TRACKING FLUID EVOLUTION USING FLUID INCLUSIONS IN SYNKINEMATIC FRACTURE CEMENTS: PICEANCE BASIN, COLORADO

Natural opening-mode fractures in Piceance Basin “tight gas” sandstone reservoirs are partially cemented with crack-seal quartz cement bridges that precipitated synkinematically with fracture opening. We combine high resolution SEM-CL imaging of quartz bridge cements with microthermometry and Raman microspectrometry of fluid inclusions trapped in the crack-seal cement layers to constrain the pressure-temperature-pore-fluid chemical (PTX) evolution during fracture opening and cementation.

Homogenization temperatures of aqueous fluid inclusion assemblages (FIAs) at the MWX/SHCT well site show a thermal history from ~145°C to ~185°C in Cozzette Sandstone of the Isles Formation and ~141°C to ~177°C in the overlying Williams Fork Formation. Analyses in Corcoran Sandstone at the Grand Valley site indicate a thermal history from ~150°C to ~172°C. Aqueous fluid inclusion salinities are low, ranging from 2-3 wt% NaCl equivalent at all examined sites, and are similar to seawater salinities. Coexisting aqueous and hydrocarbon inclusions indicate the homogenization temperatures represent true formation temperatures of fracture opening, and that hydrocarbon charge occurred coevally with fracture opening. Raman spectroscopic analyses indicate that vapor bubbles in aqueous fluid inclusions are adequately modeled by a composition of pure methane, confirming that these inclusions were trapped under methane–saturated conditions. We used the CH₄ Raman peak position of the vapor bubble to determine the pressure of the aqueous inclusions at room temperature. Based on integrated microthermometry, Raman spectroscopy, and equation of state modeling, we calculate pressures at trapping for the observed inclusions. Preliminary results indicate trapping pressures ranging from ~70 MPa to ~100 MPa, suggesting fracture opening occurred under significant pore fluid overpressures.

According to available burial histories for these well sites, the highest fluid inclusion temperatures are the same as maximum burial temperatures, and constrain fracture opening and associated hydrocarbon charge between ~42 to ~10 Ma.