--> Revisiting the Upper Cretaceous Niobrara Petroleum System in the Rocky Mountain Region: Mark W. Longman, and Barbara A. Luneau

AAPG Rocky Mountain Section Meeting

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Revisiting the Upper Cretaceous Niobrara Petroleum System in the Rocky Mountain Region: Mark W. Longman, and Barbara A. Luneau

Abstract

Our 1998 publication based on a regional study of the Niobrara Formation slightly preceded a burst of successful horizontal drilling and much new research into Niobrara stratigraphy and depositional processes. Isopach and facies maps we published as well as our depositional models for the Niobrara have proved useful, and we may have been ahead of the learning curve in attributing the chalk and marl benches primarily to current flow patterns in the Western Interior Seaway rather than eustatic sea level changes. Some lessons learned about the Niobrara in the 21 years since that paper was published are the focus of this presentation. Our depositional model claimed that current flow patterns in the long, narrow Western Interior Seaway, specifically the flow direction of tropical Gulfian versus colder arctic currents, was the primary control on chalk vs. marl deposition. In the Denver Basin, for example, when warmer Gulfian waters flowed northward, they brought into the seaway the abundant coccoliths, copepods, and planktonic foraminifers that form the chalkier intervals. When arctic waters flowed predominantly southward in the seaway, carbonate productivity was limited and marlier, more organic-rich facies were deposited. Changes in water depth had little to do with the resulting Niobrara rock types, but the associated eddies, swirls, and current vortices did significantly impact chalk vs. marl deposition on a fine scale. New evidence supporting our interpretation is five-fold: 1) Welldocumented high-resolution interfingering of the chalk and marl facies on a scale of centimeters or less, which is far too thin to be controlled by sea-level fluctuations; 2) Thin (<2 cm) organic-rich marls within the clean chalk benches that cannot be the product of sea-level changes; 3) Lack of evidence for the eustatic sea level changes along the seaway’s margins (e.g., in Utah and Kansas); 4) Interbedded chalk and marl facies in the Fort Hays Member, which are commonly attributed to Milankovitch cycles, with significant currentinduced scours and up to a meter of relief, and 5) Documentation of modern ocean current flow patterns on deep-water hemipelagic deposits off New Zealand’s South Island and in the Mediterranean that have produced bedforms now shown to occur in the Niobrara on high-resolution cross sections (e.g., in the Denver Basin).