Characterization of the diagenetic mechanisms controlling dolomite formation in the Middle Bakken member at Elm Coulee field, Montana, A

Abstract

Elm Coulee is an active oil field in the Williston Basin. The Bakken Formation at Elm Coulee field consists of three members: (1) an upper organic-rich black shale, (2) a middle member ranging in presentation from a silty dolostone or limestone to sandstone lithology, and (3) a lower organic-rich black shale. Dolomitization has generated enhanced secondary porosity in the Middle Bakken Member (MBM), and this has facilitated production at Elm Coulee field. The dolomite present in the MBM is largely secondary in nature. When calcite is replaced by dolomite, there is a reduction in interlayer spacing in the mineral’s crystal structure. The subsequent reduction in crystal size forms additional pore space during replacement. The more diagenetically-introduced porosity present, the higher the likelihood that these will become connected effective pores. Connected pores act like microfractures, which increase the permeability of the MBM and lead to increased production. Other Bakken fields across the Williston Basin show lower percentages of dolomite and higher silt content, meaning they tend to produce more so from coarser grained facies/natural fractures. In general, the Bakken, Sanish, and Three Forks reservoirs have very low measured core porosity, which is another reason that productivity is generally expected to be through natural and artificial fracturing. This variance in reservoir rock necessitates a detailed understanding of the diagenetic stages and resulting in the enhanced secondary porosity that made the MBM so productive. To that end, a detailed characterization of the dolomite seen in Elm Coulee field is required.