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

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 (a) 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.