Fluvial Facies and Rock Property Modeling of a Tight-Gas Reservoir in Green River Basin, Wyoming
Jonah Field produces gas from over-pressured fluvial channel sandstones of the Upper Cretaceous Lance Formation in the Green River Basin of Wyoming. Porosity in the sandstones ranges from 2 to 12%, while permeability is in the range of 0.005 to 1 mD. Because the rock properties are driven by the facies, accurately modeling the channel facies is critical for hydrocarbon reserve assessment and flow characterization. A workflow of modeling facies and petrophysical properties was developed for an integrated reservoir characterization.
Regional geology and seismic attributes indicated that the sandstone channels were deposited in low energy meandering to anastomosing rivers with a general N-S orientation. Core analysis showed two types of channel facies: isolated channels and amalgamated channel complexes. Gamma-ray and resistivity logs were used to predict facies at the log scale. An object-based stochastic modeling technique was used to build the facies model, honoring the fluvial channel orientation and dimension characteristics, the NTG by geologic unit, and the facies data at the wells.
The facies model was subsequently used to guide the petrophysical property modeling. The dependencies between rock properties were investigated, and a workflow of modeling porosity, NTG, water saturation and permeability was developed using geostatistical techniques. The final model honors the dependencies between these rock properties, and was used for the flow simulation of the reservoir.