Origin and maturity of natural gases are interpreted by the molecular and isotope data. Based on the carbon and helium isotope data, no indication of abiogenic gases has been found in commercial natural gases. Biogenic gases are classified into microbial and thermogenic gases by molecular and isotope compositions. However, secondary alteration (mixing, migration, microbial degradation, etc.) sometimes largely affects the compositions of natural gases. Microbial degradation especially alters both molecular and isotopic signatures for reservoired gases. Thus, although the prime order of isotopic and molecular fractionation in gases is due to genetic phenomena, secondary effects must be taken into account by putting together various pieces of information. If the secondary alteration is small, carbon isotope compositions of thermogenic hydrocarbons are largely controlled by the maturity. Berner and Faber (1996) developed isotope/maturity models for methane, ethane and propane based on open-system dry pyrolysis experiments for Type II and III kerogens, and instantaneous kinetic models. The model for Type II kerogen was successfully applied to natural gases in Northeast Japan. The estimated maturity of gases in the Niigata Basin is generally higher than the Akita-Yamagata Basin. Besides the maturity estimation, the application of the model enables detection of microbial degradation, and mixing between microbial and thermogenic gases.
The Pattani Trough is an elongate N-S trending Tertiary rift in the Gulf of Thailand. In the Pattani Trough, large quantities of gas and condensate are charged in the central trough and lesser oil is accumulated in the frank areas and northern trough. The stratigraphic units of the trough which have a total of about 10,000m of the Tertiary sediments, were initially developed from the Oligocene to present time. The initial syn-rift sediments of Oligocene consist of lacustrine deposits, and thick early to middle Miocene sediments are composed of fluvial and partially deltaic/marginal/marine deposits. The gases in the Pattani Trough are derived from the thermal cracking of type III kerogens and, the cracking of pre-existing oils. The oils are waxy and heavy, and interpreted to be lacustrine algal origin. Two petroleum systems with two different source facies, the Oligocene lacustrine algal facies and the Miocene fluvial coaly facies, are operating in the trough. Two-dimensional basin modeling was conducted to examine generation, migration and accumulation of hydrocarbons across the trough. Oil generation from Oligocene source rocks initiated in the early Miocene, and gas generation mainly occurred in the middle to late Miocene in the central trough. CO2 contents generally increase with depth from a few percent to around 25% in the trough. Occasionally some wells encounter gas reservoirs containing CO2 above 60% and as high as 91%. Anomalously high CO2 in the Platong gas field was found, and its distribution was identified to be large plume-like shape related with a nose structure. Based on carbon isotopic analysis, anomalously high CO2 has inorganic origins which include magmatic activity and decomposition of carbonates. Basement lithologies and high thermal activities in this region suggested that anomalously high CO2 was derived from thermal decomposition or hydrothermal dissolution of carbonate basements.
Recently source rock potentials on coals in Japan have attracted interests by the discoveries of Yufutsu oil/gas field and a new gas-field at offshore in Sanriku District, in which coals were assumed as their source rocks. The gas generation mechanism and gas potentials of coal as a source rocks in the petroleum system is discussed in this paper. On the van Krevelen diagram, several Japanese coals are plotted in the area between type II and type III organic matter. Petrographical and ultimate analyses on coals revealed that the high hydrogen contents of Japanese coals are caused by abundant degradinite, a maceral of vitrinite group. The degradinite is observed such mixture as very small fragments of the virinite (type III organic matter) and exnite (type II organic matter) under the microscope. The oil generation is assumed to begin at Ro=0.5, but only dehydration and decarboxylation continue until Ro =1.0, and the demethylation occurs later along the curve on van Krevelen diagram, resulting in the decrease of H/C atomic ratio. It is difficult for the degradinite in the oil window (Ro= 0.5 to 1.0) to generate hydrocarbons because of its difficulty of primary migration from coals. The compositions of gases from Japanese coal mines showed that CH4 content in hydrocarbon was more than 99% with a few exceptions. It suggests that little hydrocarbons are generated and migrated from coals in the oil window. Coals and terrestrial organic matters have been treated as gas source rocks only, and acknowledged to generated gas after Ro = 0.5 in the previous studies . However in the petroleum systems for the basins of non-marine or deltaic environment, hydrogen rich coals, such as Japanese coals, should be awarded that non-migrating oil-drops in micron size could have generated light hydrocarbons in the gas generation window when a thermal clacking of oil and bitumen begun.
