Shibata and Iwafune district of northeastern Niigata prefecture is one of the main Japanese oil / gas fields. However, the source rock distribution and hydrocarbon generation potential are not necessarily known, because this field is located at marginal area of the Niigata oil / gas fields. We performed geological route-map survey and collected 21 argillaceous black color rock samples from the Middle Miocene Nanatani and Teradomari Formations deposited in the northeastern Niitaga basins. CHNS elemental and Rock-Eval analyses were performed on these black mudstones. The Teradomari Formation showed high potential of hydrocarbon generation (TOC = 0.65-3.4%, S2 = 0.2-5.8 mgHC / g) and planktonic origin with some inputs of land plant fragments. On the other hand, the Nanatani Formation showed medium to low potential of hydrocarbon generation (TOC = 0.3-3.2%, S2 = 0.3-1.4 mgHC / g) and planktonic origin with abundant higher plants. HI values have a correlation with TOC in the Teradomari Formation. C/S ratio showed that the Teradomari Formation deposited in anoxic to oxygen poor environment and the Nanatani Formation deposited in freshwater to suboxic seawater. All samples except for the lower most one mudstone in Nanatani Formation showed immature zone for oil / gas generation.