GEOCHEMICAL AND PETROPHYSICAL CHARACTERIZATION OF THE BAKKEN SHALE IN MOUNTRAIL COUNTY, NORTH DAKOTA

Bakken Formation is one of the most prolific tight plays in the Williston Basin. However, due to its heterogeneity and very low porosity and permeability, the Bakken recovery factors remain small. The knowledge of pore types and size distribution are crucial in understanding flow dynamic pathway (fluid storage, flow and transport). This study aims to characterize rock mineralogy and its effect on pore types and size in the Bakken Shale.

This study utilized two wells in Mountrail County, North Dakota. Samples from the Upper and Lower Bakken Shale were prepared using ion milled technique for better imaging on scanning electron microscope (SEM), bulk mineralogy was acquired by x-ray diffraction (XRD) method. This study also involved total organic carbon (TOC) content quantification, nuclear magnetic resonance (NMR) spectroscopy on transverse time (T₂) and helium porosimetry techniques.

Preliminary results show that three types of pores exist within the Bakken Shale; mineral matrix (interparticle and intraparticle), organic matter and fracture pores. The identified pore types were controlled by the properties of individual mineral within the rock sample. The dominant minerals in the study wells are quartz (54%, 59%), clay (30.5%, 19.8%) and pyrite (3.0%, 9.0%), while TOC content of the study wells varied between 12.0 and 15.75 (wt%). Sample with higher content of pyrite has more interparticle pores compared to the sample with higher clay content. This is because brittle minerals (for example, pyrite) can help preserve interparticle pores than clay minerals. Sample with higher TOC content, having a shallower depth, has more organic porosity as pores are preserved under shallow burial condition. Hence, there is positive relationship between TOC and porosity, with NMR T₂ result for the study wells dominated by micropores.