SHIBAOKA & BENNETT have developed a method for estimating the maturation level of organic materials in a sediment, where the lag of increase in maturity behind increase in geotemperature accompanied by subsidence of a sediment is taken into consideration. They have also presented a diagram which correlates vitrinite reflectance with geotemperature and time. The diagram, however, cannot be used if the burial history of a sediment is not reconstructed in detail, as in other nomograms ever published. In this paper, the burial history of a sediment is assumed to be represented by a parabolic curve, and the increasing process of maturity shown by SHIBAOKA & BENNETT is methodologically employed. Within the limitations, a mathematical method for estimating the maximum paleo-geotemperature by which an already uplifted sediment had been affected is newly described on the basis of a set of data, i.e., burial time and reflectance of vitrinitic phytoclasts included dispersedly in surface rock-samples. As a result, the following statements can be made: 1) The increase of maturity of organic materials during the uplifting process of a sediment is very small as compared with that during the subsiding one. In case that a sediment had been affected by Tmax at the middle period of the whole burial time, organic materials in it had already gained the maturity of ca. 90% to the final one there. In that case, Tmax can be approximated from the measured vitrinite reflectance (R0) by NEWTON'S method on the basis of the following formula; log (0.90*R0)=0.0045*Tn+0.149*log tn-0.87, where Tn and tn are the formulae containing Tnax only as a variable, respectively. For example, in case that the burial time of a sediment is 20m.y., they are given by Tn=0.10+0.995Tmax, tn=1+2p+23*p+26*p+210*p+......+245*p, where p is presented by p=(20-Tmax)/500. 2) The calculated values of Tmax are substantially independent of the period when a sediment had been affected by Tmax if both the burial time and R0 are the same. It is ascertained that Tmax calculated by this method on the basis of actually measured reflectance of vitrinitic phytoclasts occurred in the Lower Miocene sediments distributed in the western part of Tokyo is harmonized with paleo-geotemperature estimated from natural zeolite occurred in the same sediments.
The deep-gas field in the Sekihara-Katagai area recently discovered is situated 10 kilometers southwest of the Nagaoka City. The reservoirs of the gas field are rhyolites of the Nanatani Formation of middle Miocene. Lower formations than the Nanatani Formation are not clear as these have not yet been drilled. The Teradomari and Shiiya Formations of upper Miocene and the Nishiyama, Haizume and Uonuma Formations of Pliocene-Pleistocene overlie the Nanatani Formation, and the first three are mainly composed of several kinds of andesites in this field. These andesites are more strongly altered in compared with those occured in the Higashiyama anticline area. The Nanatani Formation is composed of mudstones and green tuff, which is basalt, andesite and rhyolites. Rhyolites are the thickest member and from petrographical characters are classified into the following four types: rhyolite A, B, C, and D. Rhyolite A contains hyaloclastites and pyroclastics and rhyolite B, C, and D are lava. Rhyolite C is perlitic and rhyolite B and D are characterized by needle-like plagioclase in groundmass. SiO2 and K2O contents (calculated as H2O free) of rhyolite B and C are 66-77 and 0.16-2.9 weight per cent respectively and the latter is much smaller than those of fresh rhyolites. Good reservoirs are rhyolite B, C and D, whose pores are not primary ones such as cooling cracks and joints of rhyolite bodies, but are secondary pores formed by hydrothermal alteration. Under hydrothermal condition, leaching of K and Fe in glass of rhyolite and recrystallization of quartz and albite from glass have taken place. Secondary pores are druse (vug) and channel coated by quartz, and micropores between crystal grains of quartz and/or albite. Although many problems remained unsolved, relation between formation of rhyolite reservoirs and primary migration of oil, gas and water are infered as follows: (1) formation of elongated volcanic piles by products of submarine eruption of rhyolites and deposition of mudstones (source rock) around the piles, (2) generation of hydrocarbon with compaction of mudstone and abnormal increasing of temperature in related to volcanism, (3) primary lateral migration of oil, gas and water into secondary pores of rhyolite B and C and enclosure of them beneath impermeable rock (cap rock), which was transformed from rhyolite A by argillization.