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3D Geomodeling of the Lower Cretaceous J Sandstone, Wattenberg Gas Field, D-J Basin: Stratigraphic and Structural Compartmentalization Quantified for Reservoir Simulation

Sonnenfeld, Mark D.1, Edmund R. “Gus” Gustason2, Hai-Zui Meng1, Steven R. Clawson2 
1, Inc, Littleton, CO
2 EnCana Oil & Gas (USA) Inc, Denver, CO
3 Aspect Resources, Denver, CO

Recent 32-acre infill drilling within the Wattenberg Gas Field's Lower Cretaceous J-Sandstone encountered wide-ranging bottom-hole pressures suggestive of small-scale compartmentalization. Results from core-based, sequence-stratigraphic and structural studies of the J Sandstone reveal a complex interplay among depositional and erosional processes, pedogenesis, diagenesis, and syndepositional and post-depositional faulting that contributes to both regional and small-scale compartmentalization. A nine-section 3D geomodel was constructed to determine gas-in-place and simulate gas production. Multiple sources of heterogeneity were quantified and tested for sensitivity in dynamic simulations including: deltaic shingles within the Fort Collins Member, aerial permeability anisotropy accounting for strike-parallel (depositional) preferred permeability likely within wave-dominated deltas, poro-perm relationships constrained by facies, and stratigraphic truncation and diagenetic porosity degradation by the Horsetooth lowstand surface of erosion (LSE).

Stratigraphic grids constructed at log resolution (2 feet) facilitated geostatistical interpolation of log properties, starting with gamma-ray, because it was most abundant, followed by co-located density and resistivity. The 3D log property model was processed to generate a 3D volume of shale, effective porosity, bulk volume water, and ultimately bulk volume gas. For efficient fluid-flow simulation, the static geomodel was up-scaled into 12 flow units, while maintaining aerial cell sizes.

After initial simulations, it became apparent that greater reservoir compartmentalization was needed to match the relatively high bottom hole pressures of certain recent wells. Small-scale reservoir compartments are controlled by syndepositional and postdepositional faulting, facies changes and diagenesis. Faults “inferred” by contouring the Mowry Shale at 10-foot intervals proved critical to matching observed pressure compartments.