From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.
Bureau Seminar, February 28, 2014
Shirley P. Dutton
Senior Research Scientist, Bureau of Economic Geology
University of Texas at Austin
AAPG Distinguished Lecturer
Modern seismic methods have revealed large, deep structures in the northern Gulf of Mexico. Wilcox Group sandstones are deep (>4.5 km) to ultradeep (>6 km) exploration targets below the present-day shelf and deepwater Gulf. At these depths, reservoir quality is a critical risk factor. Petrographic study of onshore Wilcox sandstones, combined with burial-history modeling, provides insight into the main controls on regional variation of reservoir quality.
Wilcox sandstones were sourced by continental-scale drainage systems that terminated in deltas in Texas and Louisiana. Seismic data were used to identify three major sediment fairways that carried sandstone from the shelf into the deep basin in the northwestern Gulf. Sandstone composition and diagenetic history were determined by point counts of thin sections from onshore Wilcox samples from depths of 0.2– 6.7 km, at temperatures of 25–230°C. The sandstones are mostly lithic arkoses and feldspathic litharenites; metamorphic and volcanic rock fragments are the most abundant lithic grains.
Primary, intergranular porosity was lost during diagenesis mainly by compaction (grain rearrangement and ductile-grain deformation) and quartz cementation. Pores in Wilcox sandstones changed from a mix of primary and secondary pores and micropores at lower temperatures to predominantly secondary pores and micropores at temperatures >150°C as primary pores were occluded. Primary porosity, which affects permeability the most, decreased from an estimated 40% at deposition to 5–8% by 125°C and 1–2% at temperatures >200°C. Pore-type evolution with temperature alters the porosity–permeability transform.
Burial-history modeling reveals large differences in thermal history of Wilcox sandstones across the study area. Thermal maturity of Wilcox sandstones 6 km below the deepwater Gulf of Mexico is similar to that of onshore Louisiana Wilcox reservoirs at 3.4 km (both have Ro ≈ 0.7%). Geothermal gradient in the deepwater Gulf is lower than onshore, and >3 km of allochthonous salt has been in place above the Wilcox for the past 30 m.y. Salt transmits heat effectively, thus reducing temperature and diagenesis in underlying Wilcox sandstones and preserving reservoir quality. During the past decade numerous oil discoveries have been made in Wilcox sandstones in the deepwater play. In contrast, Wilcox sandstones 8.2 km beneath the Louisiana shelf have higher thermal maturity (Ro > 2.7%). Allochthonous salt ~2.5 km thick was present for an estimated 8 m.y., but that time was apparently insufficient to lower thermal maturity. Deep, high-temperature Wilcox sandstones below the shelf have higher reservoir-quality risk.