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3-D Multi-Scale Lithofacies Models of the Upper and Lower Bakken Shale Members of the Williston Basin in North Dakota, USA

Abstract

Integrated 3D shale lithofacies modeling is important to visualize distribution pattern of different lithofacies, interpret depositional environments, and understand hydrocarbon potential of mudstone formations. Shale lithofacies modeling is a complex multi-class and multi-scale problem. The main objective of this study is to identify different shale lithofacies in the Bakken Formation of the Williston basin in North Dakota, to better understand their depositional controls on mineralogy and organic matter content, plus provide a quantitative geologic framework at core, well, and regional scales. Shale lithofacies is defined using quantitative mineralogy, Total Organic Carbon, and various petrophysical properties derived from core data (such as X-ray Diffraction, X-ray Fluorescence, pyrolysis, and secondary X-ray emission spectroscopy), advanced geochemical spectroscopy logs, and conventional logs. After core-based observation and identification of lithofacies, machine learning algorithm, such as Support Vector Machine (SVM), is used to recognize the pattern of different shale lithofacies, associated with basic petrophysical parameters from conventional well log suites from ~1,200 wells. A set of ten petrophysical parameters are used as input to the SVM algorithm, which can directly output shale lithofacies with high accuracy in wells, with or without core data and advanced geochemical logs. Geostatistical algorithm, such as Sequential Indicator Simulation, is used to generate 3D stochastic geocellular lithofacies models (~1.72 million cells) of the upper and lower Bakken shale members, spanning over an area of ~13,000 sq. miles. The results show that the Bakken shale members are vertically and laterally heterogeneous at core, well, and regional scales, but can be classified into five different lithofacies. Organic-rich shale lithofacies outweigh the proportion of organic-lean shale lithofacies. It appears several factors, such as source of minerals, paleo-redox conditions, organic matter productivity, and preservation etc. controlled the Bakken shale lithofacies distribution pattern. Silica in the Organic Siliceous Shale (OSS) lithofacies is related to both biogenic and eolian action. Organic-rich shale lithofacies are found to be deposited mostly in euxinic (high sulfur) and anoxic conditions, whereas organic-lean shale lithofacies are considered to be deposited in dysoxic-redox condition. OSS lithofacies shows positive relation to hydrocarbon production.