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Abstract: Structural Compartmentalization and Reservoir Fluid Distribution in the Terry and Muddy (J) Sandstones, Denver Basin, Colorado


Five basement wrench fault systems have been mapped on top of the Muddy (J) Sandstone in the central Denver basin. Major faults are 10-15 miles apart, but the intervening areas are cut by numerous synthetic and antithetic faults. In a nine township area (including Denver International Airport), 10-90 ft thick Muddy (J) Sandstone reservoirs are compartmentalized owing, at least in part, to one of these structural trends. Near-vertical faults which form compartment boundaries trend both northeast and north-northwest and have offsets of 10-90 ft. Evidence for compartments includes gas opposite oil at the same structural elevation across faults, and gas downdip from oil. Faulting and associated fracturing may enhance production or provide seals by silica diagenesis, gouge, and/or juxtaposition of sandstone against shale.

Similar fault patterns divide the Upper Cretaceous Terry Sandstone into a number of fault blocks of variable size in the sandstone trends of the Latham Bar/Hambert/Aristocrat fields. Vertical offset is on the order of 10s of ft. Evidence which suggests faults are sealing include: (a) higher normalized GORs for structurally higher wells within small single fault blocks and/or for wells in upthrown blocks than for wells in downthrown blocks, (b) gas-oil contacts at different elevations across blocks, (c) log bulk densities which are higher in wells cut by faults (suggesting a relatively high-density cement-fill), and (d) one core which exhibits calcite-filled fractures/faults.

Since numerous faults are now known to penetrate the entire Cretaceous section in the Denver basin, the sealing fault block compartment model presented here should be applicable to other Cretaceous reservoir intervals in this, and perhaps other, Rocky Mountain basins.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado