TEMPERATURE AND FLUID COMPOSITION CONDITIONS OF DOLOMITIZATION ALONG THE KENTUCKY RIVER FAULT ZONE

Fault-related dolostone bodies in the Appalachian basin are of increasing interest because of their potential as hydrocarbon reservoirs, and many appear to owe their existence to local perturbations on the regional thermal and hydrologic regime. The temperature and fluid composition conditions under which vug-filling dolomite and calcite formed in the Ordovician Calloway Creek Limestone and Cambrian Knox Dolomite are being investigated using petrographic and fluid inclusion analyses of core samples from Clark County in central Kentucky. The well intersected the Kentucky River fault, which forms the north boundary of the Rome Trough in the area. Breccia in the fault zone is cemented primarily with calcite, whereas dolomite occurs above and below the fault zone in the Clays Ferry Formation, Garrard Siltstone, and Calloway Creek Limestone; and the Knox Dolomite, respectively.

Notwithstanding the approximate 800 ft (244 m) of stratigraphic separation between the Calloway Creek and Knox dolostones, the sequence and types of carbonate cement are remarkably similar. In both units matrix dolomite forms an interlocking mosaic of anhedral crystals. Vuggy pores in the matrix dolomite are cemented with saddle dolomite and later sparry calcite.

Aqueous homogenization temperatures for primary inclusions in saddle dolomite in the Calloway Creek and Knox are approximately 100°C. Bulk salinities, based on final ice-melting temperatures, are also similar and equal approximately 15 wt.% NaCl equivalent. Aqueous single-phase inclusions of secondary origin in calcite in the Calloway Creek post-date the calcite, but nonetheless suggest a cooling trend after dolomitization. A post-dolomite cooling trend is also suggested by a mean aqueous homogenization temperature of 87°C from possible primary inclusions in calcite in the Knox. Stratigraphic analysis in the area suggests that maximum burial for these rocks is less than 1 km. Assuming a geothermal gradient of 30°C/km and a surface temperature of 20°C, predicted maximum burial temperatures are cooler than homogenization temperatures measured in the saddle dolomite. The temperature difference suggests that dolomite formed under possible hydrothermal conditions. Homogenization temperatures from later calcite, though still anomalous, suggest a cooling trend.