Diagenesis of the Sappington Formation in Southwest Montana: Implications for Reservoir Quality in the Middle Member of the Bakken Formation

Abstract

The middle member of the Devonian/Mississippian Bakken Formation is a primary target interval for hydrocarbon development in the Williston Basin, in part because of its proximity to source and seal intervals. Production from the middle member would not be possible without the formation of enhanced secondary porosity and the formation of microfractures during different diagenetic stages. The Sappington Formation in southwestern Montana is contemporaneous to the Bakken Formation and exhibits facies similar to the Bakken Formation facies in the Williston Basin, with a lower and upper organic rich mudstone, and a middle member that contains dolomitic siltstone to very-fine sandstone. Here we present results from a detailed diagenetic study of the middle member of the Sappington Formation along a 17 km outcrop transect in the Bridger Range in southwest Montana to distinguish the diagenetic changes associated with lateral lithologic heterogeneities observed on this reservoir scale. The diagenetic history of the Sappington Formation begins with mechanical compaction followed by several phases of mineralization. Amongst the mineral cements observed in the Sappington Formation are rhombic ferroan dolospar grains with non-ferroan rims, and calcite often replaced by dolomite. Other authigenic phases include quartz overgrowth, and euhedral pyrite crystals, and a series of clay minerals including illite and chlorite. In particular dolomitization and dedolomitization resulted in the formation of enhanced secondary porosity. In sections with high dolomite content, porosity and permeability are higher than sections with lower dolomite content. Microfractures are also present in the Sappington Fromation in both, the shale intervals and the middle Sappington interval. The formation of microfractures is likely related to tectonic forcing, pressure release from dewatering during an earlier diagenetic phase, or due to high lithostatic pressure during the hydrocarbon expulsion process. The results of this study indicate a complex diagenetic history for the Sappington Formation in southwest Montana. Understanding the distribution of the facies and the diagenetic stages that have occurred within the Sappington Formation can help to determine the reservoir heterogeneity along a single well path and help to find new drilling targets in the middle Bakken formation in the Williston Basin.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015