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Geologic Controls on Oil Production from the Niobrara Formation, Silo Field, Laramie County, Wyoming

Carrie Welker, Tom Anderson, and Lisa Stright
University of Utah

The Niobrara Formation, an interbedded source-rock and low-porosity chalk/limestone deposited during the Late Cretaceous in the Western Interior Seaway (WIS), is an important hydrocarbon play throughout the Rocky Mountain region. The interbedded chalks and marls contribute to the petroleum system potential of the Niobrara. Ductile marl units have higher organic carbon content, and act as both a source and seal while most reservoir capacity is in the brittle chalk benches. Silo Field, located in the Denver-Julesburg Basin in Laramie County, Wyoming, has been producing from the Niobrara Formation since 1981. Vertical wells were drilled in the 1980s, followed by horizontal drilling in 1990, and finally, horizontal drilling using modern technology began ~2009. Cumulative production to date is 10.8 MMBO and 9,751 MMCFG. At Silo Field, the Niobrara is ~300 ft. thick, is at depths between 7500-8500 ft., and consists of the lower Fort Hayes Limestone and the upper Smoky Hill Member, which contains alternating chalk and marl sections. The middle B chalk bench is the main production target. Despite over thirty years of production history at Silo Field, it is not well understood why only a few wells are top producers while neighboring wells have very poor production rates. Though the Niobrara has been the topic of much previous research, little attention has been paid in analyzing relationships between geological trends and production data in a quantitative manner. Our objective is to identify geologic factors that contribute to productive wells or groups of wells ('sweet spots') at Silo Field. We will identify completion practices in order to differentiate whether successful production is due to geological variables like mineralogy, distance from faults, fracture intensity, interval thickness, and porosity; or to how wells were managed. We will present the correlation between production and geologic variables determined from core, well logs, cross-sections and maps, with an emphasis on the B chalk. Our goal is to build a predictive geologic model of spatial and stratigraphic heterogeneity to test whether a relationship exists between geologic variables and production. Results from this study may contribute to understanding other Niobrara plays in the Denver-Julesberg basin like the nearby Wattenberg and Hereford fields in Colorado, and may also define what makes the Niobrara Formation unique compared to other source rock reservoirs.

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