A COMPARISON OF THE FRACTURE AND FLUID HISTORY OF TWO ANTICLINES, THE THOMAS FORK ANTICLINE IN THE WYOMING SALIENT AND THE BERWICK ANTICLINE IN PENNSYLVANIA APPALCHIAN

Understanding the development of fracture connectivity and fluid evolution in growing anticlinal structures is important for modeling hydrocarbon migration during folding. In this study, two anticlines in two separate fold belts are compared. The Thomas Fork anticline in the Wyoming salient was folded during the Cretaceous. Samples were taken from the carbonate Jurassic Boundary Ridge and Watten Canyon Fms. The Berwick anticline is located in the Pennsylvania Appalachian Valley and Ridge province and was folded during the Late Paleozoic Alleghenian Orogeny. Samples were taken from the clastic Devonian Marcellus through Catskill Fms.

In the Thomas Fork anticline, fractures are typically filled with one or more stages of calcite. Fluid inclusions are uncommon. Oxygen and carbon stable isotope analyses are used to constrain the fluid history. Host rock samples have 1.7-2.7‰ δ¹³C and -8.0 to -5.1‰ δ¹⁸O. Five vein sets are present and are primarily pre-folding to early syn-folding as poles to veins rotate to near vertical upon unfolding. The 087±14° striking set occurs only on the west limb of the fold, while the 349±8° striking set only on the east, and exhibit closed system behavior with isotope values similar to the host. The 310±12° striking set occurs on both limbs, and is due to reopening of early stylolitic cleavage. An open fluid system is indicated by a difference of 2.3-9.5‰ δ¹⁸O and 0.0-0.8‰ δ¹³C from the host. The cleavage-parallel veins are cut by a 066±8° striking set with a difference of 7.7‰ δ¹⁸O and 0.2‰ δ¹³C from the host. On the west limb of the anticline the 066±8° striking set also cuts a 030±10° striking set with open system behavior.

In the Berwick anticline, one or more stages of quartz mineralization are present and contain common two-phase fluid inclusions. In the Devonian section, the dominant fracture sets are: a pre-folding 308±10° striking set, a pre- to early syn-folding 341±5° striking set. Each of these have T_h values of 110-156 °C and 9.2-14.0 wt. % NaCl equiv. salinity. An early syn-folding 358±8° striking set has early inclusions with T_h = 162-200 °C and 9.0-11.5 wt. % salinity, and later inclusions with T_h = 218-240 °C and 22.4-25.6 wt. % salinity. A syn-folding 075±15° striking set is typically not mineralized. In both anticlines, the fracture connectivity increases early during the folding.