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The Upper Cretaceous Mesaverde Group in the Piceance Basin, Colorado, is considered a continuous basin-centered gas accumulation in which gas charge of the low-permeability sandstone occurs under high pore-fluid pressure in response to gas generation. High gas pressure favors formation of pervasive systems of opening-mode fractures. This view contrasts with that of other models of low-permeability gas reservoirs in which gas migrates by buoyant drive and accumulates in conventional traps, with fractures an incidental attribute of these reservoirs. We tested the aspects of the basin-centered gas accumulation model as it applies to the Piceance Basin by determining the timing of fracture growth and associated temperature, pressure, and fluid-composition conditions using microthermometry and Raman microspectrometry of fluid inclusions trapped in fracture cement that formed during fracture growth. Trapping temperatures of methane-saturated aqueous fluid inclusions record systematic temperature trends that increase from approximately 140 to 185°C and then decrease to approximately 158°C over time, which indicates fracture growth during maximum burial conditions. Calculated pore-fluid pressures for methanerich aqueous inclusions of 55 to 110 MPa (7977-15,954 psi) indicate fracture growth under near-lithostatic pressure conditions consistent with fracture growth during active gas maturation and charge. Lack of systematic pore-fluid-pressure trends

Natural fractures form preferred pathways for basinal fl uid fl ow and associated heat and mass transport. In gas sandstone reservoirs with low matrix permeability, fractures provide fl ow pathways between organic-rich source and reservoir layers during gas charge, and between matrix pores, hydraulic fractures, and the well bore during production. While the formation of natural fractures has previously been associated with gas generation and pore-fl uid pressure increase through a process referred to as natural hydraulic fracturing, other driving mechanisms such as stress changes by tectonic or exhumation processes remained viable alternatives. To test whether these mechanisms contributed to fracture development, we investigated the spatial and temporal distribution of fracture formation and its relationship to gas generation, migration, and charge in sandstone of the Cretaceous Mesaverde Group across the entire production interval on a basinwide scale. Using fl uid inclusion microthermometry of crack-seal fracture cement formed concurrently with fracture opening, we observed temperature trends that, when compared with temperature evolution models of the formation, date fracture formation between 41 and 6 Ma in the northern and between 39 and 6 Ma in the southern Piceance Basin. The onset of fracture formation 20-30 m.y. prior to maximum burial eliminates changes in stress state asso ciated with exhumation as a mechanism for triggering the onset of fracture formation. Instead, calculated paleo-pore-fl uid pressures of 40-90 MPa (5800-13,000 psi) during fracture opening and the presence of methane-rich inclusions in fracture cement suggest that fracture formation was aided by high pore-fl uid pres-For permission to copy, contact

Halite-bearing inclusions may display three different modes of homogenization during heating from room temperature: (A) halite dissolution (Tm halite) followed by liquid-vapor homogenization (Th L-V), (B) simultaneous Th L-V and Tm halite , or (C) Th L-V followed by Tm halite (Fig. 1). Fluid inclusion researchers use microthermometric data from halite-bearing inclusions to estimate fluid compositions and temperatures and pressures of formation of the inclusions (Roedder, 1984). Compositions and trapping pressures may be estimated with reasonable precision (and accuracy) for inclusions that homogenize via modes "A" and "B" because experimental (