Effect of Mineralogy, Pore Geometry, and Fluid Type on the Elastic Properties of the Bakken Formation

Abstract

ABSTRACT: The velocity-porosity relationship depends on many factors such as grain shape, sorting, chemical composition, and diagenesis processes. Fluids always occupy the pore space and pore shape plays a significant role in rock stiffness. The Bakken Petroleum System (BPS) in the Williston Basin, North Dakota, consists of the Bakken formation and three folks reservoirs. Bakken formation is divided into three members: Upper Bakken (UB), Lower Bakken (LB), and Middle Bakken (MB), while Three Forks (TF) formation divided into five different units. Although clastic formation’s pores spaces are homogeneous, carbonates present pores heterogeneity which makes their elastic properties estimation complex. This heterogeneity is represented by different pore shapes captured from thin sections, where the aspect ratio (α) defines multiple pore types such as cracks, intergranular, and moldic pores. Furthermore, the pore filling material is a mixture of gas, oil, water, and kerogen in organic-rich shale. This study aims to analyze the effect of mineral composition, pore shape, and fluid type on rock properties of the BPS using various rock physics models. Our results showed that both compressional and shear velocities decreased for all fluid types. We also observed that filling pores with different fluids affect the elastic properties differently, based on their pore geometry, porosity, and lithology. 1. INTRODUCTION The Bakken Petroleum System (BPS) of the Williston Basin is at the forefront of this new oil boom and contains mature source rocks (the Bakken Shales) and proven reservoirs (Middle Bakken and Three Forks). The BPS is composed of three different formations in ascending order: Three Forks, Bakken, and Lodgepole (LeFever & LeFever, 2005; Nordeng et al., 2010). This would include the upper and parts of the middle Three Forks (Bazzell, 2014). The Bakken formation is divided into nine lithofacies where the upper (UB) and lower Bakken (LB) members are formed from the same lithofacies which is a thinly laminated black organic rich mudstone, while the other eight were identified within the middle Bakken (MB) member (Smith & Bustin, 2000). The UB and LB are mainly composed of illite, silt and clay-size quartz, and orthoclase feldspar. The MB is mainly composed of limestone and sandstone (Smith & Bustin, 2000). The middle Bakken member is characterized by a bigger grain size (sandstone and oolite) and more significant calcite percentages. The Three Forks (TF) consists of mudstones, silty to sandy dolostones, and anhydrites (Gutierrez & Sonnenberg, 2013). The TF Formation is formally divided into five units in which Units 4 and 5 are the primary producing units (Baah, 2015).