THE FRACTURE AND FLUID HISTORY OF THE BEAR CREEK ANTICLINE IN THE IDAHO PORTION OF THE NORTHERN WYOMING SALIENT

Understanding the development of fracture sequence, connectivity, and fluid evolution in growing anticlinal structures is important for modeling hydrocarbon migration during folding. In this study we examine the Bear Creek anticline in the Idaho portion of the northern Wyoming salient of the Sevier fold and thrust belt. The Bear Creek anticline is ~7km wide and ~50km long, trends 330 ° and plunges to the SE/ and NW. It is located in the hanging wall of the Absaroka thrust sheet and was emplaced during the Cretaceous Laramide orogeny. The rocks are moderately to highly fractured with one or more stages of blocky calcite filling the fractures (veins). Vein samples were taken from the carbonate rocks of the various member of the Jurassic Twin Creek Formation and the Lower Triassic Thaynes Formation.

Three major mineralized fracture sets and associated cleavage sets are found on both limbs of the anticline but are locally restricted by formation: An early pre-folding vein set strikes 080°±15° and is associated with a stylolitic cleavage striking 360°±10°. A pre- to syn-folding vein set strikes 040°±15° and is associated with a stylolitic cleavage striking 332°±10°. Third and fourth vein sets are interpreted to be syn- to post-folding reopening of earlier stylolites and strike 330°±15° and 005°±10°.

Fluid inclusions are uncommon in the calcite veins, therefore δ¹⁸O and δ¹³C isotope analysis is used instead to track differences in the vein materials in different formations or different stress events. Overall, most veins exhibit closed system behavior with vein compositions being depleted by only 0.3 to 1.0 ‰ δ¹⁸O and having nearly identical values of δ¹³C with the host rock. For example, all veins in the Watton Canyon Member are similar to the host rock. However, several veins have values indicating open system behavior. Two samples in the Giraffe Creek and Slide Rock Members and one in the Leeds Creek Member have δ¹⁸O values ~2 ‰ difference from the host rock. In contrast, in the Thaynes Formation, most veins exhibited significant isotopic difference from the host rock.

In general, fracture fluid connectivity in the Twin Creek during folding was restricted to similar fluid reservoirs, most likely within a short stratigraphic distance and within the formation. The Thaynes, however, has evidence for possible ‘exotic’ fluid infiltration.