Reservoir rocks of this province are composed of sandstone, tuffaceous sandstone, tuff, tuff breccia, agglomerate and lava. It is very striking that many of reservoirs were originated from volcanic rocks. Pyroclastic rocks of liparite or rhyolite, dacite and andesite in the "green tuff" have been proved to form excellent reservoir rocks. Characteristics of volcanic rock reservoir are that they have effective pores originating from fissures and/or vugs besides intergranular pores. This is indicated by comparison of porosity values from Sonic Log and Formation Density Log on sandstone reservoir and volcanic rock reservoir. The writer attempted, in this paper, to review the resuls of exploratory wells and the state of hydrocarbon production from the "green tuff" in some areas where much efforts have beer devoted to exploration of the "green tuff". In oil and/or gas fields which are producing hydrocarbons from the "green tuff", lava and tuff breccia constitute reservoir rocks, but tuff, especially fine or bentonitic tuff which surrounds lava and tuff breccia, plays the role of cap rock. Large hydrocarbon deposits in the "green tuff" have been found in the area colse to abundant source rocks. Petroleum or protopetroleum should have migrated into lava or tuff breccia through tuff. The time of migration should be just after tuff was deposited, when tuff had not yet been compacted or altered so severely so that petroleum could not permeate it. Fumdamental requisites for hydrocarbon deposit in the "green tuff" are as follows. (1) Contemporaneous source rocks should exist close to the "green tuff". (2) "Green tuff" should have formed relative high at the time of migration and it has developed into anticline or anticlinal structure. (3) Good porosity of any kind should exist in the "green tuff". (4) Fine or muddy tuff in the "green tuff" should have turned into cap rock contemporaneously with the progress of hydrocarbon migration.
Compressibility at hydrostatic pressure up to 2000 bars were studied on dry samples of about 50 kinds of rocks collected from both surface exposures and exploratory wells in Japanese oil bearing areas. The results indicate that at porosity less than about 30% (for argillaceous rocks) or 15-20% (for arenaceous rocks), compressibility changes regularly according to porosity, while at more than the above porosity compressibility becomes as large as that of liquid. Meantime, porosity of the cores from recent deep drillings in centre of Niigata sedimentary basin were studied. In conclusion, following three stages of compaction were recognized. Stage 1: The mineral grains within the rocks are not touched each other. The rocks behave like soil and physically rather liquid-like. Stage 2: The mineral grains come into contact. The rocks consist of the mechanically stable framework of these grains. Physically, they are plastic solid. Stage 3: Authigenic minerals appear among the mineral grains. Cementation is common in every part of the rocks. In argillaceous rocks, primary porosity is around 80%, and from 80 to 30% belongs to stage 1. At 5 to 15% porosity, it changes from stage 2 to stage 3. In arenaceous rocks, primary porosity is approximately 50%. Stage 1 is considered to be between 15 or 20% and 50%. From 15 or 20% it becomes stage 3. Stage 2 is possibly absent in arenaceous rocks.
According to the results of a series of high pressure experimentation, following empirical equation was proved to be valid between porosity n and strength σs. n=Aoe-bσs……(1) where Ao and b are constants or coefficients. This equation is quite compatible with equation (6) derived from well-known compaction eqution (2) and indicates a close relation between "strength coefficient, b" and compaction coefficient c. In was pointed out that the strength coefficient b is an indication of somewhat softness of the rock structure against failure and hence closely related to strength of boundness or cementation of rock. Since the boundness or cementation of the clastic rocks is formed probably by strong effect of both pressure and temperature, therefore the coefficient b can be a parameter to indicate accumulative energy related to the above factors acted upon the rocks concerned during long geological age. By this reason, further, the coefficient b could be an indication of physical potentiality for oil and gas fields. To prove this idea, strength coefficients b were studied in several oil-bearing sedimentary basins in Japan. The results are given in Tab. 3, which are agreeable with geological data and mode of occurrence of oil and gas.
In this experiment, the resistivity variances of reservoir rocks such as sandstone, which had been prepared were examined under every confining air pressures. As preliminary experiment, the comparisions between two electrode and four electrode method of resistivity measurement were studied on two samples under atmospheric pressure. In the experiment concerning with change of resistivity under high pressure, core sample which are saturated with electrolitic solution was subjected to high pressure by air in autoclave and we measured the variations of resistivity at any pressure within 200kg/cm2 during pressure increased or decreased. In this way, we could obtained the result that the resistivity variances were not so much and they were maximum 25% to minimum 5%. The relations between resistivity and pressure were looped by hysteresis and the variances were most intensive under 40kg/cm2 during pressure up and down until 200kg/cm2 in vessel. On the other hand, remaining saturattion variances of cores were less than about fifteen percent at this time.