AAPG Bulletin, V. 84, No. 7 (July 2000), P. 905-922.

New Exploration Concepts for the Edwards and Sligo Margins, Cretaceous of Onshore Texas (E&P Notes) ¹

Dale A. Fritz,² Terry W. Belsher,³ James M. Medlin,³ John L. Stubbs,³ Robert P. Wright,³ and Paul M. (Mitch) Harris⁴

©Copyright 2000. The American Association of Petroleum Geologists. All rights reserved.
¹Manuscript received August 13, 1999; revised manuscript received November 15, 1999; final acceptance December 15, 1999.
²Chevron U.S.A. Production Company, Houston, Texas. Present address: Santa Fe Snyder Corporation, Houston, Texas.
³Chevron U.S.A. Production Company, Houston, Texas.
⁴Chevron Petroleum Technology Company, Houston, Texas.
We thank our coworkers for support in various stages of our studies. Scott Butler was a strong source of support for the work done on this project, and we are all indebted to him for his perseverance with management to allow these concepts to be developed and pursued. Glenn Priess helped acquire data and Juan Narbona drafted the figures. We would like to acknowledge John Sangree for his initial overview, encouragement to pursue a deep Cretaceous play, and early recognition of the key sequence boundary in the Sligo. We are also indebted to Brad Macurda, Bob Goldhammer, and Clyde Moore for their insights on the Lower Cretaceous of the Gulf Coast. Seismic data are credited to Seismic Exchange, Inc., and CGG American Services. Chevron granted permission to publish the manuscript. Comments by AAPG reviewers Scott Montgomery, Neil Hurley, and Lee Billingsley improved the manuscript greatly. 

ABSTRACT

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 particular the Edwards and Sligo formations. Two-dimensional (2-D) and three-dimensional (3-D) seismic data with lithologic and biostratigraphic information led to a detailed sequence stratigraphic framework. This framework resulted 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 more 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 grainstone deposits are located far seaward of the commonly recognized margin and numerous exposure surfaces occur in the shelf deposits. Proximity to faulting after burial ensures the development of a plumbing system that enhances secondary porosity and provides a migration pathway for hydrocarbons.

A Sligo debris play is based on a sequence boundary within the upper part of the Sligo in ferred 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 shelf margin. Rapid deposition may have helped preserve porosity within the thick debris wedge. Data from reservoir analogs confirm that downslope carbonates can retain reservoir-quality porosity. Facies variation and slump faulting on the foreslope creates the potential for trapping, and juxtaposition to deep-water carbonates sets up the source and migration pathway. This undrilled wedge extends for hundreds of miles along the Sligo margin.