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Abstract

We have quantified the effects of diagenesis on quartzose sandstones from a large range of locations and moderate- to great-burial depths in the Greater Green River Basin, southwest Wyoming. The quantification has allowed us to make basin-scale conclusions regarding mechanical compaction, intergranular pressure solution, and quartz cementation and changes in their relative importance to porosity decline during progressive burial to depths of 7000 m (22,966 ft). We use a point-counting technique which combines cathodoluminescence and light microscopy to quantify the abundance of quartz cernent, intergranular volume (IGV = intergranular porosity + cements + matrix) and grain contact types (point vs. pressure solution contacts). We compare our data to published thermal history data to establish controls on porosity in quartzose sandstones with thermal maturities equivalent to vitrinite reflectance values of 0.4 to 2.1% R0, which, on average, corresponds to present burial depths of 1500 m (4921 ft) to 7000 m.

The results suggest that most mechanical compaction and intergranular pressure solution has occurred at depths shallower than our sample range, which, correcting for erosion, means shallower than approximately 2000 m (6562 ft). The observation that IGV does not decrease with depth below 2000 m adds to a growing body of evidence that intergranular pressure solution is only rarely an important moderate-burial to deep-burial porosity reduction process. Compaction, except by stylolirization, in quartzose sandstones does not usually continue below shallow burial because the combined effects of mechanical compaction, pressure solution, and small amounts of shallow quartz cementation produce stable grain-packing arrangements. Mechanical compaction, that is, grain position rearrangement, somelimes aiderj by grain fracturing and rotation, results in IGV loss to approximately 30% and pressure solution further reduces IGVs to an average of 22.2%, with very little variability (σ = 3.8%). Neilher grain size nor abundance of grain-coating clays correlates with pressure solution abundance. Electron microscopic examination of materials along the surfaces of macroscopically-visible stylolites indicates that most of the material, which consists of carbonaceous matter and clay minerals, was deposited as a thin layer in the sandstone. This observation suggests that the amount of dissolution along a stylolite is best measured by the volume of stylolite cones ralher than the thickness of stylolite seams.

The most important consideration for porosity prediction below 2000 m is the distribution of quartz cement, which generally increases in abundance with thermal maturity as measured by vitrinite reflectance and, to a lesser extent, with present depth. The average quartzose sandstone with a thermal maturity equivalent to 1.5% R0, corresponding on average to maximum burial to –5000 m (16,404 ft), has <2% porosity, 17.4% quartz cement and a few percent carbonate and clay cement. We calculate that 7.6% quartz cement can be provided internally to the average quartzose sandstone from intergranular pressure solution, stylolitization, feldspar dissolution, carbonate replacement of quartz and feldspar grains and clay mineral transformations. The remaining –10% quartz cement observed in the average deep low-porosity sandstone is episodically imponed to the sandstone during moderate to deep burial. We present data on aluminum abundance in quartz cement; these data support the hypothesis that quartz cement episodically precipitates over a large portion of a sandstone’s burial history.

Given that a large amount of silica transport is required to import 10% quartz cernent and that most cementation is occurring in the thermobaric hydrologic regime, we suggest that a significant proportion of the observed quartz cernent may be imponed during episodes of rapid fluid flow related to deep-basin mineral dehydration, tectonic forces, hydrocarbon generation and migration and the consequent breakage of overpressured compartments and flow along faults and fractures. The background quartz cementation is provided by internal silica-sources and compaction-driven flow, local density-driven flow and diffusion from silica-sourcing beds adjacent to the sandstones, where present. To predict deep porosity and permeability distribution, one can take advantage of the fact that most silica must be imported; where high flow rates are unlikely to have occurred and where adjacent beds have little silica-exportation potential, high porosity can occur at great depths.

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