History of Fracture Development and Diagenesis in Piceance Basin Tight Gas Reservoirs: Insights from Fluid Inclusion and Fracture Scaling Analyses
Timing of the development and cementation of fracture porosity, as well as the reservoir-scale distribution of fractures has important implications for exploration and production in low permeability tight gas sandstone reservoirs. Natural opening-mode fractures in Piceance Basin reservoirs display evidence of multi-step opening by the crack-seal mechanism of deformation from SEM-CL imaging of synkinematic quartz bridge cements. These diagenetic cements are spatially discontinuous within individual fractures, with significant porosity preserved between bridges. On a reservoir scale, these fractures follow power-law aperture-size distributions. Scanline data from twelve samples indicate that as fractures widen, the power law slope decreases and approaches a value near -0.5, observed in populations with the largest fractures. This uniform power law exponent could provide a useful tool for fracture intensity prediction using small datasets of fracture size. Fluid inclusion assemblages (FIAs) hosted in crack-seal cement deposits within quartz bridges record the overall range of pressure-temperature-composition conditions during fracture opening. Preliminary analyses of the homogenization temperatures of aqueous FIAs in Cozzette sandstone at the MWX site near Rulison indicate a thermal history from ~145°C to ~185°C. Analyses from Corcoran sandstone in the Grand Valley field indicate a thermal history from ~150°C to ~172°C. Coexisting hydrocarbon gas inclusions indicate these temperatures represent true formation temperatures of fracture opening, and that gas charge occurred coevally with fracture opening. According to available burial histories for these rocks, the highest fluid inclusion temperatures are the same as maximum burial temperatures, and indicate a maximum timing for fracture opening between ~42 to ~7 Ma.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009