The purpose of this study is to evaluate possible petroleum source rocks in the Neogene Tsugaru basin. Evaluation is based on three criteria: organic richness, organic maturity, and organic type. One hundred forty-one samples of mudstones from seven transects were analyzed. Samples from the middle Miocene Kodomari Formation and younger sections generally contain more than 0.5% total organic carbon and amorphous organic matter, but are thermally immature. Samples of the lower Miocene Odose and Isomatsu formations obtain maturities equivalent to the threshold of intense oil generation, but do not contain sufficient type or quality of organic matter to be considered an oil source.

Although this study failed to identify an effective source rock, it does suggest that a possible effective source may exist off the west coast of Honshu and in the eastern part of the Tsugaru Peninsula. In these areas, organic maturation of the possibly organic-rich Neogene sediments may have proceeded further than in regions investigated in this study as a consequence of a Neogene sedimentary package that is more than 2000 m thick.