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Abstract

Burial diagenesis of Frio sandstones deduced from detailed study of one small area of the northern Texas Gulf Coast (Brazoria County, Milliken et al, 1981) is regionally valid with only minor modifications.

Quartz is most commonly the first cement of volumetric significance to form, and constitutes 2.5% of the average sandstone volume. The average δ518O of quartz cement is +31 o/oo± 1.5 o/oo (SMOW), indicating precipitation at considerably cooler temperatures than most clay mineral transformation takes place. Calcite is the dominant cement in Frio sandstones, constituting about 5% of the total sandstone volume, and most commonly postdates quartz precipitation. Calcite more depleted than −10 o/oo (PDB) is uncommon, and most calcite has a δ18O of −7.2± 2 o/oo (PDB). δ13C values cluster closely around −4 ± 2 o/oo (PDB). Because of relatively constant isotopic composition, and relatively invariant iron and manganese content in calcite, both areally and with depth, both quartz and calcite cements appear to have been emplaced under relatively invariant chemical conditions prior to hydrocarbon migration. Detrital K-feldspar is essentially absent below 12,000 ft, and the zone of plagioclase albitization extends between about 9000 and 12,000 ft. Virtually no unaltered detrital feldspars are present below 12,000 ft in any of the samples examined, K-feldspar having been mostly dissolved and plagioclase albitized.

The volume of water required to precipitate quartz and calcite cements far from the apparent sources of material, generate secondary porosity and alter all detrital feldspars regionally in this thick sandstone sequence far exceeds the volume of pore water deposited with, near, or beneath the sands. Active thermally driven convection is a plausible (though unproven) mechanism for moving such large masses of dissolved components (and hydrocarbons) through the sandstones.

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