An integrated approach to understanding the petroleum system of a frontier deep-water area, offshore Japan

The Naoetsu Basin is one of several oil- and gas-producing provinces in Japan where thick Neogene strata are deposited, and oil and gas are produced from both onshore and offshore shelf areas. It is believed that the Naoetsu Basin extends into deep-water areas, but exploration activities were limited until 2000, except for regional 2D seismic surveys. After acquisition of a 3D seismic survey in 2001, the first two wells were drilled in 2004 in the deep-water area of the Naoetsu Basin. One well encountered an oil zone.

Multi-dimensional basin modelling was conducted to understand the petroleum system in the deep-water area of the Naoetsu Basin before and after the wells were drilled. The accuracy of basin modelling depends on the accuracy of the input data as well as the calibration process. However, even after the drilling campaign, only two wells were available in the deep-water area of the Naoetsu Basin. Therefore, the integration of various kinds of data, information and different techniques, such as 3D seismic, sedimentological and geochemical data, was carried out in this study. Development of sandstone networks, as well as the presence of major and minor faults, was identified on 3D seismic data. Oil and gas migration were constrained by geochemical data, such as carbon isotope on gases collected at the sea bottom and fluid inclusion chemistry.

Understanding of the petroleum system was increased significantly by this kind of integration, although the deep-water area of the Naoetsu Basin still remains a frontier area for oil and gas exploration. It was found that the petroleum system active in the deep-water area of the Naoetsu Basin is very effective. Even though maturation of the source rock has occurred only since the Pliocene, oil and gas generated and migrated very rapidly, first horizontally along sandstone networks and then vertically through faults reaching a level just below the sea bottom, with the result that the hydrocarbon trap has leaked.