Print this pageArchitecture and Permeability Structure of a Compound Valley Fill
WILLIS, B. J.
Compound valley-fill sandstones have complex internal architectures formed during multiple episodes of valley incision and filling. These complexities make it difficult to predict the distribution of rock properties and production behavior of this reservoir type. A cross section of a compound valley fill in the Lower Cretaceous Fall River Formation is exposed in Red Canyon, on the southwest flank of the Black Hills of South Dakota. A study of bedding architecture, facies organization, and permeability structure of the strata exposed in this cross section provides a detailed view of reservoir-scale heterogeneities that are predicted to influence fluid flow in subsurface analogs.
This 4-km-wide, 30-m-thick compound valley-fill sandstone cuts into a succession of meter-thick, upward-coarsening, marine shore-zone strata and muddy alluvial-plain deposits. The sandstone is partitioned into four distinct fills by major erosion surfaces, and each of these fills contains many stacked meters-thick channel bodies. Each fill records a dramatic shift in depositional environments as relative sea level fell and then rose at an accelerating rate. Meter-thick sheetlike fluvial channel deposits lower in fills are coarse-grained sandstone, whereas deposits higher in fills tend to be upward-concave channel sandstones, 10-m-thick channels with heterolithic inclined beds, and decimeter-thick wave-deposited sheet sandstones. Fills have statistically distinct permeability populations and internal trends. The second fill has a substantially. greater permeability mean and variance than the first, and it shows the greatest vertical and lateral changes. Both these fills are dominantly fluvial. The third and fourth fills have successively lower mean permeabilities and are progressively more heterolithic and estuarine in character. This retrogradational pattern juxtaposes poor-reservoir-quality valley fills on older, higher quality, valley-fill sandstones. Smaller scale (tens of meters) permeability structure within fills reflects the dimensions and internal facies of the channel bodies.