The MITI Sanriku-Oki well confirmed the presence of the gas-charged Cretaceous to Eocene formations, and proved the gas potential of those formations in the Pacific Ocean off Sanriku province of the northern Honshu Island. This study deals with the geologic structure, tectonic evolution and hydrocarbon systems of the Cretaceous to Eocene formations, which were deposited in the forearc basin setting. The area of investigation is from off Sanriku province of northern Honshu to off Hidaka province of Hokkaido Island. Structural interpretation was made utilizing 11,190 line kilometers of seismic sections. Based upon the structural interpretation, it was made clear that the severe folding, uplifting and erosion that gave rise to the trapping structures took place during Oligocene, and that fracturing of the basement volcanic and granitic rocks occurred during middle Miocene. In the Sanriku-Oki Sub-basin off Sanriku province, the A3-Coal-Marker (upper Cretaceous) and B2-Top (middle Eocene) horizons form a N-S trending synclinorium about 70km wide that gently plunges to the north. Coaly gas prone source rocks intercalated between the B2-Top and A 3-Coal-Marker are considered to have been matured since Oligocene in the axial part of the synclinorium. Several domal traps, fault dependent traps and stratigraphic traps beneath the Oligocene unconformity (sub-crop traps) are possibly gas charged. Combined gas in place in those traps can be as much as 15 TCFs. In the Yufutu-Oki Sub-basin off southern Hokkaido Island, coaly gas prone source rocks are buried deep in the Hidaka Trough and have been well maturated since late Miocene. Several basement paleo-highs on the Tomakomai Ridge are possible hydrocarbon traps if weathered or fractured. Combined gas in place in those basement traps can be as much as 6.4 TCFs.
The petroleum system of the Ishikari-Hidaka basin was investigated using the geochemical data and basin modeling of the Yufutsu oil/gas field. The Yufutsu field is located on the Tomakomai ridge, a north-south trending basement high at the center of the Ishikari-Hidaka basin. In the Yufutsu field, gas and pale yellow-orange waxy condensate oil are produced from so called "Deep Reservoir", the Cretaceous granitic rocks and the conglomerate member of the Eocene Ishikari Group. In addition, bitumen is observed on cores of Deep Reservoir. Further, black oils are found in the "Shallow Reservoirs", the upper Eocene coal bearing member of the Ishikari Group, the upper Oligocene Minami-naganuma Formation and the Miocene Takinoue Formation. Oils from the Shallow and Deep reservoirs are correlated to the coal bearing member of the Ishikari Group, from biomarker and carbon isotope data. However, there is subtle compositional difference between oils in the Shallow and Deep Reservoirs. From higher sterane C29-20S/(20S+20R), Ts/(Ts+Tm), Diahopane/C30-hopane, Oleanane/C30hopane ratios, oil from the Deep reservoir is highly matured. On the other hand, oils from the Shallow reservoirs are of moderate maturity. From basin modeling at the main kitchen to the east of the Yufutsu field, the time of peak oil generation was 10-15 Ma. The distribution of oils in the Shallow and Deep Reservoirs are explained as follows : 1) At earlier stage, moderately matured oil was generated in the main kitchen, and some oil migrated into the Deep Reservoir. 2) At later stage, with increasing burial, gas generation and cracking of retained bitumen began, and the gas and condensate migrated into the Deep Reservoir. As a result, de-asphaltation occurred. At the same time, coal and coaly shale around the Yufutsu field became mature, and black oil migrated into sandstone in the Ishikari Group. For oils in the Minami-naganuma and the Takinoue Formations, the timing of accumulation is not clear.
The Offshore Joban Basin, which constitutes the southern-most part of the forearc basins located along the Pacific coast of the northeast Japan, extends about 170 km in NNE-SSW direction with 50 km width. The Upper Cretaceous and younger sediments distribute widely with maximum thickness of more than 5,000 meters. The Iwaki-oki gas field located in the Offshore Joban Basin, which is the only commercial offshore gas field in the Pacific Ocean off northeast Japan, has been producing gas since 1984. The Paleogene and Maastrichtian coals and coaly mudstones, deposited in a confined basin along continental margin, are the most likely source rocks of the gas. The basin modeling simulation in the basin depo-center west of the gas field estimates present vitrinite reflectance (Ro) values at the source rock horizon to be in the range from 1.0 to 1.3%. The simulation also indicates that maturation of the source rocks were accelerated by rapid subsidence since Miocene, and that peak gas generation and expulsion occurred during middle Pliocene. Therefore, the basin depo-center is considered as the kitchen area. Main reservoirs are the shallow marine sandstones intercalated in the Lower Miocene and the basal part of the Oligocene. The former is the producing reservoir of the Iwaki-oki gas field. Both sandstones are sealed by the extensive and thick mudstones. Primary hydrocarbon traps are NNE-SSW trending anticlines, which were formed before Middle Miocene. These anticlines are cut by NNE-SSW trending faults at their flanks. As the reservoir sandstones are about 2,000 meters vertically apart from the source rocks in the kitchen area, it is considered that expelled hydrocarbon migrated vertically through faults. Although forearc basins are not generally considered to be prospective for hydrocarbon exploration, there could be relatively good hydrocarbon system existing as shown above in the Offshore Joban Basin.
