In this paper, the origin of carbon dioxide and nitrogen in natural gases produced from the Erawan gas field is discussed on the basis of carbon isotopic data and several geological evidences. Maximum contents of carbon dioxide and nitrogen in natural gases recovered from three wells at the northwestern flank of this field, are 59.72% and 21.38%, respectively. As a result of carbon isotopic studies of methane, ethane and carbon dioxide in 11 gas samples of those wells, natural gases are divided into two groups, such as group A and group B. The group A is characterized by heavy methane (-30 to 33‰ PDB) on carbon isotopic compositon, and by high contents of carbon dioxide and nitrogen. Gases of this group are a mixture of magmatic gases and organic-origin gases. The magmatic gases consisting of mainly carbon dioxide, nitrogen and heavy methane have migrated into the present reservoirs from the pre-Tertiary basement through the east-dipping faults cutting the basement. The group B is characterized by normal contents of carbon dioxide and nitrogen, and by lighter methane (-38 to -41‰ PDB) on carbon isotopic composition. Gases of this group have been generated from the organic matters in the Tertiary sediments by thermal maturation.
Several hydrocarbon deposits have been found in volcanic rocks of the Nanatani stage (so-called Green Tuff) in the Niigata basin, Japan. Their characteristics are summarized as follows. 1) Hydrocarbon deposits are formed in anticlinal structures and are mostly located close to depressions. 2) Oil accumulations have been found in anticlinal structures which are formed in the early stage. 3) All hydrocarbon deposits except for the Shiunji gas field are located in the Chuetsu sub-basin. 4) Larger oil and gas fields are located in or close to the kitchen areas of oil and gas. This suggests the short lateral migration of hydrocarbons together with the distribution of oil and gas in neighboring areas of the Mitsuke oil field. 5) In larger gas fields, the Teradomari formation shows a typical abnormally high pressur e, whereas the Geen Tuff is hydrostatical or slightly highly-pressured. On the other hand, formation pressure in the Mitsuke oil field increases toward lower straigraphic horizons that is similar to the pressure distribution in the Shiiya and Nishiyama reservoirs. These pressure patters are closely related to the manner of accumulation of oil and gas. 6) The migration of oil and gas in larger oil and gas fields is thought to have occurred during and after the Nishiyama stage. 7) The distributions of oil and gas in the Green Tuff depend rather on the kinetics of hydrocarbon expulsion from source rocks than on the type of organic matter. The kinetics of expulsion is controlled by the relative positions of the hydrocarbon generation zone and the overpressured zone. The Niigata basin can be sub-divided into three parts according to their geologic conditions: Kaetsu sub-basin, Kakuta-Sanjo uplift and Chuetsu sub-basin. Considering the combination of trap, source rock, reservoir and migration mechanism, it is concluded that the Chuetsu sub-basin is the most prospective for future exploration.
Three Kinds of experiments on formation damage are carried out at simulated in-situ geothermal conditions. The most severe test conditions are of Temperature of 250°C and pressure of 8.1MPa. Bentonite and sepiolite muds are used throughout the tests. The first test is measurement of mud properties at room conditions after aging for 24hours. The second test is measurement of rheological properties by a capillary tube. Rapid gelation of both muds is observed above 200°C. The third test is measurement of permeability recoveries of models invaded by muds. As simulated formation models, slits packed with sand grains and Berea sandstone cores are used. Permeability impairment of the models are measured after aging for 24 hours. Formation damage appears above 150°C in some cases. Severe cases of permeability recoveries below 10% of original values are observed above 200°C.