Unconventional petroleum system analysis using a three-dimensional basin model: Mancos shale, Uinta Basin, Utah

We present a three-dimensional (3D) basin model of the gas- and oil-bearing Mancos Shale in the Uinta Basin, Utah from 100 Ma to present. With few analogs in the public literature, this model of the Mancos Shale provides a rare detailing of a 3D unconventional petroleum systems model, as well as a dissection of the model's sensitivities. As with conventional models, basin-specific parameters like heatflow and erosion have a significant impact on the model predictions of generated hydrocarbons and their lateral distribution. Similarly influential are the equations and methods used to model source rock phenomena, including mechanical compaction, thermal maturation, adsorption, and migration. From this investigation, we find that incorporating detailed source rock facies heterogeneity trends, as described by recent stratigraphic analysis, has a substantial impact on estimated hydrocarbon volumes and phase distributions. Honoring facies heterogeneity of the Mancos Shale results in larger predicted hydrocarbon generation (+35% oil, +82% gas) and in-situ volumes (+16% oil, +20% gas) compared to results of a more classically homogenous, but otherwise identical model. Model predictions suggest that more than 90% of the hydrocarbon volume generated in the Mancos Shale has migrated out of the formation. Original gas in place (OGIP) estimates for the Mancos Shale are comparable to other major unconventional systems; however, model predictions indicate that gas is relatively well-distributed throughout the ~4000 ft of Mancos Shale stratigraphy, resulting in predicted volume-yields of less than 20 Mscf/acre-ft for much of the basin. Modeled adsorption rates in the Mancos Shale are generally high, with an average of 7% adsorbed gas and 79% adsorbed oil by volume. Modeled oil volume-yields are typically less than 20 bbl/acre-ft within the Mancos Shale except in areas of the eastern basin where oil volume-yields of 20-35 bbl/acre-ft coincide with transformation ratios between 25-50% and the presence of sediment-starved shelf facies association.