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Characterization and 3-D Reservoir Modeling of Fluvial Tight-Gas Sandstones in the Williams Fork Formation, Rulison Field, Piceance Basin, Colorado, U.S.A.

Marielis F. Vargas1, Matthew J. Pranter2, and Thomas L. Davis3
1 PDVSA, Puerto La Cruz, Venezuela
2 University of Colorado, Boulder, CO
3 Colorado School of Mines, Golden, CO

Tight-gas sandstones in the Piceance basin are estimated to hold more than 300 Tcf of gas. This resource is highly concentrated in the southern portion of the basin in stacked lenticular sandstones of the Upper Cretaceous Williams Fork Formation at Rulison field and surrounding fields. The distribution and connectivity of the fluvial tight-gas sandstones significantly impact reservoir productivity and ultimate recovery. The main reservoir interval includes low permeability fluvial sandstones, shales, and coal. Relatively high well densities (10-acre spacing) are necessary to intersect the relatively isolated fluvial sand bodies and deplete the reservoir. Heterogeneity of these fluvial deposits include larger scale stratigraphic variability associated with vertical stacking patterns and structural heterogeneities associated with faults that exhibit lateral and reverse offsets. Within the various types of fluvial sandstones, internal heterogeneities caused by fractures and lateral accretionary bedding are also significant. This study involves characterization of the stratigraphic and structural framework of the Williams Fork Formation for a portion of Rulison field and focuses on 3-D reservoir modeling of the gross sandstone distribution and associated sand-body connectivity (geobody distribution and connectivity). Results indicate that sandstone connectivity increases with net-to-gross ratio and at lower net-to-gross ratios (<30%), differences exist in the cumulative volume of connected sandstone bodies between the indicator- and object-based lithology models. At and above a net-to-gross ratio of approximately 30%, many of reservoir sand bodies are more interconnected. Therefore, the type of lithology modeling methods that are used for fluvial tight-gas sandstone reservoirs with lower net-to-gross ratios are important.