ABSTRACT

The calcite- and pyrite-cemented Eocene Carrizo sandstone on the flank of Butler salt dome in East Texas was formed by processes similar to those that form calcite cap rocks throughout the Gulf Coast. Isotopic evidence indicates that the bacterial degradation of hydrocarbons combined with the venting of deep sour gas and the dissolution of anhydrite from the dome resulted in the precipitation of these cements. Identification of the origins of these cements has lead to a better understanding of the paleofluid-flow associated with the dome.

The Carrizo is a diagenetic quartz arenite with 42-50% concretionary calcite and pyrite cements. The calcite cement is bound to the northwest by a pyrite-cemented normal fault radial to the dome while the pyrite is found on both sides of the fault.

Calcite ¹³C(-18 to -37 (PDB)) and ¹⁸O(-6 to -90 (PDB)) show a mixed source of both waters (meteoric and deep) and hydrocarbons (oil, gas, and lignite). Heavy pyrite ³⁴S (12-15 (CDT)) represents H₂ supply from deep sour gas.

Deep waters, methane, and H₂S migrated up the dome flank and out the fault into the Carrizo, already containing oil and lignite, and pyrite precipitated along the fault and in adjacent sediments. Bacteria oxidized the hydrocarbons and, with the meteoric and formation waters, formed bicarbonate. Calcium-bearing fluids from the dome moved into the Carrizo, and calcite began to precipitate. However, pyrite cement along the fault prevented northwestern migration of the fluids, confining calcite precipitation to the southeast.

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