Investigation of high pore fluid pressure in the Uinta Basin, Utah. Final report

摘要

High pore fluid pressures, approaching lithostatic, are observed in the deepest sections of the Uinta basin, Utah. The authors analyzed the cause of the anomalous overpressures with a 3-dimensional, numerical model of the evolution of the basin, including compaction disequilibrium and hydrocarbon generation as possible mechanisms. The numerical model builds the basin through time, coupling the structural, thermal and hydrodynamic evolution, and includes in situ hydrocarbon generation and migration. They used the evolution model to evaluate overpressure mechanisms and oil migration patterns for different possible conceptual models of the geologic history. Model results suggest that observed overpressures in the Uinta basin are probably caused by ongoing oil generation in strata with specific conditions of permeability, relative permeability, TOC content, and oil viscosity. They conducted a sensitivity analysis that suggests for oil generation to cause overpressures, the necessary conditions are: oil viscosity is (approximately)0.05 cP or higher, intrinsic permeability is (approximately)5 (times) 10(sup (minus)18) m(sup 2) or lower, and source rock TOC values are (approximately)0.5% or higher. The authors also analyzed hydrocarbon migration patterns in the basin and how they are affected by the basin's structural history. Oil migration patterns produced by the model are consistent with published oil production maps: oil moves from the deep Altamont source rocks toward Redwash, the eastern Douglas Creek Arch area, and southward towards the Sunnyside tar-sands and Book Cliffs. Peak oil generation occurs from the time of maximum burial in the mid-Tertiary ((approximately)35 to (approximately)30 Ma). Most differential uplift of the basin's flanks probably occurs well after this time, and most oil migration to the basin's southern and eastern flanks occurs prior to uplift of these flanks. Model results show that if the basin's flanks are uplifted too soon, reduced temperatures increase viscosity enough to inhibit migration of oil to the southern and eastern flanks.