Integrated Geochemical and Petrophysical Characterization of the Upper and Lower Bakken Mudrock Facies

Abstract

The Bakken Formation of the Williston Basin is one of the largest hydrocarbon producers in the U.S. Although several studies have characterized the middle Bakken, the mudrocks of the upper and lower Bakken are understudied. The upper and lower Bakken are both low-oxygen, high-TOC, visually cryptic mudrocks. Like most mudrock successions, however, these rocks are complex mineralogical assemblages that contain a detailed record of the depositional and oceanographic history of the basin. Integration of geochemical analysis, visual core description, and borehole geophysical logs reveals marked changes both within and between the upper and lower Bakken. These observations offer new insights into changes in sediment flux and sea floor oxygenation during Bakken deposition. The goal of this study is to define variations in the rock attributes of the upper and lower Bakken that may relate to reservoir performance, e.g. organic matter distribution, pore characteristics, and brittleness. These attributes correspond to mineralogy and sediment type, which can be defined at high resolution by X-ray fluorescence major element analysis and X-ray diffraction techniques. XRF data provide a superior record of vertical mineralogical facies stacking. XRF trace elements reveal information about oceanographic oxygenation and circulation, which affects the production and preservation of organic matter. Stable isotopes of nitrogen and carbon will help define changes in nutrient supply, which may relate to significant changes in TOC and mineralogy, and serve as a potential basis for local and regional correlation. The study is based on seven cores from North Dakota and Montana. Four of the cores form an east-west cross section in the southwestern part of the basin, which is an approximate proximal to distal succession; the other three cores offer off-axis support. Preliminary study of thin sections from these cores reveals microfractures and some sedimentary structures such as thin laminations, along with abundant pyrite, particularly in the lower Bakken. Bioturbation is rare in the upper and lower Bakken, but common in the coarser-grained middle Bakken, which shows that it represents a much higher-energy, better-oxygenated environment than the mudrocks. Integrated characterization of the Bakken mudrocks provides important clues about both basin history and refined targeting for hydrocarbon exploration.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015