Horinouchi oil field was discovered in 1952 as a first pay inland oil field in Japan. Four oil reservoirs lie at depths of about 500 to 1, 000 meters. It is proved by the flow test that reservoir II is a fractured type zone saturated insufficiently by oil and is a water drive reservoir.
It is one of the most difficult problems to drilling engineers how to control the drilling mud characteristics in drilling low or high pressure zones, having shale zones and sticky zones. The writer has succeeded to drill the above mentioned zones by the use of lime starch mud in Innai R 126.
In Japan, since several years ago, methane gas deposits have been progressively explored and exploited. The gas in these deposits occurs free geologically from petroleum, and is, in most cases, dissolved in formation water. under hydrostatic pressure which approximately corresponds to depth. To understand the genetic process and present phase of these deposits, much geochemical study has been conducted as well as geological survey. As a result of that, a close relationship between gas potentiality and some chemical components of gas and also accompanying water has been determined. Recently the writers and their associate developed a special method of geochemical survey of core and applied the method to 14 gas wells shown in Table 1. In this paper the writers describe briefly the method and the results of tests with special reference to the bearing of such survey on the problems of formational process of the deposits. Among various analyses of geochemical core survey, chemical analysis of interstitial water in core is most frequently made at present. As is shown in Fig. 1, the routine is as follows: A. fresh mudstone core is sampled; the interstitial water is extracted through dilution; and such chemical compositions of the water as Cl', pH, excess base, KMnO4 consumption, NH4+, etc., are analysed by titration or colorimetric method. Although the differences in the way of treating cores and conditions of extraction cause considerable variation in measured values of these compositions (See, Figs. 2, 3, 4, 5), the routine procedures give almost satisfactory results with little variation. The writers applied the method to some gas wells and structure borings in the Southern-Kanto gas field, which covers Tokyo and its vicinity (Fig. 6). They found four types of variation of chemical characteristics of sediments, (which they call "chemofacies", ) with increase in depth. The four types are graphically shown in Fig. 8. The relationship of each of these types to the sedimentary basin and also to the gas deposit is illustrated in Fig. 7. Although chloride in interstitial water has genetically no relation to methane, in the gas deposits of the Tertirry marine sediments, it is well observed that chloride concentration has a positive correlation to gas potentiality through the secondary destruction phenomenon of gas deposit. Ammonium concentration in cores may not only be used as a measure for the secondary destruction phenomenon, but may also have genetic relation to methane. Thus, ammonium is regarded as a more essential component than chlorine. Between gas deposit and zone of meteoric water, there is usually a remarkable water zone of brown color with low chlorinity and much humic matter, HCO3' and KMnO4 consumption. In the Niigata gas field, paleontological study shows that depositional environment changed from marine to brackish water during late Pliocene to Quaternary. Geochemical data for an exploration well drilled in the field is presented in Table 2. The fact that the upper part of the sediments has considerably high potentiality in spite of its low salinity, may suggest the presence of secondary-formed gas. In conclusion the writers stress that the geochemical survey of cores including interstitial water analysis is very important to understand the present phase of gas deposit in its formationdestruction series and is an indispensable means to reveal the genetic process of the deposit.
The writer studied the development of gas fields and its utilization as town gas, as there are many shallow natural gas pools in Japan. The results are as follows:- 1. It needs 6 wells (depth 100m, production 200m3/d each) and a gas tank of 500m3 capacity to distribute 1500m3 natural gas to a town every day. 2. It is better to distribute the gas from both sides of a town than from one side, from the standpoints of production and distribution. 3. The construction costs to supply 1500m3/d of natural gas are as follows (a rough estimate):- Cost of drilling and production 6, 377, 000 Yen, , , , gas tank 3, 930, 000, , , , , , machines for distribution * 1, 194, 000, , Total 11, 501, 000, , Besides, additional 23, 000, 000 Yen will be needed to supply gas to 1, 000 houses on the construction of pipe lines. So the total cost will amount to 33, 501, 000 Yen. 4. As mentioned above, this industry needs comparatively high cost for intsallation, but little running cost. And as well as it is reasonably managed, there is a possibility of success as an enterprise.