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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
DOI:10.1306/05171211149
Testing the
basin
-
centered
gas
accumulation model using fluid inclusion observations: Southern Piceance Basin, Colorado
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Andras Fall,1 Peter Eichhubl,2 Stephen P. Cumella,3 Robert J. Bodnar,4 Stephen E. Laubach,5 Stephen P. Becker6
1Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, University of Texas at Austin, Austin, Texas; [email protected]
2Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, University of Texas at Austin, Austin, Texas; [email protected]
3Endeavour International Corporation, Denver, Colorado; [email protected]
4Fluids Research Laboratory, Department of Geoscience, Virginia Tech, Blacksburg, Virginia; [email protected]
5Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, University of Texas at Austin, Austin, Texas; [email protected]
6ExxonMobil Upstream Research Company, Houston, Texas; [email protected]
ABSTRACT
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.
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