Diagenetic controls on evolution of porosity and permeability in lower Tertiary Wilcox sandstones from shallow to ultradeep (200–6700 m) burial, Gulf of Mexico Basin, U.S.A.

Abstract

Reservoir quality is a critical risk factor in deep to ultradeep reservoirs at depths >4.5 km. Analysis of Paleogene Wilcox sandstones on the upper Texas Gulf Coast provides insight into the evolution of reservoir quality during shallow to ultradeep burial diagenesis. Reduction of porosity and permeability with burial in Wilcox sandstones was evaluated using subsurface samples from 200 to 6700 m, at temperatures of 25–230 °C. Diagenesis and petrophysical properties were interpreted from petrographic data and core analyses. Wilcox sandstones are mostly lithic arkoses and feldspathic litharenites having an average composition of Q 59F 22R 19. 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, composition does vary with sequence-stratigraphic position; lowstand slope-fan deposits contain more rock fragments than do deposits from highstand or transgressive systems tracts. Given observations from this onshore dataset, Wilcox sandstones deposited in deepwater environments in the Gulf of Mexico are likely to contain more rock fragments than their linked highstand equivalents. Pore types in Wilcox sandstones change from a mix of primary and secondary pores and micropores at shallower depths to predominantly secondary pores and micropores in deeper sandstones. Wilcox sandstones show steady porosity reduction from an average of 33% at 38 °C to 12% at 132 °C, with little additional loss at higher temperatures. By 132 °C, most primary pores have been lost by mechanical compaction or occluded by quartz cementation. Volume of secondary pores remains nearly constant despite dissolution of 3.5% average potassium feldspar during deep burial. Volume of this late dissolution of potassium feldspar is offset by precipitation of ankerite, albite, illite, and minor quartz in secondary pores. The proportion of microporosity, which occurs in authigenic clays, altered grains, and matrix, increases in the deepest sandstones. Change in proportion of pore types and reduction in total porosity with increasing temperature changes porosity–permeability transforms. Because most deep to ultradeep Wilcox sandstones are dominated by secondary pores and micropores, porosity–permeability transforms have lower slopes than those for shallower sandstones.