Seismic investigation and attribute analysis of faults and fractures within a tight-gas sandstone reservoir: Williams Fork Formation, Mamm Creek Field, Piceance Basin, Colorado

The seismic-reflection characteristics, distribution and orientation of faults, and fracture intensity of the Williams Fork Formation at Mamm Creek Field vary stratigraphically and with lithology and depositional setting. The fluvial, marsh, and shallow marine deposits of the Williams Fork Formation were deposited within alluvial-plain, coastal-plain, and shallow-marine environments. The deposits produce significant amounts of natural gas from Cretaceous-age tight-gas-sandstone reservoirs that are moderately porous but exhibit low matrix permeability. Faults and fractures provide conduits for gas migration and enhance permeability and reservoir productivity.;Key stratigraphic units, fault and fracture characteristics, fracture intensity, and the controls on fracture distribution were evaluated by using p-wave seismic data and derived seismic attributes in conjunction with well logs, borehole-image logs, and core data. Amplitude dimming, poor amplitude coherency, and offset reflections characterize the alluvial-plain and coastal-plain deposits. More continuous and moderate-to-high amplitude reflections are present in the lower Williams Fork Formation, which is characterized by coastal-plain and shallowmarine deposits.;An ant-tracking workflow and interpreted seismic-amplitude data and curvature attributes indicate that fault characteristics are complex and vary stratigraphically; the lowermost lower Williams Fork Formation is characterized by north-northwest- and east-west-trending small scale thrust and normal faults. The uppermost lower Williams Fork Formation and the middle and upper Williams Fork formations exhibit north-northeast- and east-west-trending arrays of fault splays that terminate upward and do not appear to displace the upper Williams Fork Formation. In the uppermost Williams Fork Formation and Ohio Creek Member, north-northeast-trending discontinuities are displaced by east-west-trending events and the east-west-trending events dominate.;Fracture analysis based on ant-track and t* attenuation seismic attributes suggests a nonuniform spatial distribution of fractures. In general, higher fracture intensity occurs within the southern, southwestern, and western portions of the area, and fracture intensity is greater within the fluvial reservoirs of the middle and upper Williams Fork formations. Greater than 90% of natural fractures occur in sandstones and siltstones. In-situ stress analysis, based on induced-tensile fractures and borehole breakouts, indicates a north-northwest orientation of present-day maximum horizontal stress, an approximate 20-degree rotation in the orientation of Shmax with depth, and a sudden stress shift in the Rollins Sandstone Member.