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.

In order for oil to accumulate in economic quantities, it first has to be generated and expelled from source rocks in sufficient quantities. In spite of long term efforts, the mechanism of oil expulsion from the source rocks is not completely understood. For modelling expulsion, the adoption of pressure-driven multiple-phase fluid flow governed by Darcy’s law is widely accepted. However, relative permeabilities for fine-grained source rocks, which is an essential parameter for this model, are very difficult to specify. The conventional reservoir-rock curve is obliged to be used for the modelling. Simplification of the relative permeability model is generally used for one-dimensional basin modelling. The 1-D model also requires substantial optimization of the key parameter, saturation threshold. Laboratory measurement of relative permeabilities for fine-grained rocks is very difficult, therefore, we carefully interpreted the analysed laboratory data on various sandstones to establish a relationship between relative permeability curves and pore geometry parameters, with a view to extrapolate the relationships developed in sandstones to fine-grained source rocks. It was found that the relative permeability curves, or irreducible water saturation are controlled by two factors ; grain size and clay content. Since the total surface area of the pore system becomes greater as the grain size decreases, we consider surface water adsorbed on the grain surface as a part of irreducible water. Since clay contains micro-porosity which cannot be displaced by oil due to high capillary pressure, we regard this to also play a role. Prediction of relative permeability curves for fine-grained rocks by both processes results in the curve with high irreducible water saturation. The new relative permeability curves were tested by both one-dimensional and two-dimensional basin modelling. Test results indicate that the new curves can reproduce expulsion efficiency, locations of accumulations and leaking through cap rock, which is consistent with actual observations.

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The petroleum system is defined by Magoon and Dow (1994) as a natural system which includes all geological and geochemical elements and processes essential to oil and gas accumulation. The concept itself is not new, but they reviewed and introduced a simple method to evaluate the petroleum systems. Basin modeling is one of computer simulation techniques which model generation, migration and accumulation of oil and gas in a sedimentary basin. In this paper, the emphasis is focused on that multi-dimensional basin modeling is one of the best techniques to evaluate the petroleum systems, since it can integrate many processes with quantitative evaluation on the history of sedimentary basins. Explorationists generally have several hypotheses or scenarios by the evaluation of present data. The artificial experiment by basin modeling can be compared with these hypotheses. Since each module forming whole basin modeling package is developed by the