Problems on Primary Migration of Oil: A Review

Abstract

Primary hydrocarbon migration involves major two kinds of problems, namely, driving forces as the cause of migration and the forms in which they migrate. Although there is not so much divergent in the opinion of the former problems, the mechanisms concerning with the latter problems are still debated hotly by many petroleum geologists and geochemists. The essential debation rises between advocates of water-controlled migration which involves movement of an aqueous medium and of separate phase transport that operate independently of water movement. In this paper, various primary migration mechanisms are reviewed from the above viewpoint, and then the special attention is drawn to certain aspects of primary migration in Japanese Tertiary oil fields and the two possible mechanisms for the migration are proposed. One possible mechanism, which involves iiew concepts, consists of oil phase transport at a relatively shallow depth. Based on the re-consideration of the "structured water theory" by Dickey (1975), the amount of "free water" can become extremely small at a relatively shallow depth-about 40 per cent porosity level depth, that is, about 800-1300m depth in case of the Japanese Oil Tertiary, and at that depth, sediments still contain hydrocarbons even immature and poor quantity, and they often come up considerable amounts as good as "source rock" ones at peak generation stage. Namely, in this case, it is suggested that the immature oils formed at shallower depth than the so-called "principal zone of petroleum formation" allow the possibility of primary migration by oil-phase transport. The other possible mechanism also belongs to the category of oil-phase migration, but the type of hydrocarbons available to migrate is different from the above ones. In this case available hydrocarbons are ones generated from matured kerogen of organic-rich source rocks. In Japanese oil fields, most of the rocks belonging to the Onnagawa and the lower part of the Funakawa formations and their equivalents are considered to be source rocks, based on the organic richness, oil-source rock correlation, and the maturity and the type of kerogen. Consequently, it is suggested that most of the commercial oils in Japan were derived from those source rocks and that kerogen-laminae observed in those rocks took an important role as a pathway of separate phase oil.