Carbonate Mudrock Microporosity Classification and Characterization through Core Examination: Upper Cretaceous Niobrara Formation, Denver-Julesburg Basin, Colorado & Wyoming

Peter D. Pahnke, Tom Anderson, and Scott Ritter
Energy & Geoscience Institute

Multiple efforts at describing and classifying pore types in mudrocks have been made (Slatt & O'Brien, 2011, Loucks et al., 2012). These efforts have been focused primarily on the Barnett, Woodford, Bossier and Pearsall formations, which are silica-rich and mainly gas producing unconventional reservoirs. Carbonate rock pore type classifications have been proposed by Choquette & Pray (1970), Lucia (1995) and recently by Lønøy (2006). While these classifications have been successful in characterizing conventional reservoirs, little knowledge exists regarding pores in carbonate-rich mudrocks. The Loucks et al., 2012 classification is consistent with previous schemes used in conventional carbonate reservoirs as well as descriptive and simple to use. This study focuses on fundamental pore characteristics and looks to extend conventional carbonate classifications to the Rocky Mountain Region Niobrara Formation. The Niobrara is a self-sourced tight petroleum resource play producing oil and natural gas from low permeability chalks and marls. Located throughout much of the Rocky Mountain Region, it was deposited along the eastern margin of the Western Interior Seaway during a major Late Cretaceous marine transgression (Longman et al., 1998). The Niobrara is divided into two members: the Fort Hayes Limestone and the Smoky Hill Member. The Smoky Hill is the thicker portion, consisting of three interbedded chalk bench and organic-rich marl sequences appropriately named A, B, & C. The B Bench is currently the main production target. Having been referred to as North America's 'next Bakken', the Niobrara is an evolving resource still in the early stages of understanding and development lacking investigation of key reservoir parameters. Thus, the Niobrara is a perfect candidate for this study. A detailed description of Niobrara pore types contributes to the overall understanding of how to best develop this emergent economic resource and provides a platform of study for the investigation of similar carbonate-rich resource plays. Data for this study come from core and outcrop samples collected from locations throughout the Denver-Julesburg Basin. Here the major focus is on core from Colorado's prolific Wattenberg Field. Pore type characterization incorporates petrography using thin section, QEMSCAN®, SEM and FIB-SEM analyses. SEM and FIB-SEM samples were prepared using Argon-ion milling techniques intended to create a polished surface devoid of major surface variations.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013