The Coalbed Methane (CBM) in the Northern Bowen Basin, Australia was explored by MGC Resources Australia Pty. Ltd. (MGCRA) from 1990 to 1995 in order to confirm the volume and cost of CBM as a raw material sufficient for the methanol production plant. Unfortunately this project did not get promising results. The failure causes are attributed to the unexpected low permiabilities, heterogeneous permiability distributions, existence of secondary minerals in the coal cleats or fractures and of undersaturated gas adsorption zones. To overcome these problems and to improve the CBM productivity, some R & D ideas are discussed in this paper regarding the exploration methods, permeability improvement, well completions and others.
Several natural gas and iodine companies have developed the Mobara gas field for 70 years. In this field, the natural gas dissolved and the iodine are contained in the formation brine. More than 1000 wells were recorded by electrical logging that consists of sponteneous potential and resistivity data when they were drilled. In this study, subsurface geology of the Mobara gas field is integrated with stratigraphy of the Kazusa Group by correlating well logs. Productivity of natural gas and iodine is indicated on a contour map, then the relationship between geology and productivity is evaluated. Main conclusions are as follows: (1) Distribution of turbidite sandstones indicates that the area is covered with the spread of a submarine fan. In the Umegase and the Otadai Formations, many turbidite successions are deposited in the submarine fans. The central parts of the submarine fans are located at the southwestern part of the Mobara area. (2) In the central parts of the submarine fans of the Umegase and the Otadai Formations, both the gas water ratio and the iodine chlorine concentration ratio are relatively high. It means that productivity of natural gas and iodine corresponds with the area with the thickest part of the turbidite sandstones. Consequently, the central parts of the submarine fans are highly productive. (3) The Shibahara area, which is the most productive area for natural gas, is located on the west side of the normal faults. The depths of the production zone are relatively shallow (between 200m and 600m below sea level). It may serve as evidence that the faults and the depths of the gas reservoirs influence natural gas productivity.
Generation of "bacterial" methane by Archaea and its accumulation mechanism into interstitial water in early diagenesis were investigated by a mass balance calculation for observed isotopic compositions and geological evolution through aggradation from bathyal sediments for several hydropressured (so-called "dissolved-in-water type") natural gas fields in Japan. Hydrogen isotopic ratios of methane from the Minami-Kanto and Niigata gas fields reveal that CO2-reduction was the main reaction of the methane formation, and its carbon isotopic ratios show a concentration between -67 and -66%_0 relative to PDB. Model calculation in a semi-closed system based on coexisting carbon isotopic compositions of methane and dissolved inorganic carbon for the Minami-Kanto gas field reveals the considerable contribution of carbonate minerals to both compositions. For example, if CO2 had been supplied only from organic matter, the efficiency of methane generation would have been as low as 40%. Whereas, if methane generation rate had been equal to the input rate of CO2 from organic matter, the dissolution and precipitation of carbonate minerals and/or isotope exchange reaction between dissolved inorganic carbon and carbonate minerals should have occurred about nine times faster than methane generation. From a geological standpoint, it is inferred that the existence of thick marine argillaceous sediments rich in terrigenous organic matter and carbonate minerals was important for the generation of archaeal methane and adsorption of iodine. Accumulation of methane and iodine into interstitial water must have been promoted when interacting solid/liquid ratio of sediments was increased by the burial suppression of interstitial water caused by basin subsidence, sedimentation of sandstone, recovery of abnormal compaction and/or dehydration of clay minerals and other causes. Interstitial water dissolving methane and iodine in thick marine argillaceous rocks was removed to the upper sand/silt or mud interbeds, which had higher sedimentation rates, with the compaction and burial of solid sediments.
21st century is considered as an era of natural gas. The reasons of this could be derived from its efficiency, environmental advantages and new technological development such as micro gas turbine, gas-combined cyclic power generator etc. In Japan, however until now, natural gas resources have been limitedly utilized for electric power generation by import of LNG. It is known that Russian federation may contain the world largest gas resources at the present time and the future. In particular, substantial gas potential is highly expected in the eastern Siberia and the Far East region. Due to insufficient infrastructures from the existing gas fields to potential market, these fields have not been fully developed. For the future strategy of energy supply to Japanese market, these gas resources could be one of important candidates. In this paper, the author summarizes the world gas demand and supply, and introduces the hydrocarbon systems of representative oil and gas fields of the region.