A recent trend in exploration is the tendency toward objectives in remote areas, such as the deeper water environment and in deeper depths given the current conditions of vibrant oil and gas demand. The terms "Deeper" and "Ultra-deep" wells are generally defined as those wells drilled deeper than about 4, 500m and 6, 000m in depth respectively. There are some new exploration opportunities to be reviewed in the US Gulf of Mexico and the Rocky Mountains region as examples of these changing trends. In the US Gulf of Mexico a deep gas exploration in shallow water areas attracts a great deal of players including the "majors". In the intra-mountain basins of the Rocky Mountain region, there are aggressive exploration and development activities underway in the basin-centered gas and other objectives regarded as unconventional resources. A technological break-through is a key issue to enable deeper drilling in all cases. We hope the new waves of technology from North America, mainly the USA, will extend all over the world in the near future. As it is vitally important for us to preserve the earth's resources, it is necessary for us to observe and review the new trends and opportunities for exploration in mature oil and gas producing areas and for development of unconventional gas resources.
Southern central Hokkaido is an important hydrocarbon province and over 100 wells were drilled by METI and private companies in the past 50 years. The rea can be divided into three zones(Zone I, II and III) from the structural styles and stratigraphy, both of which are revealed by the extensive exploration activity. Zone I is characterized by basement horst structures and located in the west of the area. Zone II also has basement horst structures but its shallow sediments are thrust faulted. The thrust faults were originated from the east. East of the area is Zone III and characterized by a thrust-and-fold belt. The structural styles of these zones are in accordance with the tectonic history proposed in studies of surrounding regions. However, the results of palaeostress analysis using core fractures in Zone I and in-situ stress analysis using well logging data in Zone II revealed that the stresses couldn't be fully explained by the tectonic history. This suggests that the nature of structural development processes is far complicated from our ectonic image and we have to continue our effort to construct comprehensive geologic model for exploration of this area.
Northern Honshu, Japan, is a classical example of a trench-arc-back arc basin system, and provides exceptional opportunities for better understanding plate subduction. In this paper, data from deep seismic profiles, petrology and petroleum geology are used to construct a geologic framework for the late Cenozoic development of the back arc area of central northern Honshu. At this time, two rift systems can be recognized in northern Honshu: the Yamato basin rift system along the eastern part of the sea of Japan and the northern Honshu rift system in the western part of northern Honshu island. The eastern part of the Yamato basin rift system is marked by half grabens bound by west-dipping Miocene normal faults, with the thickness of the crust decreasing toward the west due to early Miocene crustal stretching. The northern Honshu rift system is younger, forming in the middle Miocene, and produced the Akita-Yamagata sedimentary basin by a simple shear type of rifting. Basalts were erupted in conjunction with rifting. This produced a high thermal regime, and a shallow brittle/ductile detachment formed along the rift axis with associated thinning of the upper crust. Middle Miocene basin development was mainly controlled by normal faulting along the detachment. Based on our balanced geologic cross-section, the total amount of Miocene extension of the northern Honshu rift system is ca. 40km. Due to subsequent shortening since Pliocene, the low-angle detachment was reactivated as a thrust.
MITI* & METI* wells in the Niigata basin have been providing invaluable data to consider the conditions of future potential hydrocarbon traps in the deeper part of the basin. In the MITI-Niigata Heiya well, the large amount of core data provided rock properties which are required to consider the standard lithostatic pressure of the basin. The results of the MITI-Mishima well gave us useful indicators concerning maturity level of source rocks as well as realization of difficulties to interpret seismic sections of steep folded zone. The MITI-Higashiyama well, which was drilled in the western part of the Higashiyama anticline, showed the existence of faulted block structures below the thick and steep surface limb. Various in-situ stresses were measured in the METI-Niitsu well to take advantage of the rock mechanics techniques in petroleum geology. Using the MITT & METI wells together with the other wells drilled by private oil companies, this paper presented some assumptions in order to discuss the seal potential, the structural analysis and the fracture distributions in the deeper part of the Niigata basin. Attractive seal potential areas were presumed by superimposing the thicker areas of Teradomari Formation and the comparatively thinner distribution areas of agglomerates in Shiiya-Nishiyama Formations. Breakout analysis, fault pattern analysis of regional outcrops as well as Green-Tuff horizons and lost-mud phenomenon in wells indicate that the sub-surface structuring was controlled by the echelon faults which were constructed by sinistral strikeslip movements. The different ways of fracture development in acidic rock and in mafic rock of the deeper part of the basin support the idea that the ductility contrast of rocks, creates the environment in which remnant stress and the distribution of natural fractures are controlled in deep-sited condition. *MITI: The Ministry of International Trade and Industry, **METI: The Ministry of Economy, Trade and Industry
Distribution of overpressures in a sedimentary basin is recognized as an important factor in the evolution of hydrocarbon provinces. Theoretical consideration of the mechanism of overpressuring reveals the importance of pressure seals, which prevent flow of formation fluids, for generating them. From this point of view, pressure differences in pressure regimes can be more logically interpreted as indicating lack of hydraulic connection and no fluid flow between them. This leads us to the concept of pressure compartments characterized by pressure seals in three dimensions. Pressure compartments are found worldwide and their distribution (i.e. compartmentation) affects the fluid movements within a sedimentary basin. Because pressure seals occur in fine-grained, extremely low permeability rocks, they are basically excellent caprock in hydrocarbon traps. However, when the internal fluid pressure in a compartment is close to the fracture pressure of the seal, it is interpreted that the seal had fractured and the trapped hydrocarbons had leaked from the compartment during geologic time. Some methods predicting formation pressure by using wireline log data such as sonic transit time are applicable to evaluate the above overpressure conditions. In this study, a modified method is developed in order to obtain more accurate estimations and applied to a sedimentary basin in Niigata area, Japan. The results of pressure analysis clearly illustrated the state of compartmentation of the basin. They also reveal that seal failure by hydraulic fracturing possibly controls the stratigraphic distribution of hydrocarbon accumulations. Thus, the knowledge and understanding of subsurface pressure regimes and basin compartmentation give us a new point of view to evaluate petroleum systems, especially hydrocarbon migration and entrapment.
