Insights into Niobrara Stratigraphic Architecture and Diagenesis, Wattenberg Field, Denver Basin, Colorado

Abstract

The Niobrara Formation consists of deep-water chalks and marl units in the Denver Basin. Chalk units are generally considered the reservoir rock while the marls contain higher total organic carbon (TOC) contents and are considered the source beds. Chalk and marl units consist of microscopic coccoliths, forams (dominantly pelagic), inoceramus and oyster fossils, kerogen, clay, silt, and fish bones. Chalks have 70% and above carbonate content whereas the marls range in carbonate content from 30 to 70%. Kerogen, clays and silt comprise the remainder of the mineralogical composition. The Niobrara is subdivided into two members: Smoky Hill and Fort Hays Limestone. The Smoky Hill is informally divided into A, B, C intervals in descending order. The chalks are separated by marl intervals. Dramatic thickness changes occur in the chalk and marl units in Wattenberg Field. Thickness variations result from subaqueous erosion over topographic highs, onlap and downlap of various units indicating bottom current activity and topography, convergence of section (i.e., higher rates of sedimentation in one area compared to an adjacent area), possible chalk-pellet bar formation, and compensatory deposition. Many of these causes of thickness variations can be related to intrabasin basement structural movement or differential sedimentation. The most significant erosional event removes the A chalk and parts of the A marl over an interpreted west-east paleostructure (the Wattenberg High). Differential sedimentation patterns may arise from bottom current activity either filling in topographic low areas or creating subaqueous bar-like features. Niobrara porosity is reduced dramatically by burial diagenesis. Progressive mechanical and chemical compaction and cementation associated reduce porosities within the Niobrara chalks and marls. Mechanical compaction consists of grain reorientation and breakage. Compaction is recognized by flattened fecal pellets and collapsed foraminifera tests. Chemical compaction consists of dissolution along microstylolites and grain-to-grain contacts and results in a ‘welded’ fabric. Calcite is reprecipitated as overgrowths on coccoliths grains and void-filling crystalline mosaics (chambers of foraminifera, oyster fragments, and fractures).

AAPG Datapages/Search and Discovery Article #90266 © 2016 AAPG Pacific Section and Rocky Mountain Section Joint Meeting, Las Vegas, Nevada, October 2-5, 2016