AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Regional Composition and Reservoir Quality of Wilcox Sandstones, Northwest Gulf of Mexico
(1) Bureau of Economic Geology, The University of Texas, Austin, TX.
Wilcox sandstone reservoirs in the deepwater Gulf of Mexico are a significant new oil play, and exploration is testing the potential for deep Wilcox gas reservoirs below the shelf as well. Combined petrographic study of detrital mineral composition and diagenesis of onshore Wilcox sandstones in Texas and Louisiana and burial-history modeling provide insight into potential regional variation of offshore Wilcox reservoir quality.
Lower Wilcox sandstones from the upper Texas coast, which were deposited in the Houston delta system, have a current average composition of Q61F25R14. Prior to diagenetic modification, mainly by dissolution and alteration of feldspars, original detrital composition was estimated at Q55F32R13. Lower Wilcox sandstones from the Holly Springs delta system in Louisiana have an average composition of Q62F15R24 now, and Q55F24R21 originally. These two delta systems were the main sources of Wilcox sandstones in deep shelf and deepwater Gulf of Mexico trends, and the difference in feldspar content may help to distinguish sediment pathways.
Onshore Wilcox sandstones from both upper Texas and Louisiana show the same trends of porosity and permeability versus temperature. Thus, the slight difference in detrital mineralogy between the two delta systems apparently did not result in significant differences in reservoir quality. Preliminary burial-history modeling suggests that reservoir quality of deepwater Gulf of Mexico Wilcox sandstones at 27,000 ft in Walker Ridge is similar to that of onshore Louisiana sandstones at 11,000 ft (Ro = 0.7%), whereas Wilcox sandstones beneath the Louisiana shelf at 27,000 ft have significantly higher thermal maturity (Ro > 2.4%), similar to that of onshore Texas sandstones at 21,000 ft.