ABSTRACT

The Eocene Carrizo Sandstone adjacent to the Butler salt dome in the East Texas basin was cemented locally as the result of processes similar to those that have created salt dome calcite cap rocks throughout the Gulf Coast. The geometry of this anomalous cemented zone was established using Carrizo exposures in a large quarry and cores in the contiguous area. The cemented zone is delimited by a steeply dipping normal fault that is radial to the dome; the cements extend southeast of the fault for about 750 m. The Carrizo is a fine- to medium-grained quartzarenite to sublitharenite with 42 to 50% cement, principally poikilotopic calcite with local pyrite. Pyrite concretions are found on both northwest and southeast sides of the fault, as well as along the fault plane, whereas the calcite cement is present only to the southeast.

¹³C data for calcite (-19 to -37) suggest that bicarbonate was supplied mainly by bacterial oxidation of methane, whereas ¹⁸O data for calcite (-6 to -9) indicate that meteoric water was the principal diagenetic fluid. ⁸⁷Sr/⁸⁶Sr values for calcite ( 0.7073) are within the range for Middle Jurassic seawater, suggesting that dissolution of anhydrite from the Louann Salt provided the Ca. Pyrite is more ³⁴S-rich (³⁴S range of 10 to 16) than typical sulfides in Gulf Coast salt dome cap rocks and are interpreted to represent H₂S supply from deep sour gas reservoirs.

Petrographic and isotope evidence suggests that early H₂S supply from deep sour gas reservoirs was channeled upward along the flank of the dome and outward along the radial fault, resulting in early pyrite precipitation along the fault zone and in adjacent sediments. Early pyrite cementation along the fault may have formed a barrier to subsequent fluid circulation, blocking the northwestward movement of both Ca-rich waters from the dome and deep natural gas moving along the fault, thus confining calcite precipitation to the southeast. Bacterial oxidation of this thermogenic gas resulted in calcite precipitation, although mixing with shallow ground water bicarbonate may account for the range in ¹³C values in the calcite cement.

We propose that this type of cementation occurs in similar settings associated with hydrocarbon leakage to shallow levels, including adjacent to many salt domes in the Gulf of Mexico Basin, and may represent a previously unrecognized self-sealing type of hydrocarbon trap.