Geological and geochemical assessment of the Sharon Springs member of the Pierre Shale and the Niobrara Formation within the Cañon City embayment, south-central Colorado

Abstract

The Cañon City Embayment, located in south-central Colorado, is one of the oldest and longest oil producing regions in America. Production began in 1862 after the discovery of an oil seep emanating from the Jurassic Morrison Formation. This discovery led to an unsuccessful hunt for the oil spring’s source. The first oil field discovery occurred in 1881, founding the Florence Oil Field. This discovery led to a boom in drilling and production and the further discovery of the Cañon City Field in 1926. Production soon declined, but steady and continuous production occurs to this day. With the upswing caused by the discovery of unconventional petroleum systems, renewed interest led to higher drilling rates within the Cañon City Embayment. As of 2015, more 16.4 MMBO has been produced in the region. Present day production focuses on the fractured Pierre Shale and Niobrara petroleum systems, though exploration is expanding into the Greenhorn Formation. Deposition of both the Late Cretaceous Niobrara Formation and the Sharon Springs Member of the Pierre Shale occurred during transgressive phases within the Western Interior Cretaceous (WIC) Seaway, however significant geochemical and biological differences exist between these formations. Niobrara deposition occurred when warm Gulfian currents dominated the WIC. This resulted in abundant fecal pellet deposition and robust pelagic foraminifers dominated by Heterohelix globulosa, Globigerinelloides ultramicrus, Hedbergella, Gümbelina and two Archaeoglobigerina species. However, the Sharon Springs is primarily argillaceous in composition with lesser amounts of biogenic calcite and silica. Larger foraminifers within the Sharon Springs are primarily arenaceous while foraminifers of the same species found in the Niobrara are dwarfed in the Sharon Springs. Dwarfism of foraminifers species and the presence of dispersed diatoms, which flourish in cold waters, indicate a shift in paleocurrents. Paleogeographic maps of the Middle Campanian show a southerly restriction of the WIC during the time of the southerly Claggett transgression. The influx of cold, southerly waters resulted in an environment conducive to diatoms and environmentally stressful to foraminifers. Lithological and geochemical evidence from the Sharon Springs indicates that the cold-water Claggett transgression resulted in a stratified water column in the middle of the WIC, as well as increased organic matter production. Biomarker analysis shows the presence of isoreneieratane indicating photic zone euxinia and therefore a fairly shallow chemocline within the WIC during the deposition of the Sharon Springs. Petrographic analysis of preserved organic matter show clumped floccules or flattened amalgamations of floccules in laminated facies. While dispersed amorphous organic matter is also present, flocculation had a major influence on organic matter preservation. Two possible depositional mechanisms, dependent on water density, could have resulted in these water conditions. If density contrasts existed between the cold water influx and the warm waters present during the deposition of the Niobrara, the cold waters would have progressed along the bottom of the basin leading to upwelling along the basin margin and basin stratification. If no significant density contrast existed, caballing would occur at the mixing front, leading to downwelling, rapid transport of organic matter to the sea floor and basin stratification. High energy deposits located at the base of the Sharon Springs resulted in reworking of bentonites. Lithological and mineralogical differences between the reworked bentonites and the more typical ash fall bentonite deposits located at the top of the Sharon Springs indicate that the reworked bentonites are not a significant drilling hazard. The reworked bentonites are a greenish-gray cohesive bentonite which shows distinct fining upward. These cohesive bentonites are typically associated with debrites, most likely forming from a cogenetic sediment gravity flow. Thin-section analysis shows fining-upward and reworking of particles in the cohesive bentonites indicating deposition under turbulent conditions. Clay analysis shows very low smectite and primarily kaolinite comprising the clay portion. At the top of the Sharon Springs is a light gray, friable bentonite deposit with abundant mixed-layer illite/smectite, discrete illite and kaolinite, which is more typical of an ash fall deposit. While the typical ash fall deposits are drilling hazards, the minimal amounts of swelling clay within the reworked bentonites imply significantly lower hazards at the base of the Sharon Springs. The reworked bentonites also provide implications for regional stratigraphic correlations since reworking may remove lateral continuity. Following deposition of the Pierre Shale, the Laramide Orogeny reactivated dormant structural features of the Ancestral Rockies resulting in formation of the Cañon City Embayment. The asymmetrical, synclinal nature of the basin results in variation in the Sharon Springs source rock maturity across the basin with the most mature rocks in the west along the Chandler Syncline. Biomarker analysis suggests some lateral hydrocarbon migration within the Pierre Shale petroleum system. This system is conventional in nature with hydrocarbons contained within the fracture network overlying the Sharon Springs. The system is not overpressured and is driven by gravity drainage and solution gas, therefore intersection with abundant natural fractures is essential for economic wells. However, RMS amplitude anomalies are highest where faults permeate from the Niobrara into the Pierre Shale indicating hydrocarbon migration from deeper formations. Genetic relationships from biomarker comparison between the Niobrara Formation, Sharon Springs Member and oil produced from the Pierre Shale Formation supports hydrocarbon migration from Niobrara source rocks.