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How Tight Was The Williams Fork Tight Gas Reservoir at the Time of Gas Generation and Fracturing, Piceance Basin, Colorado?


Basin-centered gas accumulations are typically characterized by regionally pervasive, abnormally pressured, low permeability gas-saturated zones that commonly lack downdip water. The main trapping mechanism for the gas is thought to be the low permeability of the reservoirs themselves, which lie close to the basinwide gas kitchen. The main source of gas for the Williams Fork reservoirs is the Cameo coals within underlying coastal plain deposits. Significant thermal gas generation from these coals began in the early Eocene at the time of deepest burial. By that time diagenesis is claimed to have resulted in significant reduction in pore space and lowered the permeability. As a result of the reduced permeabilities gas cannot escape from individual sandstone intervals and eventually the reservoir become overpressured, assisting in the fracturing process. Although the permeability is thought to be very low, no studies have yet attempted to quantify the porosity and permeability of the tight gas sandstones at the time of gas generation and fracturing. By using a diagenetic modeling approach, we model the evolution of parameters that control the reservoir quality including quartz cementation, porosity, and permeability in an attempt to quantify these parameters at the time of peak gas generation and likely time of fracturing adopted from fluid inclusion studies (41 – 6 Ma). Modeling results indicate that at the time of early gas generation (51 Ma), sandstones had primary pores varying from 2 to 8% depending on grain composition, amount of clay coats etc. At about 35 Ma when the rocks reached their maximum burial depth and stabilized thermally, quartz cement filled the available pore space reducing the porosity and permeability in the sandstones closest to the gas generating Cameo interval. Permeability of most of sandstones was less than 1 mD during gas generation which is in agreement with the theory of basin-centered gas accumulation that requires low permeability sandstones in the vicinity of gas producing intervals for gas accumulation. Some of the sandstones were still diagenetically evolving during early fracturing which probably affected their rock mechanical properties; and therefore, the fracture distribution patterns. The best approach for prediction of fracture network characteristics is achieved by adjusting rock properties to those in effect during fracturing.