Structurally-controlled hydrothermal diagenesis of Mississippian reservoir rocks exposed in the Big Snowy Arch, central Montana

Abstract

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, which often takes place within preexisting, sealed reservoirs to permanently store greenhouse gasses that are detrimental to the global climate. Within the Big Snowy Arch, central Montana, reservoir units that are targets for carbon sequestration have experienced Laramide and younger deformation and widespread Eocene igneous activity, which introduced a heating mechanism for hydrothermal fluid flow and created anisotropy in Mississippian strata. One particular region of interest is the western flank of the Big Snowy Mountains, which contains a northeast-southwest 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 carbon dioxide-rich brines, leaving a distinct assemblage of collapse breccia rich in hydrothermal minerals, such as saddle dolomite and sulfide precipitates. To determine the degree of structurally-induced anisotropy within the reservoir units, field techniques (detailed structural measurements and lithologic descriptions) coupled with analytical methods (X-ray diffraction spectrometry, stable carbon and oxygen isotope analyses, secondary electron imagery, and petrography) were utilized. These techniques presented concrete evidence of hydrothermal mineralization and episodic fluid flow within the brecciated region of the fault zone. These areas are major avenues of enhanced porosity and permeability in the subsurface, which has important applications at some sites in Montana where carbon sequestration is under consideration (e.g., Kevin Dome).