Glen A. Luebking1, Mark W. Longman2, and W. Joseph Carlisle3
1Savannah Exploration, Inc., Littleton, CO
2Consulting Geologist, Lakewood, CO
3Independent Geologist, Billings, MT
ABSTRACT: Unconformity Related Chert/dolomite Production in the Pennsylvanian Amsden Formation, of Central Montana and the Utilization of the Drill Stem Test in Finding New Hydrocarbon Accumulations
Wolf Springs Field (North and South Pools) and Wolf Springs South 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 MMBO from the Pennsylvanian Amsden Formation. Amsden reservoirs typically produce from fractured and brecciated cherts and dolomites that occur in several laterally persistent and mappable benches. 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 Amsden deposition, or during the pre-Middle Jurassic unconformity 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 have played a major role in shaping their reservoir quality.
The Wolf Springs Fields are unconformity-related stratigraphic, or combination structural and stratigraphic traps. The fields are located on a structural closure on the Wolf Springs Anticline, where porosity and permeability development exhibits a strong northeast - southwest orientation, perpendicular to structural strike of the anticline. The solution collapse breccias pinchout laterally into either dense dolomites, or anhydrite-plugged collapse breccias, whereas the overlying shaly dolomite breccia located below the Piper unconformity and the Jurassic Piper limestone provide an effective top seal.
The development of the solution-collapse breccias were controlled by diagenetic processes associated between the pre-Middle Jurassic unconformity and the underlying chert/dolomite benches in the Amsden.
The Amsden represents a "Low Contrast" reservoir that is not easily identified on electrical logs. These logs have thus far proven to be good qualitative tools (stratigraphic correlation), but poor quantitative tools (pay/reservoir identification). The best exploratory tool identified has been the Drill Stem Test. By careful analysis of wells tested with this tool, it is possible to identify hydrocarbon shows where only mud has been recovered. It is also possible to identify wells with excellent reservoir development that had previously been identified as being "tight".
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado