Visual and analytical comparisons of upper Pronghorn “shale” cores from a west-to-east transect, McKenzie and McLean Counties, North Dakota
Core acquisition in the Williston Basin has not only provided detailed analytical data on reservoir and source rocks, but has also helped to quantify and correlate complex lithologies that make up the Upper Devonian lower Bakken and Pronghorn members of the Bakken Formation. Given the variety of nomenclature applied to the Pronghorn, the authors have subdivided the formation into an upper and lower interval where the upper interval consists of black “shale” similar to the lower Bakken and the lower consists of mixed limestone, siltstone, and sandstone. A variety of analytical data and stratigraphic relationships of the upper Pronghorn from six cores along a 52-mile-long west-to-east transect provide a rich suite of information for regional comparisons. The upper Pronghorn is classified as a silty organic mudstone with rare thin bioturbated limestone beds near the base. The upper Pronghorn varies in thickness from 0 to 31 feet along the transect. Visually, the differences between the lower Bakken “shale” and upper Pronghorn are subtle. Upon close inspection of the core you can discern visual differences and the contact between the two that is not readily apparent from core photos. Analytical and visual data from this transect clearly show the boundary between the base of the lower Bakken and the upper Pronghorn interval is defined by: 1) lower TOC, 2) lower silicon, 3) more bioturbation 4) coarser quartz grains (vf sand) and 5) more common silt laminae in the upper Pronghorn. The boundary is also easily recognized on wireline logs. The upper Pronghorn is thickest in the westernmost cores and is absent in the two easternmost cores of the transect. Compositionally, the upper Pronghorn mudstone facies in this transect is composed of quartz (18–25% eolian and biogenic quartz), dolomite (12–31%, detrital and authigenic), illite/mica (17–27% increasing westward), K-spar (9–16%), albite (1–5%), calcite (3%, mostly biogenic), pyrite (1–3%, authigenic), minor mixed-layer illite/smectite, and chlorite (2–7%). The upper Pronghorn is part of a transgressive sequence that reaches a maximum in the lower Bakken “shale.” The upper Pronghorn is not as laterally continuous as the lower Bakken “shale” and varies significantly in thickness across the basin. Although the upper Pronghorn is not as organic-rich as the lower Bakken “shale,” it is a source rock and its presence is an important part of the Bakken petroleum system.
AAPG Datapages/Search and Discovery Article #90193 © 2014 Rocky Mountain Section AAPG Annual Meeting, Denver, Colorado, July 20-22, 2014