Subsurface fluid migration through cracks is controled by the state of stress. It is widely believed that reverse-faulting regime of stress prevails in folding. However, our recent field observations revealed that there are unexpectedly many strike-slip faults in the central part of the Niigata fold belt. A latest stress inversion technique showed that the sigma-3 axis was horizontal and parallel to fold axis at least the observed shallow levels of the crust in the area. This paper firstly reviews the methodological development of stress inversion for the last 30 years. Secondly, the application of the multiple inverse method, one of the latest inverse methods, to the Niigata fold belt is introduced. It is suggested that vertical fluid migration was more rapid than expected in the area due to the opening of pre-existing vertical fractures by the horizontal sigma-3 axis.
Methylphenanthrenes are one of the aromatic compounds commonly found in oils and rock extracts. Although methylphenanthrene-indices(MPI) have been proposed since the 1980's as organic maturity parameters up to high maturity level, quantitative characteristics as indicators and detailed mechanisms of isomerization have not yet been solved. Therefore, we determined a pair of preliminary kinetic parameters for methyl-shift reaction of α-isomer to β-isomer of methylphenanthrenes, based on artificial heating experiments. The experiments were performed on Sarukawa-oils with water, clay-catalyzer and Ar-gas at a series of high temperatures(300, 350, 370, 380, 400 and 450°C) for 0.5 to 40 days. The kinetic parameters obtained are 57.2kcal/mol(239kJ/mol) for apparent activation energy and 2.23×1012s-1 for frequency factor, respectively. Using these kinetic parameters, a computer simulation was performed on MPI3 transformations. The results suggest that MPI3 is sensitive to temperatures between 150 to 230°C at heating rates possible in sedimentary basin, and MPI3 shifts towards the lower vitrinite reflectance stage when heating rates are lower. According to the results of the simulation, Tertiary mudstones from catagenetic zone show that the heating rate in Niigata district was low in the deep portion(4, 200-5, 500m) of the Tojo-TS1 and high in the middle to deeper portion (3, 200-4, 400m) of the METI-Tomikura. In contrast, in Cretaceous mudstones from the metagenetic to metamorphic zone(up to about 300°C) in the Muroto district, some methylphenanthrenes survived and MPI3 decreased with increasing maturity. These results could offer key information to decipher the mechanisms of MPI transformations.
How to deal with and to evaluate fission track data of zircons from the point of view of users were presented. Analytical diagrams including a grain-age histogram, a correlation between spontaneous and induced track counts, a correlation between spontaneous and induced track densities, an age spectra, a radial plot and a uranium-concentration histogram, were explained to read grain data. Grain-age distributions were simulated for a sample with an identical age but various conditions of track counts per grain. Data from about 2, 500 samples previously determined were presented and categorized for assessment of unknown samples. These are important as a guide to evaluate fission track data with relatively large uncertainties. Fission track dating of zircon is performed as follows: 1) assessment of a sample, 2) dating and analysis of grain data, and 3) interpretation of ages. For sample assessment, it is important that grouping of zircons in the sample based on their appearances such as color, morphological features and fission track densities on external surfaces. For interpretation of fission track ages, thermal history analysis with track length distributions is useful. Crosscheck of fission track ages between external and internal surfaces of zircon is also important. For examples, three samples from deep boreholes of the Niigata sedimentary basin were investigated and the meaning of zircon ages was discussed.
Massive Methane hydrate (hereinafter referred to as "MH") is interpreted to be widely distributed in the offshore region of the Japanese islands. Trial calculations indicate the amount of MH is estimated to be enough to supply the domestic natural gas demand for some a hundred years. The Advisory Committee of the METI (Ministry of Economy, Trade and Industry) prepared "Japan's Methane Hydrate Exploitation Program, July 2001" with the aim of producing natural gas from the MH. The action plan "MH Resources Research and Development" (2001-2016) is currently being carried out. The existence of MH in sandy beds in the offshore region of the Japanese islands was confirmed for the first time in 2000 through the METI Exploratory Test Well "Nankai Trough". The well clarified that the net thickness of MH-rich layers is 12 to 14m in total. Several thin MH layers are also present at a depth between 1, 100-1, 140 m deep from the sea surface. Some thicker sand layers (12-14m thick) associated with a submarine fan that are characterized by the high concentration of MH were identified between 1, 140 m and 1, 213 m deep from the sea surface. In the lower interval of these thicker sand layers, each sand bed is less than lm in thickness and was correlated between test wells. Wire line logging with Combinable Magnetic Resonance* (CMR) data showed that the MH saturation was 60 to 80% in the MH containing layer. The bottom depth of high MH concentration deposits coincides with the depth of BSR on the basis of Vertical Seismic profile* (VSP). *: Sclumberger trademark