THE FRACTURE AND FLUID HISTORY OF TWO ANTICLINES IN THE NORTHERN WYOMING SALIENT

Fracture connectivity and fluid evolution in growing anticlinal structures is important for understanding and modeling hydrocarbon migration during folding. In this study we examine two closely-spaced anticlines in the northern Wyoming salient. The first fold is the northern plunge-out of the herein-named Astoria anticline which is in the hanging wall and northern terminus of the Darby thrust. Less than 10km to the west, the Little Grays River anticline is in the hanging wall of the Elk Mountain thrust. Both folds were deformed during the Cretaceous Laramide orogeny. The rocks are moderately to highly fractured with one or more stages of blocky calcite filling the fractures (veins). In the Astoria anticline, vein samples were taken from the carbonate rocks of the Giraffe Creek, Leeds Creek, Boundary Ridge, Rich, and Watten Canyon Members of the Jurassic Twin Creek Formation. In the Little Grays River anticline, samples were taken from the Giraffe Creek Member. Fluid inclusions are uncommon in all samples. Instead, oxygen and carbon stable isotope analyses are used to constrain the fluid history.

In the Astoria anticline, the host rock has a range of 1.8-3.0‰ δ¹³C VPDB and -10.8 to -7.5‰ δ¹⁸O VPDB. Six vein sets are present. Veins within sets striking 091±11°, 118±16°, 060±10, and 030±11° all indicate a closed system environment based on little isotopic variation. A vein set formed parallel to bedding and another set striking 178±3° indicate open systems based on their isotopic variation compared to the host rock. In both of these sets, isotopic variation from host rocks ranges from 0.5-4.8‰ δ¹⁸O and 0.21-.82‰ δ¹³C.

In the Little Grays River anticline, host rock samples have a range of 1.7-2.3‰ δ¹³C VPDB and -10.8 to -9.2‰ δ¹⁸O VPDB. Four vein sets are present and are primarily pre-folding to early syn-folding as poles to veins rotate to near vertical upon unfolding. There are two cross-fold vein sets striking 045±8° and 075±10°. A vein set oblique to the fold strikes 003±11°, and a strike and cleavage parallel set strikes 331±9°. Vein calcite from all sets exhibit differences of up to 2.0‰ δ¹⁸O and up to 0.2‰ δ¹³C from the host rock indicating a moderately open fluid system. Increased fracture connectivity may be related to increased fracturing and fracture reopening at the location of the sampling sites near the crest of the fold structure.