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The Steele/Niobrara of Central Wyoming: Insights into Hydrocarbon Generation-Induced Regional Over-Pressure

Schmude, David and Mark Tobey
EnCana Oil & Gas USA, Denver, CO

     The Steele/Niobrara and its equivalent units are important source rocks throughout the Rocky Mountain region. In central Wyoming the interval is in its early life cycle of maturation and hydrocarbon generation, and in certain areas, sub-surface over-pressuring within this package is associated with a maturation thresh hold for hydrocarbon generation and saturation. Vitrinite reflectance data demonstrate relationships between present day thermal maturity / top of overpressure and an increase in the paleo-thermal gradients below top of overpressure at several locations. The former relationship is believed be related to kerogen conversion and hydrocarbon expulsion. The latter relationship is believed to be related to the thermal conductivity of hydrocarbon saturation within the over-pressured interval. The data infer that the maturation of the source interval is relatively recent, and that hydrocarbon generation is dynamic present day. The data also suggest that hydrocarbon over-pressure cells within this petroleum system can be inferred from well cuttings where there is little pressure or mud log data. At one location studied, there is evidence that a paleo over-pressure boundary has breached, and moved down section.
     Thin section, fluid inclusion, and kerogen conversion kinetics work was undertaken to help understand the processes associated with the development of the overpressure seals within the Steele- Niobrara section. At one well, a larger fracture cross-cut with later open fractures, and open veins of all sizes were observed within the overpressure boundary, possibly suggesting periodic breaching and healing.


AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah