Abstract: Constraining Reservoir Models of Fluvial- vs. Wave-Dominated Delta-Front Sandstones Through High Resolution and High Density Sequence Stratigraphic Analysis, Ferron Sandstone, Utah

John A. Dewey Jr., Thomas H. Morris, Thomas A. Ryer

The Ferron Sandstone Member of the Cretaceous Mancos Shale contains world-class examples of fluvial- and wave-dominated deltas and sub-delta lobes. Superb 3-D exposures in the vicinity of Indian Canyon and Willow Springs Wash display parasequence-scale stratal units. Because fluid flow is impaired by an increase in the complexity of lithofacies architecture and the amount of heterolithics, high resolution and high density stratigraphic analysis of beds, bedsets, and parasequence-scale stratal units has been undertaken as the initial step in reservoir modeling.

Wave-dominated parasequences show a coarsening and thickening upward sequence of beds and bedsets. Sand bed thicknesses can reach 8-10 ft. and bedsets 30 ft occur. Heterolithics are less abundant in wave-dominated units. Grain sizes vary from very fine to medium sand. Detailed photomosaic mapping of the facies architecture shows that some wave-dominated parasequences extend continuously for 1-1.5 miles from their proximal to distal ends. Parasequences are bounded by fine-grained prodelta and delta plain deposits. Fluvial channels downcutting into adjacent parasequences may communicate fluids.

Fluvial-dominated parasequences are as thick as wave-dominated parasequences. Thicknesses of individual sand beds reach 3-4 ft. but these beds occur only within the distributary mouth bar facies. Bedsets attain thicknesses of 15-20 ft, but heterolithics are abundant Grain sizes vary from very fine to fine sand with medium sand being rare. In contrast to wave-dominated parasequences, fluvial-dominated parasequences are composed of isolated, smaller (150-250 ft.), and more heterolithic-rich bedsets stacked en echelon within fine-grained sediment This results in greater reservoir compartmentalization.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada