Geohistory of the Akita and Niigata Oil Fields of Northern Japan - Application of Microfossils to Petroleum Geology

Abstract

Micropaleontology has historically been used in the oil industry primarily for determination of age and correlation of exploration wells. Because planktonic microfossils generally have a worldwide distribution, they are useful for age determination. Benthic microfossils, which are influenced by environmental factors, can provide important paleoenvironmental data. Based on the occurrence of calcareous nannofossil, Helicosphaera ampliaperta, the age of the oldest Paleo-Japan Sea can be correlated to the late early Miocene NN4 Zone (between 15.6 and 18.4 million years ago). The study of calcareous nannofossils also helps to clarify some important characteristics of reservoir rocks. The basaltic volcanic reservoir rocks found in the Akita oil fields are younger than those of the Minami Nagaoka Gas field, and it is a result of the opening of the Japan Sea by its separation from mainland China and by volcanic activities. The geologic structure of the Akita oil fields was formed approximately at the Pliocene/Pleistocene boundary (1.72 million years ago), and is related to the activity on the Kitayuri thrust faults. The source rocks, which are distributed in the western part of the Akita oil fields, were situated in the mature zone. As a result, the oil from the mature zone had migrated to the reservoir rocks in the oil fields. Benthic foraminifera indicated that anoxic environment, which existed in the Miocene Paleo-Japan Sea, was conducive to the accumulation of the petroleum source rocks. Such elements as topography, currents, and anoxic environment of the sea-floor in the area near Nagaoka were suitable for the production of such organic materials as phytoplankton and for their deposition without decomposition.