligo. The youngest Edwards margin appears to have stepped seaward a distance of m ore than 3 mi (4.8 km) from the position of the margin as indicated by the Word field. This seaward shift and repositioning of the margin is shown in 3-D seismic and well data. Recognized within the progradational package are distal slope wackestones, reef and bank complexes, and back-reef lagoonal deposits that are offset seaward across sequence boundaries. Reef and grain stone deposits are located far seaward of the commonly recognized margin an d numerous exposure surfaces occur in the shelf deposits. Proximity to faul ting after burial ensures the development of a plumbing system that enhance s secondary porosity and provides a migration pathway for hydrocarbons. A Sligo debris play is based on a sequence boundary within the upper part o f the Sligo inferred from seismic geometry and surface exposures in Mexico. The seismic portrays a wedge geometry positioned downslope from the Sligo margin. Base-level change about the sequence boundary would have initiated coarse carbonate debris- and grain-flow deposition seaward of the Sligo she lf margin. Rapid deposition may have helped preserve porosity within the th ick debris wedge. Data from reservoir analogs confirm that downslope carbon ates can retain reservoir-quality porosity Facies variation and slump fault ing on the foreslope creates the potential for trapping, and juxtaposition to deep-water carbonates sets up the source and migration pathway. This und rilled wedge extends for hundreds of miles along the Sligo margin.