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Abstract

Chalk beds of the Upper Cretaceous (Coniacian-Campanian) Niobrara Formation were deposited in a shallow epicontinental seaway in the Western Interior of the United States during a major global sea-level rise. Biogenic gas is produced from the thermally immature, organic-rich chalk beds of the Niobrara in the eastern part of the Denver basin in eastern Colorado, northwestern Kansas, and southwestern Nebraska. These chalks have high porosity and low permeability. Accumulations of shallow gas are not controlled by major structural closures but by local, faulted, low-relief domal structures, or noses. Fracture stimulation, primarily with the use of foam treatments, is necessary to make gas production from these wells economically feasible.

Westward and at greater depth in the basin, however, oil is produced from much tighter, naturally fractured chalk beds that are thermally mature and capable of thermogenic oil generation.

The reservoir properties (mainly porosity and permeability) and types of hydrocarbons produced in the chalk of the Niobrara Formation from a given location within the basin are primarily controlled by diagenetic processes. Maximum burial depth (with associated differential pressure and temperature history) is the main controlling factor in reservoir quality. Thus, reservoir characteristics and source-rock potential of the Niobrara can be predicted from an understanding of the post-Niobrara depositional and thermal history of the region coupled with research that has identified systematic diagenetic changes.

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