Abstract

The Niobrara Petroleum System is a developing petroleum resource play. It is present over most of the Rocky Mountain region and is prospective in many areas. The Niobrara is self-sourced and reservoirs are low permeability chalks, shales, and siltstones. Source beds have total organic carbon contents that range from 2 to 8 wt. % and are thermally mature in the deeper parts of many of the Laramide basins in the Rocky Mountain region. Continuous accumulations can occur in these thermally mature areas.

The Niobrara source rocks contain dominantly Type II (sapropelic) oil-prone kerogens. Oil accumulations occur where source beds are in the oil generation window (e.g., western Denver Basin). Thermogenic gas accumulations are located within the gas generating window in deeper parts of basins (e.g., Piceance Basin). Biogenic methane occurs in shallow chalk reservoirs on the east flank of the Western Interior Cretaceous Basin (e.g., northeastern Colorado). Biogenic gas fields are also found in northern Montana (e.g., Bowdoin Field).

Natural fractures are important in controlling sweet spots in the play and formed by several processes. Models proposed for fractures in the Niobrara include tectonic folding and faulting (local structures, some related to deeper salt dissolution), stress relief with Neogene regional uplift and erosion, regional horizontal stress (regional orthogonal fractures), and hydrocarbon generation creating sufficient pore pressure.