In order to assess the hydrocarbon potential of the Middle Miocene (10-12Ma) Onnagawa diatomaceous sediments, outcrop samples from seven sites in the Aomori and Akita Prefectures were examined for the distribution of their major and minor elements. Three factors optimal for the good source rock generation, i. e., supply of biogenous materials, small dilution effect by terrigenous detrius, and anoxic bottom water condition, were extracted by multivariate statistical analyses. Spatial distributions of these factors indicate source facies developed in the central deep part of the basin, which was kept stagnant by restricted circulation of sea water. During 10 to 9Ma, paleoceanic condition changed drastically due to the initiation of localized upwelling.