ALBERT

Description: 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 methane-rich 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 over time suggests dynamic pressure conditions requiring an active process of pressure generation during maximum burial conditions. Such a process is consistent with gas generation within the Mesaverde Group or by gas charge from deeper source rocks along fracture and fault systems but is inconsistent with significant high-pressure generation by compaction disequilibrium during earlier stages of burial. On the basis of a comparison of trapping temperatures with burial and thermal maturity models, we infer that active gas charge and natural fracture growth lasted for 35 m.y. and ended at approximately 6 Ma. Our results demonstrate that protracted growth of a pervasive fracture system is the consequence of gas maturation and reservoir charge and is intrinsic to basin-centered gas reservoirs.

Description: Wells in the Piceance Basin show anomalous large-magnitude (up to 200 mV), large-interval (〉2000 ft [610 m]) self-potential (SP) log responses in the Mesaverde gas-producing interval that can be best explained by electrokinetic potential resulting from water flow toward producing Mesaverde wells. Water flow is compartmentalized by capillary seals that are formed when gas generated from coals saturates adjacent thinly bedded sandstones and shales. Capillary seals can be identified by shifts in the SP baseline. The first wells drilled in an area with no previous Mesaverde production have very little SP response, as is expected in tight sandstones with single- to double-digit microdarcy permeability. After Mesaverde production is established in a new area, the SP log begins to show stepwise changes to more negative values beginning in the upper Mesaverde and becoming increasingly more negative with increased depth. The magnitude of the change to more negative values increases with time in an area of active Mesaverde production; some of the more recent SP logs have negative deflections of over 200 mV. This type of SP anomaly has not been reported before, and these anomalies can be used to identify large-scale water movement within a reservoir.