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