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Reservoir quality trends in Lower Tertiary sandstones along the Texas Gulf Coast are a product of regional variations in intensity of diagenesis. The major controls on diagenesis were detrital mineralogy and regional geothermal gradient. Porosity and permeability in sandstones shallower than 3350 m (11,000 ft) are generally adequate for hydrocarbon production, whereas reservoir quality in deeper sandstones in the onshore Lower Tertiary section is highly variable. Many of these sandstone reservoirs have permeability values of less than 1 millidarcy (md), but in a few areas permeability values are higher than 1000 md.
Wilcox sandstones are poorly to moderately sorted, fine-grained, quartzose lithic arkoses, becoming more quartz-rich from the upper to the lower Texas Gulf Coast. Most rock fragments are metamorphic or volcanic in origin. Wilcox sandstones exhibit no systematic regional reservoir quality trends. Along the lower and parts of the middle and upper Texas Gulf Coast, deep Wilcox sandstones are tight, but in other parts of the middle and upper Texas Gulf Coast, porosity exists at depth. Vicksburg sandstones are poorly sorted, fine-grained lithic arkoses. Rock fragments are mainly volcanic clasts with lesser carbonate and minor metamorphic clasts. The deep Vicksburg Formation has low-quality reservoirs. Frio sandstones range from poorly sorted, fine-grained, feldspathic litharenites to lithi arkoses along the lower Texas Gulf Coast to poorly sorted, fine-grained, quartzose lithic arkoses to subarkoses along the upper Texas Gulf Coast. Volcanic and carbonate rock fragments are common in the lower Texas Gulf Coast and decrease in abundance up the coast. Frio sandstones show a systematic improvement in reservoir quality from the lower to the upper Texas Gulf Coast that is related to grain composition and geothermal gradient. Reservoir quality trends in Tertiary sandstones have been substantiated by acoustic log analysis.
In spite of variations in composition, Lower Tertiary sandstones exhibit similar diagenetic sequences generalized as follows:
Surface-to-shallow-subsurface diagenesis (0 to 1200 m ±; 0 to 4000 ft ±) began with the formation of clay coats on framework grains, dissolution of feldspar, and replacement of feldspar by calcite. Minor amounts of kaolinite, feldspar overgrowths, and Fe-poor calcite was locally precipitated. Porosity was commonly reduced by compaction and cementation from an estimated original 40% to less than 30%.
Intermediate subsurface diagenesis (1200 to 3400 m ±; 4000 to 11,000 ft ±) involved dissolution of early carbonate cements and subsequent cementation by quartz overgrowths and later by carbonate cement. Cementation commonly reduced porosity to 10% or less, but this trend could be reversed by later dissolution of feldspar grains, rock fragments, and carbonate cements. Restoration of porosity to more than 30% occurred, but some porosity was later reduced by kaolinite, Fe-rich dolomite, and ankerite cementation.
Deep subsurface diagenesis (>3400 m±; >11,000 ft±) was a continuation of late Fe-rich and Fe-poor carbonate cement precipitation. Plagioclase was albitized during this stage.
Differences in intensity of diagenetic events and depths at which they first occurred correspond to the chemical and mechanical stability of the original detrital mineralogy and to regional variations in geothermal gradient.
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