AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Testing the Basin-Centered Gas Model Using Fluid Inclusion Observations
(1) Bureau of Economic Geology, University of Texas at Austin, Austin, TX.
(2) Department of Geosciences, Virginia Tech, Blacksburg, VA.
The natural gas system in the Piceance Basin, Colorado, has been considered a basin-centered continuous gas accumulation. In these systems, continued gas generation by thermal maturation of source rocks within the low-permeability reservoir allows gas accumulation in the absence of a top seal. This model contrasts with models of low-permeability reservoirs where gas accumulates in conventional traps. To test these models, we determined the temperature, pressure, and fluid composition history during fracture opening in Mesaverde Group sandstones of the Piceance Basin based on fluid inclusion microthermometry and Raman microspectrometry. The fractures are partially cemented by crack-seal quartz cement indicative of repeated stages of fracture opening and cementation. We observe variable ratios of coexisting hydrocarbon gas to aqueous inclusions within single fractures, in multiple fractures of different depth, and in different well locations. The same variability is expressed in pore fluid pressures determined form methane-saturated aqueous fluid inclusions, with pressures ranging from ~60 to ~100 MPa without systematic trend over time. In contrast, homogenization temperatures of aqueous fluid inclusions record systematic temperature trends from ~140 to 185°C and to 158°C over time. The fluctuating pressure record indicates a dynamic system where pore pressure increases locally and temporarily to lithostatic pressures. In agreement with existing reservoir models, we interpret this pressure record to indicate migration of gas in high pressure cells that move upward by natural hydraulic fracturing. Repeated passage of pressure cells through fracture systems results in cyclic fracture opening and sealing. Based on a comparison of microthermometry data with burial models we infer that this process of gas generation, migration of high pressure cells, and natural hydraulic fracturing lasted for 33 m.y. in the Piceance Basin.