STRUCTURALLY-CONTROLLED HYDROTHERMAL DIAGENESIS OF PALEOZOIC RESERVOIR ROCKS EXPOSED IN THE WESTERN BIG SNOWY ARCH, CENTRAL MONTANA

The subsurface characterization of three dimensional structural traps is becoming increasingly important with the advent of new technologies for the sequestration of anthropogenic carbon dioxide (CO₂), which often takes place within pre-existing, sealed reservoirs to permanently store greenhouse gasses that are detrimental to global climate. Within the Big Snowy Arch, central Montana, reservoir units that are targets for CO₂ sequestration have experienced Laramide and younger deformation and widespread Eocene igneous activity, which introduced a heating mechanism for hydrothermal fluid flow. One particular region of interest is the western flank of the Big Snowy Mountains, which contains a NE-SW striking, high angle fault zone which has acted as a conduit for hydrothermal brine solutions into the overlying Phanerozoic rocks. Such fault zones often branch and bifurcate as they propagate up-section through the overburden until a loss of thermally-driven hydrodynamic pressure terminates the upward movement of CO₂-rich brines, which leaves a distinct assemblage of hydrothermal collapse brecciation rich in sulfide minerals, bitumen, and saddle dolomite. By using field techniques (detailed structural measurements and lithologic descriptions) coupled with analytical methods (X-ray diffraction and stable carbon and oxygen isotope analyses), we found evidence of episodic fluid flow within the brecciated region of the fault zone. Field reconnaissance revealed that breccia pipe alteration is confined to mechanically stiff units and therefore often follows lateral contacts in addition to vertical fractures. Laboratory studies demonstrate that hydrothermal fluids migrating through this network of brittle features likely caused early dissolution of the host rock and may have deposited secondary dolomitic cements; such is the case for strongly negative δ¹⁸O values within the brecciated and altered Mississippian limestones. We predict that these areas are major avenues of enhanced porosity and permeability in the subsurface, which has important applications at some sites in Montana where CO₂ sequestration is under consideration.