Effect of mineralogy on elastic properties of the Bakken formation: A lab scale rock physics modelling

Abstract

The Bakken formation in the Williston Basin, is one of the largest tight oil reserves in the U.S.A. and is currently the second largest oil producer in the nation. The Bakken formation is divided into the upper and lower shale members which comprise the source rocks, while the middle Bakken, which is a fractured silicious-carbonate formation, is the main producer. The Bakken has a complex mineralogy, composed of mainly quartz and clay in the two shale members and quartz, calcite, dolomite, and anhydrite in the middle member, along with several other minerals with lower volume fractions. Past studies have reported the importance of mineral composition in development, completion, and production from shales.In this study, the effect of mineral composition of the three Bakken members on rock elastic and velocity properties are investigated. Rock physics models were developed at macro (core) scales from data in Dunn and Mountrail Counties. The minerals' volume fractions from core XRD data were used for estimation of rock stiffness and velocities. The calculations were performed for the upper and lower Bakken (UB, LB) shales as well as the clastic and carbonate intervals of the middle Bakken (MB). Hill averaging was used to mix the non-clay members. Clays were added using both Kuster-Toksoz (K-T) model and differential effective medium (DEM) theory. The effect of fluid phase (assuming both homogeneous and patchy fluids) was applied using K-T (Kuster and Toksöz, 1974) and Gassmann equations (Gassmann, 1951) for frequency-dependent velocity estimation. For shales, Backus averaging (Backus, 1962) captured the upper and lower bounds of the data, whereas the K-T model developed for parallel and perpendicular to the laminations, allowed accurate curve fits to the lab data by changing the pore's aspect ratios to capture the pores heterogeneity exhibited in Bakken. The results showed that, In overall, the UB and LB shales, due to the presence of clay minerals, show lower properties than the MB and composed mainly of microfracures (pores). The MB is shown to have more intraparticle pores with stiffer properties. The estimated stiffness and velocity properties are valuable input data for the development, completion and production planning from future wells.