Diagenesis of the Sappington Formation in SW Montana: Implications for Reservoir Quality in the Time-Equivalent Bakken Formation

Abstract

Understanding relationships among diagenesis, facies, and stratigraphic architectures is critical for predicting reservoir quality and performance in hybrid plays. However, the direct analysis of these relationships in hydrocarbon reservoirs is often limited. This limitation is especially problematic in formations with limited thickness and contrast in acoustic impedance, such as the reservoirs in the middle Bakken Formation. To study the dependence of reservoir architecture and facies on diagenesis, we examined outcrop samples from the Devonian-Mississippian Sappington Formation in southwest Montana. The Sappington Formation is contemporaneous to the Bakken Formation of the Williston Basin, contains similar facies, and is well exposed along the ridge crest of the Bridger Range, providing information about the architecture and facies distribution that can't be easily acquired from the subsurface in the Williston Basin. The overall diagenetic sequence of the Sappington Formation is characterized by a series of cement forming stages dominated by euhedral, zoned dolomite rhombs, with less common quartz overgrowths, and calcite cementation. Other diagenetic phases include clay rimming, pyrite formation, and carbonate dissolution. Of particular importance is the formation of dolomite. Many euhedral and subhedral dolomite crystals in Sappington facies show a complex zonation between ferroan and non-ferroan dolomite. In the deepest sampled portion of the Sappington basin, dolomite displays up to 10 subsequent rimming stages defined by variations in the ratio of Mg to Fe. In contrast, stratigraphic sections closer to the southern landward pinch out of the Sappington Formation, contain dolomite rhombs with significantly fewer zones. Subsequent dissolution of ferroan dolomite resulted in secondary dissolution porosity across the field area. The results from this study will help to better understand the mechanisms and timing of dolomite formation, the distribution of different dolomite phases within a sequence stratigraphic framework, and their control on secondary porosity formation in the middle member of the Sappington Formation. Using these results will facilitate a more predictive model of better reservoir quality intervals within the Bakken Formation in the Williston Basin.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016