Abstract

Changes in packing and porosity resulting from compaction were studied in 40 Wilcox and Carrizo sandstone samples with burial depths from 792 to 4420 m. As measured by packing indices and loss of intergranular porosity and pre-cement porosity, sands compacted rapidly to approximately 1200 m and more slowly and variably at greater depths. Contact index (= average number of contacts/grain) and tight packing index (= average number of long, concavo-convex and sutured contacts/grain) increase and intergranular porosity and pre-cement porosity decrease logarithmically with burial depth. Because of a scarcity of deep samples, compaction behavior deeper than 3000 m is uncertain. Compaction appears to have stopped at 3000 m, which is 500 m shallower than the present depth of hard overpressure. Neither quartz cement (x = 11%) nor carbonate cement (x = 2%) was introduced shallow enough or insufficient quantity to halt compaction, although quartz cement may have retarded compaction. Compaction was the major cause of porosity reduction with depth (r = 0.79). The total whole rock porosity lost by compaction for individual samples ranges from 9 to 31%. At all depths the amount of porosity lost by grain rearrangement (9 to 27%) is more than twice the amount of porosity lost by either measureable ductile grain deformation (0 to 8.3%) or pressure solution (0 to 7.3%), although the amount of pressure solution and its influence on grain rearrangement is probably underestimated. Depth and thermal maturation, the latter represented by time-temperature, best explain compaction in these sandstones.

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