Characterization of geochemical properties and microstructures of the Bakken Shale in North Dakota

Research on unconventional shale reservoirs has increased dramatically due to the decline of production from conventional reserves. Geochemical properties and pore microstructures are known to be important factors that affect the storage capacity and nano-mechanical properties of self-sourced organic- rich shales. In this study, eleven shale samples were collected from the Upper and Lower Members of the Bakken Formation for the analysis of mineralogy, geochemical properties, and pore structure. Bulk pyrolysis analysis was conducted using the default method and two modified methods, namely the reservoir and the shale reservoir methods. Although all three methods showed the Bakken samples to be organic-rich and to have considerable remaining hydrocarbon generating potential, it was the shale reservoir method that gave the highest hydrocarbons yield because it captured most of the lighter thermo-vaporizable hydrocarbons. Thus, the shale method is considered to be more appropriate for the geochemical analysis of the Bakken samples. This method also showed that most of the remaining potential is due to the cracking of heavy hydrocarbons, NSO compounds (Resins + Asphaltenes) and kerogen. The organic matter in the samples is mixed II/III type (oil and gas-prone), is thermally mature, and plots at the peak of the oil window. The VRo-eq values, based on solid bitumen Ro measurements and conversion, ranged from 0.85% to 0.98%. The pore structures obtained from the image analysis method showed that total surface porosity of the samples ranged from 3.89% to 11.56% and that organic porosity is not the main contributor of total porosity for the samples analyzed. The pore structures of the samples are heterogeneous due to differences in lacunarity values. Results of the impact of mineralogical composition on pore structures demonstrate that clay minerals and feldspar have a positive influence on porosity while quartz, pyrite, and that TOC has a negative impact.