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Detached Silt-rich Lowstand Shoreface Deposits of the Western Interior Seaway: Known and Prospective 'Shale' Gas Reservoirs

Steven Schamel, GeoX Consulting Inc, 1265 Yale Avenue, Salt Lake City, UT 84105, phone: 801 583-1146, fax: 801 583 1356, [email protected] and S. Robert Bereskin, Bereskin and Associates, Inc, Salt Lake City, UT 84109.

Within the thick Mancos Shale (Upper Cretaceous) succession of the southern Western Interior Seaway silt- and fine sand-rich intervals possess characteristics of detached lowstand deltaic and shoreface deposits. The overall sedimentologic and stratigraphic features of these deposits are remarkably consistent: (1) relatively low-energy deposits situated 30-60 miles basinward of coeval wave-dominated highstand shorelines, (2) planar and ripple laminated, coarsening-upward silt/sand successions usually encased in organic-rich shale and mudstone, (3) sedimentologic and trace/body fossil evidence for partial deposition under comparatively shallow marine to rare subaerial conditions, (4) very low matrix porosity and permeability (less than 6% porosity; microdarcy or nanodarcy permeabilities), but considerably greater than encasing shale and mudstone, (5) relatively brittle successions due to the higher silt/sand content, prone to natural fracturing and receptive to fracture stimulation, and (6) low average TOC (about 1%) and abundance of terrigenous plant debris. These silt/sand-rich low-TOC strata are partly self-sourcing in the manner of normal gas shales, and partly charged externally in the manner of tight sandstone reservoirs. Within the San Juan Basin of New Mexico, the Lewis Shale is already a proven gas reservoir containing total gas-in place in the 1,500 ft thick interval on the order of 125 MMscf/acre or 80 Bscf/mi2. Analogous basinward silt/sand-rich successions in Utah are the Prairie Canyon and Juana Lopez Members of the Mancos Shale; numerous examples occur in other Rocky Mountain states. Where sufficiently buried to generate and/or retain natural gas, all analogues are likely 'shale' gas reservoirs of great resource importance.