Characterization of Fine Grained Lithofacies in Coeval Strata From a Superregional Nearshore to Offshore Transect in the Upper Cretaceous of the North American Western Interior Seaway
As the exploration of unconventional plays continues to expand into increasingly challenging geologic environments the ability to predict the distribution of high reservoir quality has become an area of active research. Strata deposited around the Cenomanian – Turonian (C-T) boundary have received particular attention globally. Widely regarded as one of the premier times of source rock accumulation this time period has been extensively studied regarding enhanced preservation of organic carbon. Given recent developments of C-T strata becoming world class unconventional reservoirs (i.e.: Eagle Ford) gaining a full understanding of regional variations in how these rocks both generate and store hydrocarbons requires knowledge of the detailed lithofacies variations both vertically and laterally. In this study we have logged and sampled strata spanning the C-T boundary super-regionally from the North American Cretaceous Western Interior Seaway (KWIS). Samples from both cores and exposures from central Utah to Kansas have been analyzed to 1. determine their biostratigraphic context; 2. facies attributes and 3. environments of deposition. Lithofacies were observed from strata of the late transgressive to early highstand associated with the early Turonian maximum flooding. Strata from fluvial influenced environments are highly bioturbated argillaceous mudstones with abundant plant material. At the top of the interval coarsening upwards parasequences are more apparent with the tops of the parasequences becoming increasingly silt-rich with wave rippled laminae. Away from clastic input the coeval lithofacies are mixed calcareous - siliciclastic systems. The primary carbonate components are pelagic coccoliths and foraminifera. The early transgressive deposits consist of intervals of highly bioturbated bases that evolve upward to wave rippled thin (<1 cm) graded beds. Within the late transgressive to highstand a similar stacking is observed with the tops being 10 – 20 cm thick highly cemented carbonates. From these observations we believe that strata deposited in the transgressive interval of the mixed system environments make the optimal tight oil reservoirs in this part of the KWIS. The strata studied from the more terrestrially influenced environments were too influenced by fluvial processes. It is possible that other strata along strike where riverine input was less dominant could have high reservoir potential.
AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015