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Mixed siliciclastic-carbonate system of the middle member of the Bakken Formation, Williston Basin, North Dakota
Listiono, Geraldus Adi
Listiono, Geraldus Adi
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2016
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2016-11-26
Abstract
The Middle Member of the Bakken Formation (MM-BF) is a mixed siliciclastic-carbonate unit deposited in the shallow, epeiric shelf setting of the Williston Basin during the Famennian-Tournaisian Transgression. The MM-BF in North Dakota consists of six distinct units (MB-A to F) but primarily can be divided into: (1) lower variably calcareous dolomitic siltstone, (2) middle calcareous grainstone-sandstone, (3) and upper argillaceous dolomitic siltstone. Stratigraphic analysis of subsurface core reveals that the lower siltstone unit (MB-A, B, & C) was deposited in a tide-dominated, wave-influenced environment as part of a highstand to falling stage systems tract. The middle MM-BF sandstone (MB-D) is a lowstand systems tract (LST) deposited in tide-dominated, wave-influenced environments. The upper MM-BF siltstone (MB-E & F) contains tide-dominated environment in a transgressive systems tract. Cycle analysis of alternating siliciclastic-carbonate intervals reveals that the interplay of tides, storms, and seasonal changes in oceanic surface current played an important role in the mixing of carbonate and siliciclastic constituents within the MM-BF. The calcareous sandstone-grainstone of MB-D presents unique opportunity to investigate mechanisms of sediment delivery and siliciclastic-carbonate mixing in an epeiric shelf/low-angle ramp setting. The grain size, mineralogy, and zircon trends indicate a siliciclastic delivery system that originated from a northern ephemeral drainage system. Reworked carbonate grains were sourced from intra-basinal paleo-highs and subjected to significant storm and wave reworking. The discontinuous nature of MB-D is due to both depositional and erosional thinning. Thickness variations of this facies are similar to modern oolitic environments where shoal and bars are cut and incised by tidal channels. The base of the MB-D is a 3rd order sequence boundary (Ss5) and the top of the unit is a transgressive surface of erosion event (Ss6). Variations in thickness are related to these two services. With the presence of erosional thinning, MB-D may not represent a complete depositional environment and is a depositional or accommodational remnant. MB-D thick trends in the north-northwestern part of the study area are preserved offshore bars that developed on topographic or bathymetric highs. In the northeast- and central part of the study area, MB-D thicker sections are bats that filled bathymetric lows or are incised channels. This study also presented a comprehensive initial look at the basic reservoir properties of the MM-BF in North Dakota. Porosity-permeability-grain density cross plots along with UV fluorescence reveal that early burial calcite cement prevented dolomitization, and this cement has detrimental effects on the reservoir properties of MM-BF. Core mechanical analysis was conducted using a micro-rebound hammer, Proceq Bambino, to acquire the Leeb hardness value of the rock. Hardness refers to the measure of resistance to a permanent deformation and can lead to the evaluation of fine-scaled heterogeneity and anisotropy of the rock. The micro-Schmidt hammer data show small-scale heterogeneities within the MM-BF at the bed to lamina-scale.
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