AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Tidal and Deltaic Influence in Storm-Dominated Prograding Shoreline Deposits of Pictured Cliffs Sandstone, Northern San Juan Basin – Implication Toward Reservoir Characterization
(1) Geology, Bowling Green State University, Bowling Green, OH.
The Upper Cretaceous Pictured Cliffs Sandstone of northern San Juan basin represents a storm-dominated prograding shoreline sequence deposited during the last regressive cycle of the Western Interior Seaway. The unit produces unconventional stratigraphic traps with the underlying Lewis Shale (offshore mudstone) and overlying Fruitland Formation (non-marine coal). Most of the sandstone units show evidences of tidal effects and distal deltaic deposits. In these instances, architectural element analysis of these reservoir sandstones reveals variations in internal architecture and petrophysical characters of the sandstone units.
Outcrop investigation of the Pictured Cliffs Sandstone reveals 14 different lithofacies. These lithofacies are grouped into 4 common lithofacies assemblages: 1) Tempestite, 2) Turbidites, 3) Flood deposits, and 3) Tidalites. In tempestites and flood deposit sequences, heterolithic sandstones and mudstones are noticed by cyclic and sharp grain size variation and are identified as rhythmites. These rhythmites are asymmetric which are commonly identified as mud-drapes produced by daily tidal cycles. Monthly tidal cycles (spring and neap tides) are also recognized by thin and thick tidal bundles. Increased fine grained particles in these sandstone units would greatly reduce porosity (by 50%) and permeability. Evidence for such reduction in porosity is revealed from thin section analysis which shows iron-rich clay rims filling the pore spaces.
Evidences of distal deltaic plain deposits were also found in the lower section of the Pictured Cliffs Sandstone. These deposits are identified by non-erosional base overlain by planar-laminated sandstone (Sl) or massive sandstone (Sm) often with soft-sediment deformation structures like load casts (Sld and Smd). There is no direct evidence of a delta in the Pictured Cliffs Sandstone in the study area. But, provenance analysis and paleocurrent study suggest that there was a fluvial system and a wave dominated delta located in close proximity, northeast of the study area. These flood deposits are generally homogenous, quartz rich sandstones and have a fairly high porosity and permeability.