As the search for gas in the Gulf of Mexico focuses increasingly on reservoirs at depths more than 4.5 kilometers (14,750 feet), the most critical yet unknown risk factor is reservoir quality. Petrographic analysis of Wilcox sandstones on the upper Texas coastal plain provides insight into the evolution of porosity and permeability during burial that is useful in the exploration for deep reservoirs both onshore and in the Gulf of Mexico.

Wilcox sandstones are mainly lithic arkoses and feldspathic litharenites that have an average composition of Q59F22R19 (quartz-feldsparrock fragment). Provenance did not change significantly during Wilcox deposition in this area, nor does average sandstone composition vary among lower, middle, and upper Wilcox sandstones. However, lowstand slopefan deposits contain more rock fragments (mainly metamorphic and volcanic) than do deposits from highstand or transgressive systems tracts. Wilcox sandstones deposited in deepwater environments in the Gulf of Mexico are likely to contain more rock fragments than do their linked highstand equivalents.

With increasing burial depth, total volume of porosity decreases and the proportion of different pore types changes. Average coreanalysis porosity declines from 35% at a depth of 0.4 kilometers (1,300 feet) to 10.7% at a depth of 4.5 kilometers (14,750 feet). Pores types change from a mix of primary, secondary, and micropores (P₃₅S₃₈M₂₇) at shallower depths to predominantly secondary pores and micropores in deeper sandstones (P₇S₃₅M₅₈). At a burial depth of 3.5 kilometers (11,500 feet), most primary pores have been lost by physical compaction or occluded by quartz cementation, and secondary pores generated by feldspar dissolution compose the majority of macropores. Average permeability decreases from 976 millidarcies at a depth of 0.4 kilometers (1,300 feet) to 0.2 millidarcies at a depth of 4.5 kilometers (14,750 feet). Because most deep sandstones are dominated by secondary pores and micropores, the porositypermeability transforms for deep and ultradeep sandstones will have lower slopes than those for shallower sandstones.

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