SIMULATIONS OF THE ORIGIN OF FLUID PRESSURE, FRACTURE GENERATION, ANDTHE MOVEMENT OF FLUIDS IN THE UINTA BASIN, UTAH

The Altamont oil field in the deep Uinta basin is known to have reserv oir fluid pressures that approach lithostatic. One explanation for thi s high pore-fluid pressure is the generation of oil from kerogen in th e Green River oil shale at depth. A three-dimensional simulation of fl ow in the basin was done to test this hypothesis. In the flow simulati on, oil generation is included as a fluid source. The kinetics of oil generation from oil shale is a function of temperature. The temperatur e is controlled by (1) the depth of sediment burial and (2) the geothe rmal gradient. Using this conceptual model, the pressure buildup resul ts from the trade-off between the rate of oil generation and the flow away from the source volume. The pressure increase depends primarily o n (1) the rate of the oil-generation reaction and (2) the permeability of the reservoir rocks. A sensitivity analysis was performed in which both of these parameters were systematically varied. The reservoir pe rmeability must be lower than most of the observed data for the pressu re to build up to near lithostatic. The results of the simulations ind icated that once oil generation was initiated, the pore pressure built up rapidly to near lithostatic. We simulated hydrofractures in that p art of the system in which the pressures approach lithostatic by incre asing both the horizontal and the vertical permeability by an order of magnitude. Because the simulated hydrofractures were produced by the high pore pressure, they were restricted to the Altamont field. A new flow system was established in the vicinity of the reservoir; the maxi mum pore pressure was limited by the least principal stress. Fluids mo ved vertically up and down and laterally outward away from the source of oil generation. The analysis indicated that, assuming that one is w illing to accept the low values of permeability, oil generation can ac count for the observed high pressures at Altamont field.