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Detrital Mineral Composition and Reservoir Quality of Upper and Lower Wilcox Sandstones of the Texas Gulf of Mexico Coast

Dutton, Shirley P.1; Loucks, Robert 1
1 Bureau of Economic Geology, The University of Texas at Austin, Austin, TX.

Study of Paleogene Wilcox sandstones of the Texas coastal plain provides insight into reservoir quality of deeply buried Wilcox reservoirs in the deepwater Gulf of Mexico. Petrographic analysis of 425 Wilcox thin sections from the Texas Gulf Coast was conducted to determine detrital mineralogy, diagenesis, and evolution of porosity and permeability with burial.

Wilcox sandstones deposited on the northern Texas coast in the Houston delta system are mostly lithic arkoses and feldspathic litharenites having an average composition of Q58F22R19. Provenance did not change significantly during Wilcox deposition in this area, nor does average sandstone composition vary among lower, middle, and upper Wilcox sandstones. In contrast, upper Wilcox sandstones deposited on the southern Texas coast in the Rosita delta system are more quartz rich, having an average composition of Q69F17R14. Differences in detrital mineralogy influenced subsequent diagenesis and changes in reservoir quality with increasing temperature.

Porosity and permeability in lower Wilcox sandstones in the Houston delta system decrease relatively rapidly with increasing burial depth and temperature because of mechanical compaction of ductile rock fragments and quartz cementation. At temperatures >150°C, porosity in lower Wilcox sandstones averages 1.6% primary pores, 5.4% secondary pores, and 7.1% micropores. Permeability range is 0.0001 to 19 md, and geometric mean is 0.12 md. Porosity and permeability decrease more slowly in upper Wilcox sandstones in the Rosita delta system because they contain fewer ductile grains and, thus, undergo less mechanical compaction and because chlorite rims retard quartz cementation. At temperatures >150°C, porosity in upper Wilcox sandstones averages 2.8% primary pores, 6.9% secondary pores, and 3.4% micropores. Permeability range is 0.01 to 855 md, and geometric mean is 0.22 md. Because upper Wilcox sandstones contain a greater volume of primary, intergranular pores, they have higher permeability per porosity unit. However, at temperatures >200°C, average permeability in both upper and lower Wilcox sandstones has decreased to 0.1 md.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009