--> Mixed-Siliciclastic-Carbonate System of Facies “D” in the Middle Bakken Formation, Williston Basin

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Mixed-Siliciclastic-Carbonate System of Facies “D” in the Middle Bakken Formation, Williston Basin

Abstract

The middle Bakken reservoir interval consists of six distinct facies, namely facies A-F. Facies D is the coarsest-grained, highest-energy facies in the entire Bakken Formation. Porosity varies between 1–8%. Lithologically, Facies D varies from very fine- to medium-grained calcite-cemented, quartz-rich sandstone, to ooid-rich grainstone with abundant fossil fragments. Cycles of increasing calcite and decreasing quartz and dolomite, in conjunction with massive zones alternating with zones of high-angle cross-stratification, gives Facies D a banded appearance. The Facies D is discontinuous within the Williston Basin and reaches a maximum thickness of around 5 m in North Dakota. All these features of Facies D highlight the complexity of this mixed siliciclastic-carbonate depositional setting. This study is focused on developing a depositional model for the mixed siliciclastic-carbonate system of Facies D, which will aid in improved middle Bakken reservoir characterization and proper planning of well completions designs. Detailed core descriptions, petrographic studies, log correlations, and subsurface mapping suggest that Facies D was deposited in a middle to lower shoreface environment in a homoclinal ramp setting. Presence of oolitic grainstones and a restricted faunal assemblage indicate that it was deposited in a saline basin under arid conditions. The principal carbonate factory existed in the southern part of the basin, while the siliciclastic source was from the north. Thickened intervals of Facies D correspond to building up of carbonate shoals that are cross cut by intervening channels, similar to the ebb delta ooid shoals of eastern Abu Dhabi. Tidal currents and storm-generated flows within these channels distributed both siliciclastic and carbonate sediments across the basin. Seasonal variations in wind direction, as seen in the modern day Sunda Shelf in South China Sea, influenced current intensity within the channels. This resulted in cyclic deposition of alternating bands of oolitic grainstone and quartz-rich sandstone. Fast Fourier Transform analysis is applied to understand this complex interaction between tidal influence and seasonal variations in a mixed siliciclastic-carbonate depositional setting.