Presentation Type

Poster

Abstract

The Bear Gulch is a dolomitic limestone unit of Upper Mississippian/Lower Pennsylvanian age (~320 Ma) occurring in the Big Snowy Trough, an east/west-trending structural trough north of the Little Belt Mountains in central Montana. The Bear Gulch crops out near Grass Range, Montana, and is an informal unit within the Fergus Group. Hydrocarbon source rocks similar in age and composition to the Bear Gulch limestone have played a central role in the recent success of directional drilling and hydraulic fracturing (e.g., Bakken, Eagleford, etc.). Despite the suspected economic potential and previously-documented paleontological wealth of the Bear Gulch, no recent sedimentary analyses of outcropping Bear Gulch or its subsurface equivalents in the Big Snowy Trough have been published. The results of this study provide the basis for assessing the source rock potential of the Bear Gulch and understanding its stratigraphic affinity to other mapped units within the Big Snowy Trough. Six weeks were spent in the field collecting and cataloging rock samples and measuring laminated sections of the Bear Gulch (July-August, 2013). This was followed by laboratory analysis of the samples. Thirteen of the 75 samples collected were made into thin sections and analyzed with a petrographic microscope. Mineralogical composition, carbon content, and sedimentary fabric data were determined through X-ray diffraction, elemental analysis, and SEM studies. Using these combined data sets, sedimentary facies within the stratigraphic measured section of outcrop were defined. This information was compared to gamma ray logs from nearby oil wells and a north/south-trending cross-section across the Big Snowy Trough was constructed in order to explore the lateral variations within Bear Gulch time-equivalent strata across this structure.

Category

Physical Sciences

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Apr 11th, 11:00 AM Apr 11th, 12:00 PM

Stratigraphic and Sedimentologic Analysis of the Bear Gulch Limestone (Mississippian-Pennsylvanian) near Grass Range, Montana

The Bear Gulch is a dolomitic limestone unit of Upper Mississippian/Lower Pennsylvanian age (~320 Ma) occurring in the Big Snowy Trough, an east/west-trending structural trough north of the Little Belt Mountains in central Montana. The Bear Gulch crops out near Grass Range, Montana, and is an informal unit within the Fergus Group. Hydrocarbon source rocks similar in age and composition to the Bear Gulch limestone have played a central role in the recent success of directional drilling and hydraulic fracturing (e.g., Bakken, Eagleford, etc.). Despite the suspected economic potential and previously-documented paleontological wealth of the Bear Gulch, no recent sedimentary analyses of outcropping Bear Gulch or its subsurface equivalents in the Big Snowy Trough have been published. The results of this study provide the basis for assessing the source rock potential of the Bear Gulch and understanding its stratigraphic affinity to other mapped units within the Big Snowy Trough. Six weeks were spent in the field collecting and cataloging rock samples and measuring laminated sections of the Bear Gulch (July-August, 2013). This was followed by laboratory analysis of the samples. Thirteen of the 75 samples collected were made into thin sections and analyzed with a petrographic microscope. Mineralogical composition, carbon content, and sedimentary fabric data were determined through X-ray diffraction, elemental analysis, and SEM studies. Using these combined data sets, sedimentary facies within the stratigraphic measured section of outcrop were defined. This information was compared to gamma ray logs from nearby oil wells and a north/south-trending cross-section across the Big Snowy Trough was constructed in order to explore the lateral variations within Bear Gulch time-equivalent strata across this structure.