Shallow vs. Deep Water Origin for U.S. Cretaceous "Shale" Reservoir Successions
The conventional interpretation of fine-grained terrigenous clastic successions within the Cretaceous system of the USA is that they represent sediment accumulation in deep water settings (100s of m) distant from contemporaneous sources of coarse-grained sediment. Data from a variety of sources, however, suggest this is not always the case. Rather, many "shale" reservoirs were formed within a few km of coarser-grained sediment accumulations associated with delta systems. Data from two active plays, the Mancos Shale in Utah, and the Mowry Shale in Wyoming, are presented to illustrate these relationships. Within the Turonian Tununk Shale-Ferron Sandstone of southern Utah, interbedded sandstones and mudrocks of delta front origin can be shown to pass distally into prodelta and basinal mudrock-dominated facies over distances of <5 km. On the fringes of these delta systems, mud-dominated heteroliths preserve impressions and compressions of invertebrate fossils (ammonites, bivalves), fish bone, scale and teeth debris, and abundant early diagenetic pyrite, features that have been used to infer deep, basinal settings. Yet these same beds are often rich in macerated plant debris ("coffee grounds"), preserve wave- and combined flow-generated physical sedimentary structures, contain a trace fossil assemblage suggestive of shallow water deposition, and preserve winnowed lag horizons of gravel and fossil debris. Furthermore, bentonite beds (volcanic fallout deposits) in the succession are often locally absent due to erosion, or dispersed into a sequence of beds containing tractional sedimentary structures, indicating the operation of current flows capable of transporting sand grade material. Sediment accumulation in no more than 20-50 m water is indicated. The Cenomanian Mowry Shale in the northern Bighorn Basin of Wyoming has also been previously interpreted to record deep basinal environments, this impression reinforced by the abundance of bentonite beds within the unit, around which the lithology is typically silicified. However, the Mowry can be traced proximally into more sandstone-dominated, deltaic deposits, and it too preserves shallow-water trace fossils and sedimentary structures. Accordingly, accumulation in delta front to prodelta settings is again indicated. These examples serve to illustrate the dangers of uncritical adoption of facies models for fine-grained reservoir systems, and the need for care in the choice of appropriate analogs.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013