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Reservoir characterization and geomechanical evaluation of the Greenhorn Formation in the northern Denver Basin, Colorado
Durkee, Hannah Marie
Durkee, Hannah Marie
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2016
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2016-12-13
Abstract
Success of unconventional reservoirs in the Denver Basin has led to increased interest in the petroleum potential of the Greenhorn Formation. The Denver Basin produces hydrocarbons primarily from Cretaceous-aged strata. An increase in hydraulic-fracture technology has shifted focus from conventional reservoirs, such as the āJā sandstone, to unconventional accumulations throughout the basin. Evidence indicates that hydrocarbons exist in the underexplored formations of the Benton Group, specifically within the Greenhorn Formation. While the individual formations within the Benton Group are historically recognized as a source rocks lacking in conventional appeal, the Cenomanian-Turonian Greenhorn Formation exhibits notable unconventional reservoir characteristics and should be further explored. The Greenhorn Formation is comprised of three members: the Bridge Creek Limestone, the Hartland Shale, and the Lincoln Limestone. The Bridge Creek Limestone and the Lincoln Limestone act as reservoir intervals, while the Hartland Shale is a source, with total organic carbon (TOC) values of 2-4%, predominantly as Type II marine kerogen. Geochemical analyses of the organic-rich shales and marls within the source units indicate that hydrocarbons have been generated within the study area. Previous research on the Greenhorn Formation characterizes the source-rock reservoir properties and includes geochemical characterization and source-rock analysis. The Greenhorn Formation is mature and organic rich in Wattenberg Field and in the Northern Denver Basin, which includes Silo, Hereford, Fairway, and Redtail fields. The Greenhorn is of similar lithology and porosity to the overlying Niobrara Formation, which is the primary unconventional target within the Denver Basin. The Greenhorn contains source and reservoir intervals, Type I shows, high TOC values, elevated levels of thermal maturity, and internal fractures; all of which are indications that hydrocarbons may exist within this interval. An extensive reservoir characterization has been completed in order to understand the reservoir and completion quality of the members within the Greenhorn Formation. This multidimensional study of borehole and core data, combined with a microscope study of mechanical behavior, utilizes basin-wide correlations, reservoir characterization, and rock mechanical analysis to understand the heterogeneity of the Greenhorn Formation. The study covers multiple scales to quantify the petroleum potential of the Greenhorn Formation, and strengthen the understanding of the unit as an unconventional reservoir. Chalk intervals within the Bridge Creek Limestone and the Lincoln Limestone members exhibit comparable porosity to the Niobrara Formation, and contain microfractures for hydrocarbon storage and migration. Petrophysical, XRF elemental, and geomechanical data suggest that the Bridge Creek Member is similar to hydrocarbon-producing chalk beds in the Niobrara, and should be considered a possible unconventional target. The Greenhorn Formation represents a complex mixed siliciclastic-carbonate depositional setting with seven main facies that represent a transition from chalk to marl to mudstone. The Bridge Creek Member represents a biogenic pelagic-dominated system, whereas the Hartland and the Lincoln represent a detrital dominated depositional system with significant clastic hypopycnal influence and tempestite deposition. Depositional fabric, sedimentary structures, dominant allochems, bentonite and limestone frequency, clay characterization, bulk mineralogy, elemental properties, organic content, and porosity and permeability influence the geomechanical response of each interval. Each of these characteristics has implications for geomechanical behavior and ultimately affect the reservoir and completion quality of the study interval. Based on these analyses, the Greenhorn Formation can be broken into 4 distinct geomechanical zones with differing mechanical behavior that can be quantified based on reservoir and completion quality. While the Bridge Creek Member has the highest completion quality and is most comparable to the Niobrara, the combined analyses from this study indicate that the Lincoln Member has the highest overall potential as an unconventional reservoir throughout the study area when reservoir, source, and completion quality are assessed.
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