Organic and inorganic geochemical characterization of the Juana Lopez and Montezuma Valley members of the lower Mancos Shale, Piceance Basin, Colorado

Abstract

The Piceance Basin is a prolific sedimentary basin known for its immense accumulations of oil and natural gas from a number of producing intervals. Of these intervals, both the middle-to upper Turonian Juana Lopez and the overlying upper Turonian Montezuma Valley members (and their coeval equivalents) of the Mancos Shale are beginning to receive some attention with regards to their petroleum potential as possible targets for unconventional exploration and production. However, minimal research has been completed on the Montezuma Valley Member, in particular, of the lower Mancos Shale with regards to its source rock potential. What research has been completed has suggested that this interval, along with the underlying Juana Lopez Member, may contain some of the highest total organic carbon (TOC) values of the lower Mancos Shale throughout the Piceance Basin. This study provides a detailed description of both the Juana Lopez and Montezuma Valley members, including: (1) lateral facies heterogeneities, (2) source rock analysis to better understand the petroleum potential of this interval, (3) a chemostratigraphic profile and the use of established elemental proxies to better understand paleoconditions, and (4) the use of petrophysical analysis to better define this interval throughout the Piceance Basin to aid future exploration. The Frontier Member is a known producing interval in the northern Piceance Basin and is often grouped together with the Juana Lopez Member due to their similar petrophysical response. A chemostratigraphic characterization of these two members across the basin assists in a more defined description and understanding of these units. Additionally, chemostratigraphy highlights lateral progression in terms of facies changes between the two units. The contact between the Frontier and the Juana Lopez has been interpreted to be a regional unconformity throughout the basin and a chemostratigraphic profile across this contact does display marked differences in elemental abundances, which reflects mineralogy and thus, depositional variations. A significant increase in the overall siliciclastic content is observed, as well as, a drastic change in the provenance are reflected in a number of detrital proxies. Detailed core descriptions of the Juana Lopez and Montezuma Valley members suggest slight facies heterogeneities throughout the basin. These heterogeneities have been interpreted to represent localized restrictions due to bathymetric structures, changes in water-column chemistry, and/or lateral changes in depositional environments/conditions. The Juana Lopez and Montezuma Valley range in TOC from roughly 1.0 wt. % to over 4.0 wt. %, and 0.5 wt. % to iv over 3.5 wt. %, respectively. Pyrolysis data suggests a broad range in maturities throughout the basin, with the south, west, and northern-most margins being less mature and lying within the peak-to-late oil to early condensate windows. Chemostratigraphic analyses suggests that defined proxies for detrital influxes, grain-size, paleo-productivity, paleoredox conditions, organophillic elements, etc. do closely correlate with paleodepositional conditions. Given the relative lack of available data and the increasing interest in organic-rich units for unconventional plays, additional research to define and characterize the Juana Lopez and Montezuma Valley members of the lower Mancos Shale of the Piceance Basin was necessary.