Organic petrology of the Upper Ordovician Red River kukersite tight oil and gas play, Williston Basin, North Dakota, United States

This new organic petrographic study supplements previously published geochemical data as part of an evaluation of a conceptual Ordovician kukersite tight oil and gas play in the Williston Basin, North Dakota. The kukersite interval of the lower Red River Formation is an organic-rich (average 3.8 wt. % total organic carbon) dolomitic limestone that has been documented as the source rock for hydrocarbons produced from porous dolomite zones in the overlying upper Red River Formation in conventional traps. Basin modeling studies suggest that only a small fraction of the generated petroleum has been produced from the conventional fields, and that a significant resource may remain trapped within low-permeability carbonates associated with the kukersite source rock. As documented on immature samples, the original kerogen of the kukersite is predominately oil-prone algal and amorphous organic matter. Secondary organic matter (solid bitumen) is rare in thermally immature samples (solid bitumen reflectance [BRo] <0.30%) and accounts for 100% of the organic matter by 0.63% BRo, and completely fills the mineral interparticle pore space in the studied samples. Only a few nanopores were observed in the organic matter by scanning electron microscope examination, including the gas mature samples (up to 3.50% BRo). The organic matter in the thermally mature samples com-monly exhibited a volatile response when probed by the electron beam, indicating that some of the organic matter pores may be obscured by altered residual oil. This study may provide useful insights into the study of other Ordovician kukersites for potential tight oil and gas resources.