Unconformity-related chert/dolomite production in the Pennsylvanian Amsden Formation, Wolf Springs fields, Bull Mountains basin of central Montana

Abstract

Wolf Springs field (north and south pools) and South Wolf Springs field (a.k.a. Wolf Springs fields), located in Yellowstone County, Montana, were discovered in 1955 and 1957, respectively, and have produced more than 5.7 million bbl oil from the Pennsylvanian Amsden Formation. Amsden reservoir rocks in the area are fractured and brecciated cherts and dolomites that occur in several laterally persistent and mappable zones. The Amsden was deposited in a peritidal to sabkha setting where evaporite minerals, mainly anhydrite, were once common. These evaporites were partly replaced by silica (chalcedony and chert) soon after deposition. Later dissolution of the remaining evaporites soon after the silicification event, or during the pre-Middle Jurassic unconformity (PMJU), produced the solution-collapse chert breccias that now serve as the best reservoir facies in the field. Subtle variations in the diagenetic history of these breccias was a major factor in shaping reservoir quality. The Wolf Springs fields are unconformity-related combination structural and stratigraphic traps. The fields are located on a structural closure on the Custer anticline, where porosity and permeability development exhibit a northeast-southwest orientation perpendicular to structural strike of the anticline. The solution-collapse breccias pinch out laterally into either dense dolomites or anhydrite-plugged collapse breccias. The overlying shaly dolomite breccia of the Botts member (informal name) located just below the Piper unconformity and the Jurassic Piper Limestone provide an effective top seal. (Begin page 132) Understanding the geographic distribution of the chert/dolomite zones provides a key to exploration for these reservoirs. This must be coupled with analysis of available rocks and drillstem-test data and the integration of the regional hydrodynamic forces affecting the Amsden. These exploration tools should lead to the discovery of new Amsden reserves in the Bull Mountains basin and Central Montana platform.