Depositional system and source rock potential of the Sharon Springs Formation in Colorado

Abstract

The Sharon Springs Member of the Pierre Shale has recently been divided and reclassified as the Gammon Ferruginous Formation and Sharon Springs Formation, with the Pierre Shale being upgraded to group status. Deposition took place between 82 and 77 Ma in a conformable succession of facies following the Niobrara Highstand, and an overall progradation of siliciclastics into the Western Interior Seaway. These units gradually decrease over time in calcite content associated with chalk filled fecal pellets, to clay and quartz dominated deposits with a large constituent of pyrite. This interval has the potential to be a significant source rock where thermally mature, within the Denver and Piceance basins. Biomarker analyses on oil collected form the Florence-Cañon City Field suggest that the Sharon Springs has contributed to production from fractured members of the Pierre Shale up section. Thermal maturity indicators in biomarkers indicate that the oil produced from the Pierre is slightly more mature than source rock extracts taken from core in the field, suggesting that oil has a mixed source and likely migrated from further down dip. As a potential source rock, it is important to understand how these units were deposited and where they are most organic-rich. The previous model for Sharon Springs deposition and preservation of organic carbon was pelagic sedimentation in an anoxic, stratified and stagnant water column. Observations made in core and outcrop suggest that the Sharon Springs and Gammon Ferruginous formations were deposited by offshore sediment gravity flows. Storms likely assisted in reworking shelf muds and transporting them offshore where they are hydrodynamically sorted and often contain normally-graded beds, ripples and bioturbation. During the process of offshore transport, fine-grained sediment and organic matter which is commonly adsorbed onto clay grains or amalgamated into clay floccules, are concentrated, resulting in elevated accumulations of organic carbon. Organic carbon was then preserved through euxinic conditions below the sediment water interface due to low oxygen diffusion rates in the fine-grained material. Biogenic fecal pellets are abundant in the lower Gammon Ferruginous Formation, representing pelagic sedimentation in the most distal reaches of the basin. This interval, however, is not exceptionally organic-rich. This suggests that the prograding siliciclastic clinothems not only concentrated fine-grained sediment and organic matter at the distal portion of the deposits, but may be the primary mode of transport for organic matter to offshore regions. As the siliciclastics stepped basinward, the carbonate factory shut down and was replaced by organic-rich clay having a TOC of ~5% in the more proximal regions and reaching ~14% in the more distal areas where there is less detrital dilution. Bioturbation in the offshore deposits suggests that the water column was not euxinic or stagnant, but often oxygenated and active. Active water column mixing then resulted in high rates of primary productivity, and increased likelihood of organic matter preservation. The Western Interior Seaway was likely shallow enough (~200 m) to be frequently churned by storms, bringing nutrients into the photic zone, and resulting in algal blooms. The frequency of these storms then allowed enough organic matter to be deposited on the basin floor that preservation was likely, despite the oxygenated bottom-water conditions. This method of organic matter preservation is contrary to the pelagic sedimentation model, but explains the heterogeneity and presence of detrital feeding organisms within the organic rich units. Regional stacking patterns in the Wattenberg Field area of Weld County, Colorado show a progradation of facies to the northeast. Biostratigraphic correlation connects the progradation of these clinoforms, to prograding sandstones on the western margin of the basin associated with the Blackhawk and Castlegate Sandstone. Similar turbidite deposits and normally-graded siliciclastics have also been described within the Prairie Canyon Member of the Mancos, between the more proximal sandstones and basinal shales. This biostratigraphic and lithostratigraphic succession of facies presents a strong argument for a connection of the depositional system in eastern Colorado to the prograding units in western Colorado. Through the Wattenberg field area, however, local tectonics complicate the deposition of these siliciclastics, resulting in variability in unit thickness. This structural high is associated with faulting along the Colorado mineral belt, and has been shown to migrate to the south over time. Faulting also results in thermal conduits and a heat anomaly, resulting in the Sharon Springs and Gammon Ferruginous formations being thermally mature in this area and a potential source rock for exploration.