Importance of Core and Outcrop Investigations for Evaluating TOC Distribution in Unconventional Systems: An Example From the Mowry Shale

The Mowry Shale is a late-Albian, light-gray, siliceous shale deposited in the Cretaceous Interior Seaway and is an important source rock in several Wyoming basins. An evaluation of the Mowry Shale resource potential was conducted on the Casper Arch in Natrona County, WY. This evaluation incorporated the following components: measured stratigraphic section; complete Mowry Shale core; visual and petrographic analyses; detailed facies analysis; organic analysis by carbon coulometry; geochemical analysis by X-ray fluorescence; and complete wireline log suite. Five facies associations were identified in the Mowry Shale, including fine-grain sandstone; bentonite (ash); laminated, silty shale; laminated, silty, siliceous shale; and non-laminated, siliceous shale. Though industry-accepted parameters such as gamma-ray and density logs are often used to determine intervals of maximum total organic carbon (TOC) within resource plays, results from this study show negligible correlation between TOC and wireline log response. The maximum TOC interval is not correlative with the interval of highest gamma-ray values, thus use of natural gamma radiation does not accurately reflect the presence of TOC. Preliminary results suggest that Mowry Shale TOC is more strongly related to the presence of biogenic silica, an indicator of primary productivity in the ancient seaway. The integration of geochemical data, organic composition, and a detailed facies analysis furthers our understanding of TOC distribution within the Mowry Shale when the application of standard log parameters alone is inadequate. Findings from this study emphasize the importance of core and outcrop evaluations for elucidating parameters known to affect TOC distribution. Background and Motivation Since the late 1800’s, the Powder River Basin has produced large quantities of oil from its Lower Cretaceous sandstone reservoirs sourced by the late-Albian Mowry Shale, a biosiliceous shale which was deposited in the Cretaceous Western Interior Seaway. With more modern technological advances in hydraulic fracturing techniques, companies are considering the Mowry as a prospective target for horizontal drilling and well stimulation. Recently, the USGS published mean estimates of undiscovered resources for the Mowry at 198 MMBO and 198 BCFG (Anna, 2009). Since 2000, operators targeting the Mowry for horizontal drilling have met with varied production results. Given the tremendous source potential for the Mowry Shale, it is of interest to better understand the depositional environment and controls for organic material distribution within the basin as well as developing methods for predicting total organic carbon (TOC) when destructive and/or direct sampling of material is not possible. Well log techniques, such as the ΔlogR method, have historically been applied to wells and have been relatively accurate in estimating TOC values over a range of maturities. This approach, while successful at predicting TOC, is not intended to independently provide insight into the depositional environment or mechanism for organic matter preservation. Research presented here provides a geochemical framework for analyzing core and outcrop with the aim of identifying and interpreting geochemically ideal drilling targets within the Mowry Shale.