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) 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 barlike features.

Understanding stratigraphic architecture is paramount in targeting chalks and calculating reserves in the Niobrara Formation. Chalk thickness varies dramatically across the field. Thickness variations in the marl units is also important to understand in that the source bed capacity is a function of marl thickness and TOC content. Small scale clinoforms may be present in the C marl interval. Onlap of A marl units onto the Wattenberg High also changes source rock capacity.

Introduction

The most important mineral extraction activity over the past 50 years in Colorado has been the discovery and development of Wattenberg Field (Figs. 1, 2). The Wattenberg Field is located northeast of Denver, CO and produces oil, gas, and condensate from the following Cretaceous horizons: Dakota-Lakota, J Sandstone, D Sandstone, Greenhorn, Codell, Niobrara (A, B, C, and Fort Hays), Hygiene, Terry, and Larimer-Rocky Ridge.

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