OTR Field Trip – May 19, 2012

Title: Niobrara & Greenhorn Formations, Florence-Cañon City Area

Leader: Stephen A. Sonnenberg, Colorado School of Mines

Description:  Published in the May 2012 Outcrop, p. 26

The Niobrara Formation of the Rocky Mountain Region is currently of great interest for oil and gas exploration. The Niobrara Petroleum System of the U.S. Rocky Mountain Region is a major tight petroleum resource play. The Niobrara is self-sourced and reservoirs are low permeability chalks, shales, and sandstones. Source beds have total organic carbon contents that range from 2 to 8 weight percent. Source beds are thermally mature in the deeper parts of many of the Laramide basins in the Rocky Mountain region. Continuous or pervasive accumulations occur in thermally mature areas.

The Niobrara source rocks are dominantly Type II (sapropelic) oil-prone kerogens. Oil accumulations occur where source beds are in the thermogenic oil window (e.g., Denver Basin). Thermogenic gas accumulations occur where the source beds have entered 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., eastern Colorado). In addition shallow gas fields are found in northern Montana (e.g., Bowdoin field).

Natural fractures are important in controlling sweet spots in the play and form for several causes. Several models have been proposed for fractures in the Niobrara and include folding and faulting (local structures), stress relief with Neogene regional uplift and erosion, regional horizontal stress (regional orthogonal fractures) and hydrocarbon generation pore pressure.

The Niobrara is a technology reservoir that requires horizontal drilling and multi-stage hydraulic fracturing. The Niobrara petroleum system is present over most of the Rocky Mountain Region and is prospective in many areas.

This field trip will examine the Niobrara in outcrops between Florence and Cañon, CO. The petroleum system of the Niobrara and Graneros-Greenhorn will be emphasized. Reservoirs, source beds, and fracture models will be discussed.