In the previous paper(1), the writer estimated the volume of the downward compaction current from the overlying mudstone into the Shiunji tuff bed at Shiunji, where the basement rock type entrapment has been formed. The similar method is applied in this paper for the volcanic reservoirs in Nagaoka Plain. The volcanic reservoirs in Nagaoka Plain differ from the basement rock type existing at Shiunji, because source mudstone exists below the reservoir in the case of Nagaoka Plain. It is, however, considered that the hydrocarbons produced from the overlying mudstone, could have been entrapped in the reservoirs more effectively, for the overlying mudstone is also important as cap rock. That is to say, it is doubtful whether the cap rock overlying the reservoir existed, at the time the compaction current was expelled from the underlying mudstone. Because of the overwhelmingly greater significance of the overlying mudstone in Nagaoka Plain, coupled with the scarcity of data on the umderlying mudstone, this paper deals with only the downward compaction current expelled from the overlying mudstone in this case. The vertical mudstone porosity distributions of the main wells in Nagaoka Plain are determined by using the Sonic log data. These distributions suggest that the downward compaction current might have moved into the volcanic reservoirs from the Teradomari mudstone at the Mitsuke oil field, from the Shiiya mudstone at the Fujikawa gas field, and from the Nishiyama mudstone at the Kumoide, Sekihara and Katagai gas fields, indicating that these are the major source beds. The clear porosity differences between the mudstone which has acted as barrier against water expulsion, and the underlying mudstone, are observed at these fields, indicating preferable conditions to downward water movement, while at Nakanokuchi, Tsubame, Yukyuzan and Minami-Nagaoka there seems to be only slight downward migration. The final section of this paper treats the relations between oil-condensate gravity and depth in Nagaoka Plain. According to Holmquest(5) (1966), different source-bed lithologic types relate to distinct hydrocarbon characteristics; gray shale may produce oils with lower gravity (higher API gravity) at a given depth than black and dark shale. This implies that source beds deposited in open-marine waters produce lower gravity oil than source beds deposited under stagnant-marine conditions. The relation between the gravity of the oil or condensate produced from the Nagaoka agglomerate reservoir, and depth suggests that the Fujikawa condensate is different in source from the Kumoide condensate and the Sekihara-Katagai oils. Consequently, the major source rocks are the Shiiya or dark-gray mudstone at Fujikawa, and the Nishiyama or gray mudstone at Kumoide, Sekihara and Katagai, as assumed from the vertical porosity distributions discussed above. In addition, the crude oil produced from Kumoide SK 13 and SK 14, may be of the same origin as Fujikawa. Similarly the relationship between oil gravities and depth at Mitsuke, Kurosaka and Higashi-Sanjo indicates the difference of source beds between Higashi-Sanjo and Mitsuke or Kurosaka.
The trial combination rock bit to be used for the hard formations has had two problems in torcus, as presented on the memoir No. 1. That is, Skidding of the cutter and breakness of the middle of the bottom were reasons why it is imposible to practice effective drilling for the hard rocks. As results of the study, at last we found a way to drill the hard rocks effectively. The new trial bit (No. 5-B) consists of the diamond crown shoes, and charasteristic one cutter to crush the core in the hole, that is called D.M.C type rock bit. (PAT. 478520, pend 41-074989) Present steps of the improvement have been directed to producing the 81/2" D.M.C to use for the practical field wells.
It is very convinient to calculate the static bottom hole pressure of gas formation whithout closing well or descending measuring tools into a well. The following have been investigated: 1. The relation between the surface static pressure and the bottom hole one, about gas well. 2. The relation between the surface flowing pressure and the bottom hole one, about gas well. 3. The relation between the flowing pressure and the static one, about gas well
The general aspects of the present study are summarized in "Abstract" of the first instalment. This instalment deals with the explanation of the author's new method of bottom hole pressure build-up analysis for exponentially declining flow rate case. (to be continued)