Expression of low- to intermediate-frequency cyclicity in siliciclastic intervals of the Paradox Formation, Paradox Basin, Utah
Jennifer Eoff, Katherine Whidden, Russell Dubiel, Sarah Hawkins, Kristen Marra, Ofori Pearson, and Janet Pitman
The U.S. Geological Survey (USGS) completed a geology-based assessment of the undiscovered, technically recoverable oil and gas resources in the Paradox Basin. Ongoing research includes revised description of core from the Shafer-1 well, drilled in the northwestern part of the basin in San Juan County, Utah. Whereas only two scales of cyclicity were described previously, changes in mineralogy and depositional fabric actually reveal at least five scales of nested cyclicity. Recent paleogeographic reconstructions place the Paradox Basin at low-latitudes during the Middle Pennsylvanian, within about 15 deg of the paleoequator. At the scale of tens of meters, the lowest-frequency cycles consist of alternating salt and non-salt strata, with the latter comprised of dolomitic siltstones and organic carbon-rich shales. Subdivisions of each lithology are defined by crystal size and texture or by the presence and type of sedimentary structures, including bioturbation, for salt and siliciclastic intervals, respectively. High-frequency cycles in evaporite successions that record freshening of the saline basin contrast with those cycles that document periods of increasing basin restriction, and most of the finest-scale cyclicity within each likely resulted from local to regional variations in climatic conditions. Alternatively, retrogradational and progradational stacking of siliciclasic packages document the effects of intermediate-frequency, regional variations during eustatic flooding and highstand. The distribution of hydrocarbon source rocks within the siliciclastic intervals was controlled primarily by the varying supply of silt-sized and coarser sediment by active basinward transport processes and by the degree of bioturbation. A preliminary microstratigraphic framework aids in characterization of shales as potential source rocks or as potential self-sourcing shale reservoirs, which has important implications for exploration strategies and production techniques.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012