THE FRACTURE AND FLUID HISTORY OF THE WHISKEY FLATS ANTICLINE IN THE CRAWFORD THRUST SHEET, CENTRAL WYOMING SALIENT

Fracture connectivity and fluid evolution in growing anticlinal structures play a principal role in understanding the fluid generation and migration of fluids, including hydrocarbons, during folding. In this study, the Whiskey Flats anticline located near the Idaho-Wyoming border of the Wyoming salient was examined. The fold is in the hanging wall of the Crawford Thrust, and was deformed during the Cretaceous Laramide orogeny. This anticlinal structure is ~1.5 kilometers in width in outcrop and trends 331°. The rocks in the area are moderately to highly fractured and show one or more stages of calcite mineralization. Vein samples in the area were taken from the Watton Canyon Member of the Jurassic Twin Creek Formation, a known oil reservoir.

Several major mineralized fracture sets and associated cleavage sets are found on both limbs of the anticline: An early pre-folding vein set strikes 073°±10° and is associated with a stylolitic cleavage striking 346°±5°. A pre- to syn-folding vein set strikes 055°±5° and is associated with a stylolitic cleavage striking 335°±5°. Third and fourth vein sets are interpreted to be syn- to post-folding reopening of earlier stylolites and strike 335°±5° and 090°±5°. Two additional sets are of unknown affinity: 037°±10° and 295°±5°. Shortening by wedge faults is directed toward 055°.

Fluid inclusions are uncommon in veins in the Whiskey Flats anticline. Oxygen and carbon stable isotope analyses were done in order to understand the fluid history. The host rock δ ¹³C values ranged from 3.2 to 2.7 ‰. The δ ¹⁸O values for the host rock in the formation range from –5 to –5.5 ‰. These are significantly different from the values determined for Phanerozoic Limestone (Veizer et al., 1999), therefore there was geochemical alteration of the host rocks. In general, calcite vein δ ¹⁸O values are depleted relative to the host rock, whereas δ ¹³C values are slightly enriched with respect to the host rock. Most veins have isotopic values similar to the host rock indicating a closed fluid system However, calcite in low-angle bedding wedge faults values show large δ ¹⁸O deviations of up to 6 ‰ and up to 1.5 in δ ¹³C from the host rocks.

In general, fracture fluid connectivity in the Watton Canyon Member of the Twin Creek Formation during folding was restricted to the formation, with little influence form outside fluid sources.