The Nagaoka Plain is located in the central part of Niigata Prefecture, Japan. It is gently folded area and is bounded by intensely folded hilly areas. Many oil and gas fields have been discovered and developed in these areas. Some of them are characterized by the abundance of pyroclastic reservoir rocks. The writer studied geological development of these areas using some restored stratigraphic sections and isopach maps, and he obtained the following conclusions. (1) Gently folded anticlines in the eastern half of the plain area may have been originated from the synsedimentary upheavals (synchronous highs) which consisted of basement complex and "Green Tuff" (pyroclastic rocks of early Miocene). (2) Foldings of surrounding hilly areas may have been farmed since Nishiyama age early Pliocene). This tectonic movement was possibly caused by block movement of the basement. Thrust faults have extended along the boundary between the folding area and the gently folded area with down thrown anticlines under them. Down thrown anticlines have pyroclastic rocks in their cores and some gas reservoirs have been found in them. (3) There are two types of petroleum deposits of Green Tuff. In the first case, oil and gas migrated at the early stage into the Green Tuff which was making synchronous high by itself (Mitsuke Oil Field type). On the other hand, in the case of Yoshii Gas Field, gas migrated into the Green Tuff at the Teradomari or Shiiya age (late Miocene) when the Green Tuff got to reveal itself as the structural high (Yoshii Gas Field type).
Since 1967, the writers have studied the stratigraphical and geographical distributions of carbonate rocks of the Funakawa, Onnagawa and Nishikurosawa formations of the Neogene in the Akita oil fields, Japan. They considered the origin of these carbonate rocks from the geological and mineralogical points of view. The results are summarized as follows: 1. Stratigraphically, these carbonate rocks are generally distributed in the following formations; uppermost horizon of Nishikurosawa formation uppermost horizon of Ormagawa formation lower horizon of lower Funakawa formation uppermost horizon of upper Funakawa formation Geographically, the carbonate rocks are remarkably distributed in the Onnagawa and Nishikurosawa formations along the southern coast of Oga peninsula, in the Nishikurosawa formation of the eastern part of Kameda-machi and in the Onnagawa formation of the Southern part of Yajima-machi. 2. The detrital substances derived from the inorganic and organic matters are the main source materials for these carbonate rocks, while the chemical deposits from the sea water occupy only a part of the carbonate rocks. Depositional environments of these rocks are ranged from the inner-neritic to bathyal conditions. The supply of both calcium and magnesium is necessary for the formation of carbonate rocks from the source materials because of the poorness of these elements in them. Concerning the carrier waters of calcium and magnesium and the geologic stages of supply of these elements, it is considered that the supply of calcium and magnesium from the salt water originated from the earlier lithification during the period of syn-diagenesis is more effective to the transformation of source materials than the supply of these elements from the water originated from the ana-diagenesis, epi-diagenesis and hydrothermal alteration. Also, the additional supply of these elements from the volcanic detritus in the sea may sometimes be very important. In the studied area, the formation of chemical deposits is very rare, and genetically, this formation is mainly controlled by the following two factors; one is the abnormal increase in magnesiun and calcium contents of the sea water caused by the supply of large amount of volcanic detritus in the sea, and the other is the temperature rise of the sea water caused by the same phenomenon.
We published a paper "Some Relationships between Crude Oil Properties and Geology in the Kubiki (Katamachi) Gas and Oil Field, Japan (Part 1)", which appeared in the Journal of the Japanese Association of Petroleum Technologists, Vol. 35, No. 1. In the paper we pointed out the existence of the correlation between specific gravity of crude oils and reservoir conditions. This paper now presented here is a sequel of the "Part 1", dealing with the results of gaschromatographic analysis of the crude oils from the Katamachi field. We sampled the crude oils of the le reservoir from a number of producing wells in the field, and made a gas-chromatographic analysis of lighter cut (lower than 105°C) of the crude oils. The result is shown in Table 2 of the "Part 1". The purposes of the analysis are (1) to determine the degree of evolution of the crude oils, (2) to find out any measure that characterizes the Ie crude, and (3) to presume the direction of lateral migration of the oils. As for the subject (1) we already described in the "Part 1". With regard to the subjects (2) and (3) we adopted Silverman's idea (1965) as working hypotheses. Silverman showed that the ratio of amount of some liquid hydrocarbons to that of other hydrocarbon (excluding benzene) is kept almost constant during secondary migration and accumulation of oil, if the differences in boiling points of these hydrocarbons are small. He also suggested that a trend of decrese or increase of the relative amount of benzene may imply the direction of secondary migration, which is mainly due to the selective adsorption effect of rock-forming minerals. At first we discuss the subject (2). The ratios of amount of hydrocarbons that Silverman demonstrated are as follows: (cylcopentane+2 methylpentane+23 dimethylbutane)/3 methylpentane (23 dimethylpentane+cyclohexane)/n-heptane The ratio that characterizes the crude oil and differs from others should at least have smaller variance and bear no correlation to any other factors, i.e. specific gravity of crude oil, reservoir conditions and so on. We tested the above ratios for the crudes from the Katamachi Ie reservoir, but no satisfactory results were obtained. We further tested many other ratios that Silverman had not shown, but we could not find out suitable one to the crude oil. In the course of this study we found out that some ratios have correlationship with the specific gravity of the crude oils (Figures 10 and 11). We cannot explain the reason, but we presume the alteration of crude oils by formation water which is of low salinity and has a considerable amount of sulfate ion. Furthermore, we can recognize the tendency that a number of ratios in the east side of d-d' fault are different from those in the west side of the d-d' fault (Figures 13 and 14).
Most of the failures in primary cementing are attributable to "channeling", an imperfect displacement of mud with cement slurry. Water or phosphate added water works to some degree as a spacer between mud and cement slurry but its function as "channeling" preventive is negligible. Its use also presents a problem of pressure balance in primary cementing. The objective of the authors have been to develop "an effective preflush fluid" that solves the problem of channeling. Preflush fluid should have the following qualities: 1. Early destroying performance against mud gel strength. 2. Good suspending capacity. 3. No adverse effect on cement slurry. After repeated experiments along this line, a satisfactory preflush fluid has been developed by using a kind of surfactant. The density of this fluid may be increased up to 2.0g/cm3 adding with fly ash and barite. And such weighted fluids solve a problem of pressure barance. As this fluid has a washing effect on the surfaces of casing pipe and formation, it may be utilized as an excellent means of improving cement-bonding.