Abstract

Sandstone composition and related reservoir quality, and isotopic compositions of diagenetic minerals in sandstones and mudstones produced during the stratigraphic evolution of the Cretaceous Skull Creek, Muddy, and Mowry sediments vary in patterns that correspond to the following: (1) Skull Creek sandstones, consisting of transgressive highstand deposits, are litharenites that contain abundant kaolinite cement and local concentrations of quartz cement at a subaerial unconformity that separates the Skull Creek from the overlying Muddy Sandstone. The subaerial exposure in outcrops of Skull Creek deposits between Fort Collins and Turkey Creek, Colorado, is characterized by ¹⁸O-enriched quartz overgrowths in a 10 to 15 m thick interval, indicating that the quartz cement in the Skull Creek sandstones formed during early meteoric water diagenesis beneath a subaerially exposed surface; (2) Overlying Muddy sandstones, consisting of incised valley-fill deposits, are porous quartzarenites and sublitharenites that contain abundant quartz cement formed from evolved, warmer subsurface fluids; and (3) Mowry shales with relatively high total organic content and siderite were deposited in a condensed interval during maximum flooding. The isotopic composition of diagenetic carbonate in both the Skull Creek and Muddy/Mowry stratigraphic sequences has variations in the δ¹³O compositions that correspond to variations in total organic content. Co-varying patterns of regional stratal “geometry” and diagnetic modifications illustrate the strong impact of genetic stratigraphy on reservoir quality.