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Abstract This investigation reviews the vertical distribution of organic-rich sequences in Upper Cretaceous rocks of the northern Denver Basin. The primary objective is to provide a more complete definition of the effective source rocks in this sequence. Multiple organic-rich members in the Upper Cretaceous contribute to the effective petroleum systems in the basin. With an understanding of all possible generating source rocks in the Upper Cretaceous, an oil to source rock correlation, oil-to-oil correlation, and production allocation in the drainage reservoir is investigated.
Weld County, Colorado is the focus area for descriptions provided by samples available from the Niobrara, Carlile, Greenhorn and Graneros intervals. A review of rock extract and produced oil geochemical data made available by operators reveals discrete geochemical signatures among organic rich source rocks and corresponding production oils. The sample analyses reveal geochemical detail not previously reported in oil to source rock and oil to oil correlations in this area. This provides geochemical information about effective source rocks in the Upper Cretaceous petroleum systems of the Denver Basin.
Baseline geochemical correlations using rock and oil data provide calibration data for production allocation efforts. An automated algorithm was developed for consolidating raw geochemical data, running mixing models with variable input parameters, and specifying end-member production streams. The algorithm provides the functionality necessary for the long-term integration and comparison of results, which may be linked to mapping resources, production data and well performance over time. This study is providing building blocks for a web-based production allocation tool.
Introduction It is appropriate that the Denver Basin Niobrara chalk and marl sequences have dominated the literature for many decades as the Niobrara has been the predominant conventional and unconventional oil and gas play target. However, in the greater Denver Basin (Figure 1), many petroleum systems are effective (Higley, 2007; Clayton and Swetland, 1980). The Codell Sandstone member of the Carlile Shale has become a familiar reservoir and horizontal drilling and hydraulic fracture target over the last decade due to its juxtaposition to the chalk-dominated organic-rich systems of the Upper Cretaceous. The Codell sandstone is a clastic-dominated sequence of the Upper Cretaceous. The Codell is characterized by its widespread lateral extent and varying reservoir quality (Longman et al., 2021; Gent et al., 2021). Because the Codell is organic-lean, the hydrocarbon fluid charge into this reservoir can vary in its chemical signature depending on migration mechanisms and specific source rock charge. Upper Cretaceous organic-rich sequences in the Denver Basin include Pierre Shale, Niobrara marl sequences, collectively termed Smoky Hill members, and organic-rich sequences in the Carlile Shale, Greenhorn Limestone and Graneros Shale (Figure 2). "The science is not settled" (Sonnenberg et al., 2021, p. 349) and this study offers Greenhorn and Graneros side wall core extracts to further the understanding of organic-rich source bed contribution interpretation to the Codell reservoir.
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