The Mowry-Muddy Hydrocarbon System of the Powder River Basin; a Developing Resource Play

John R. Forster
Whiting Oil and Gas Corporation, Denver, CO

Recent successes in the Bakken Formation of the Williston Basin have generated renewed interest in oil-prone source rocks as potential producing intervals. While the Mowry is a comparable quality source rock to the Bakken Formation in the Williston Basin, the Bakken has retained much of it hydrocarbons in-situ due to the efficiency of seals adjacent to the source intervals. In contrast, the Mowry interval sourced millions of barrels of oil into the adjacent Muddy sandstones. It has yet to be determined how much insitu hydrocarbon remains within the Mowry as a possible exploration target. This is a world-class hydrocarbon system with over 549 MMBO and 1.978 TCFG produced to January 2008 in stratigraphically trapped accumulations adjacent to mature source rocks.

The Mowry-Muddy hydrocarbon system in the southeastern Powder River Basin is a continuous-phase oil accumulation sourced by the rich, dark siliceous marine shales of the Mowry Formation with adjacent continental and marine clastic reservoirs of the Muddy Formation. The Mowry Shale, like the Barnett Shale in the Fort Worth Basin, is a highly siliceous rich source rock and susceptible to fracturing creating good reservoir potential. This stratigraphic interval has potential to be a combination source-reservoir resource play. However, two marine sandstones within the Muddy, the Rozet and Springen Ranch Members, provide pathways for vertical hydrocarbon migration out of the Mowry reducing the exploration potential.

Pressure data as well as fluid distributions show a relationship between the Mowry source interval and the Muddy reservoirs. Production data and drill stem test data indicate an area of regional extent where mobile water is absent and only oil and associated gas are produced from either the Mowry or the Muddy Formations. Pressures across this area demonstrate both over-pressure and normal-pressure relative to hydrostatic gradient indicating isolation of highly pressured hydrocarbons from normally pressured saturated reservoirs. Structural as well as stratigraphic isolation occur locally and regionally.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado