Defining Morrow Sandstone Channel System in Manassas Field, Denver Basin, Colorado, USA

The Manassas oil field was discovered in 1986 in Lincoln County, Colorado (Township 14S, Range 56W) by Petro Lewis Corporation and it completed production from the Pennsylvanian Atoka Sandstone within 24 years. It is located on the theoretical NW extension of the Haswell-Salt Lake Morrow sandstone producing trend and Morrow sandstones have found in some of the available wells in the area. The productive Morrow sandstones are fluvial channel deposits that are encased by marine shales. The distribution of the Morrow channel system is difficult to define from either data obtained from sparsely located wells alone, or directly from seismic data due to low acoustic contrast between fluvial sandstones and marine shale. Therefore, this study has used a correlation of well log data to understand regional stratigraphy of the selected study area and seismic attributes were used to develop a workflow to define the Morrow sandstone channel system in the Manassas prospect. Well logs from forty wells were used to define the early Pennsylvanian strata (Marmaton, Cherokee, upper and lower Atoka, upper and lower Morrow) and underlying Mississippian unit (St. Louis). The formation data were used to visualize the regional stratigraphy using isopach maps and stratigraphic cross sections. The acoustic and density logs from the Lockwood 27-22 well located in the Manassas field, were used to generate the synthetic seismogram for the purpose of seismic horizons interpretation. The upper Morrow isochron thick was defined using upper and lower Morrow time structure maps. The channel infill system was first detected using the Chaos attribute that identifies reflectors associated with channel infills that are normally chaotic signals with low consistency. The observation was confirmed using, Variance attribute that analyzes signal coherency, and estimates trace to trace variance. The amplitude attributes (RMS Amplitude – iterative attribute, and Envelope) highlight both channel infills and bright spots. The Genetic Inversion identifies relative variations of rock properties. The Generalized Spectral Decomposition attribute was used to visualize the detailed channel morphology through generating a RGB blending model as the final step. This study shows that the Morrow channel sandstones are present across the Manassas prospect and are interpreted as a composite meandered and anastomosed channel system. The observed high amplitude variations are interpreted as tuning effects of thin beds and relative lithological changes caused by variations of rock properties such as density and porosity