From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.
Bureau Seminar, October 31, 2008
Evolution of porosity and permeability in lower Tertiary Wilcox
near surface to ultradeep burial (200 to 6,700 m), upper Texas Gulf Coast
Shirley Dutton and Robert G. Loucks
Bureau of Economic Geology
Evolution of porosity and permeability with increasing burial in Wilcox sandstones from the Gulf of Mexico basin was evaluated using samples from 200 to 6700 m. Diagenetic processes were interpreted from petrographic data (276 samples) and core analyses (>3,500 samples) from the upper Texas Gulf Coast. Upper, middle, and lower Wilcox sandstones, mostly lithic arkoses and feldspathic litharenites, have an average composition of Q59F22R19. Provenance did not change significantly during Wilcox deposition in this area, nor does average sandstone composition vary with age. However, lowstand deposits contain more rock fragments than deposits from highstand or transgressive systems tracts. These lowstand slope-fan deposits, generally in the most deeply buried Wilcox sandstones (>5.8 km), contain a higher proportion of metamorphic rock fragments and mudstone clasts than do shallower samples.
Wilcox porosity-depth trend shows steady porosity reduction to 4 km, then little additional loss at greater depths. Average core-analysis porosity declines from 35% at 0.4 km to 11.5% at 4 km. By this depth, most primary pores have been lost by mechanical compaction or occluded by quartz cementation.
Porosity decreases slowly at greater depths, and average porosity is 10% at 6.5 km. Secondary pores show no statistically significant volume change from 2.5 to 6.5 km, although dissolution of potassium feldspar between 3 and 5 km is indicated by (1) petrographic evidence of quartz cementation before some feldspar dissolution and (2) progressive decrease in potassium feldspar with increasing temperature. Volume of potassium feldspar dissolved during deep burial is apparently offset by precipitation of ankerite, albite, illite, and minor quartz in secondary pores. Porosity and thin-section data do not support mechanical compaction of secondary pores continuing during deep burial.
Micropores are the dominant pore type in deepest Wilcox sandstones, which also contain the most rock fragments. Intragranular micropores can form by weathering in the source area or during transport, and they can also be created during burial through grain alteration or precipitation of authigenic clays. Negligible porosity reduction during deep burial may be attributed to the abundance of micropores and their retention during diagenesis. Abundance of rock fragments at depth may cause this decrease in porosity loss.
Average permeability decreases steadily from 976 md at 0.4 km to 0.2 md at 5 km. Compared with worldwide reservoir sandstones, Wilcox sandstones have lower permeability per porosity unit except in the most porous sandstones (=35%). In the least-porous sandstones (2.5 to 7.5%), divergence is almost two orders of magnitude. Capillary-pressure curves indicate that Wilcox sandstones deeper than 4.5 km contain few macro-size pore throats (pore-throat radius >2.5 microns). An understanding of pore types and pore-throat size is essential to correct estimation of permeability from porosity in sandstones.