Sequence stratigraphy of the Niobrara Formation, Powder River Basin, Wyoming: A tool for reservoir quality prediction

Abstract

The Niobrara Formation is emerging as a large tight oil play within the Powder River Basin (PRB), Wyoming. Selection of landing zone and the prediction of lateral sedimentological variation across the basin is crucial for hydrocarbon production. In this paper we outline a workflow utilizing primary data-sets inclusive of facies, automated mineralogy, biostratigraphic and organic geochemistry data to build a sequence stratigraphic framework for the Niobrara. The Niobrara is late Coniacian (87.8 Ma) to early Campanian (81.5 Ma) in age and forms a 3^rd order transgressive-regressive eustatic sea level cycle. Lithostratigraphically, the Niobrara can be divided into 3 members (from youngest to oldest); A, B and C. Internal subdivision of each of these units is based on an underlying “Marl” unit and overlying “Chalk” or “Bench” unit. The Niobrara Fm. has two peak TOC intervals; one within the basal C Marl (locally restricted, peak TOC of 6.3%) and in the Upper B Bench (peak TOC of 5%). The B Bench is the principal exploration target in the Niobrara. TOC positively correlates with marine algae (% of sample count) and dinocyst/coccolith diversity (S/N), which is coincidental with an increase in carbonate content. This suggests that the increased carbonate productivity is mirrored in the increased organic bio-productivity. Peak dinocyst and coccolith diversity is observed within the Upper B Bench, representing the 3^rd order maximum flooding surface (late Santonian ~ 84.3Ma) within the Niobrara Fm. However, coccolith recovery diminishes within the cleanest “chalk” intervals (carbonate content 60-80%), which are dominantly peloidal packstones. Increased miospore recovery is found within the B Marl, which is dominated by weakly calcareous mudstones (carbonate content <20%, clay content >30%, TOC content <2%) and high bentonite frequency. This is interpreted as an internal regressive cycle. An upward increase in miospore content in the A member, above the Upper B Bench MFS, coincides with greater clastic flux and reduced TOC preservation. This suggests a progressive fall in relative sea level above the Upper B Bench MFS. As oil saturation positively correlates with TOC and carbonate content, indicating that hydrocarbons have been stored within competent source rock intervals, mitigation against stratigraphic uncertainty is key in this play. Therefore an established sequence stratigraphic framework allows for more reliable comparison of production in fields across the PRB.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019