Petrographic and Geochemical Characterization of the Upper Bakken Shale, Williston Basin

Abstract

Various studies have focused on the source rock potential and organic geochemistry of the organic-rich Bakken shales. However, only few attempts have been made to understand the sedimentary processes and paleocenographic conditions prevailing during their deposition. This paper is focused on integrating petrographic study with organic and inorganic geochemical analysis for a comprehensive characterization of the Upper Bakken Shale (UBS) in the Williston Basin. Samples from the UBS were collected on 6” intervals from 6 cores located at both basin margin and basin depocenter area. Polished ultra-thin-sections were prepared from parts of the collected samples. These petrographic thin sections were used to describe the macro-to-micro scale rock-fabric, texture, biotic content and mineralogy. This helps in defining the mm-scale heterogeneity within the UBS and also in understanding the sedimentary processes associated with the deposition of these organic-rich shales. Furthermore, source rock analysis and the X-ray diffraction were also collected on the above mentioned samples. Integration of the total organic content (TOC) and the mineralogy data with the petrographic observations were used to define the micro-facies within the UBS. Selected samples were also studied under scanning electron microscopy (Field Emission SEM & Focused Ion Beam SEM) and backscattered electron microscopy to effectively define the fabric and diagenetic features of the UBS. In this study, inorganic geochemical methods, such as induced couple plasma-mass spectrometry (ICP-MS), portable x-ray fluorescence (XRF) data and stable isotope (C and S) were also collected from the above mentioned samples. These inorganic geochemical analytical results were used as proxies to determine the influence of detrital sediment influx, paleoproductivity and paleoceanographic conditions in controlling organic content in the UBS. Finally, the petrographic observations were integrated with the geochemistry results for determining the depositional environment of the UBS.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017