Outcrop to subsurface stratigraphic analysis of the Juana Lopez member of the Mancos Shale, northwestern CO, U.S.A.

Abstract

The Turonian Juana Lopez Member of the Mancos Shale in northwestern Colorado is a calcareous, bioclastic sandstone interbedded with organic-rich shale. Facies analysis of core and outcrops across northwestern Colorado indicates the presence of proximal, sand-prone facies to the north and distal, mud-prone facies to the south. The results of facies analysis and correlations between the outcrops and core suggest the Juana Lopez was deposited on a gently dipping, shallow, ramp-like shelf with internally correlative packages that prograded into the basin from north to south. The sediment was derived from a combination of sources: detrital input from the north and abundant intrabasinally derived, calcareous, bioclastic sediment consisting largely of disaggregated inoceramid prisms and fragments of other bivalves. The Juana Lopez has gained increased attention in terms of its petroleum potential, and recent research has indicated that within the Piceance Basin, it contains some of the highest total organic carbon (TOC) values within the Lower Mancos Shale. Source rock analysis conducted as part of this study confirms that the Juana Lopez is a high-quality source rock. Outcrop TOC values from the southern margin of the Piceance Basin, despite being weathered and oxidized, are good to excellent (median: 3.23 wt. %; range: 1.75–4.0 wt. %), and S2 values are poor to good (median: 5.40 mg HC/g TOC; range: 1.54–9.47 mg HC/g TOC). Source rock quality increases from north to south with increasing distance from the sediment source and increasing paleowater depth resulting in decreasing depositional energy.