Several large hydrocarbon deposits in deep-seated volcanic-rocks in "Green Tuff" horizon have been discovered around Kashiwazaki to Nagaoka cities in Niigata prefecture. This paper studies the properties of porosity and the distribution of rock facies in the volcanic-rock reservoirs. In the Minami Nagaoka gas field, rhyolite reservoirs, having total thickness of 1, 000m and gas column of more than 800m, occur at a depth of 3, 800m and deeper below the surface. The rhyolite is divided into lava, pillow breccia and hyaloclastite in terms of rock facies which resulted from drastic chilling and shattering in submarine volcanism. Pore spaces in the rhyolite are classified into several porosity types in terms of their origin, shape and size. The properties of porosity is closely related to each rock facies. The favourable rock facies as reservoirs are pillow breccia and lava facies. Meso- to micro-sized vugs play an important role in total porosity, while micro-sized secondary fractures are effective to permeability. In order to predict productive zones, it is important to understand the relavant volcanic-rock facies.
There are some important oil or gas fields of volcanic reservoir in Japan. Emprically, two basic rules on those have been pointed out by some investigators. (1) The reservoir characteristic of volcanics is better than clastics. (2) Acidic volcanic rocks have much more opportunities to reserve hydrocarbon than basic volcanic rocks. These two rules are thought to come from the potentiality of fracturing under tectonic stress. The rock properties of volcanics ranging from acidic to basic are studied under high confining pressure and compaired with clastic rock properties which were studied by HOSHINO et al. (1972). Four interesting results are obtained. (1) Ductility of volcanics is smaller than clastics under the subsurface condition. (2) Volcanics have more opportunities to be deformed with favorable fracture types for reservoir than clastics. (3) Difference of volcanic rocks type is not so clear on these two results, however, porosity is one of the most important factor for the ductility and fracture type. (4) Young's modulus of acidic volcanic rocks are bigger than basic volcanic rocks. Hereby, we concluded that the differences of rock property between volcanics and clastics are the important keys to the empirical rule (1). The difference of Young's modulus may suggest something to the rule (2), however some problems are still remained for the future.
The efficiency with which oil and gas can be stored and recovered depends not only on the characteristics of fluids in pore spaces but on the properties of pore systems in reservoir rocks. The evaluation of reservoir rocks has been determined by the conventional methods of petrography, X-ray mineralogy and core analysis up to now. In addition to these methods the more detailed and executive estimation for the reservoir quality is effectively performed by means of i) measuring the pore size distribution (i.e, capillary pressure), ii) impregnating blue-colored epoxy resins into pore spaces including in reservoir rocks, and iii) scanning electron microscopic observations of pore systems. Oil may migrate through water-filled pore spaces in order to accumulate in reservoir rocks, and the migration may be chiefly promoted by buoyant forces but is inhibited by the capillary pressures which must be overcome for an oil globule or filament so as to pass from a rock pore through a pore throat to an adjacent pore space. For a laboratory use capillary pressure can be easily transformed to an equivalent pore-throat size with the equation, Pc=-2γcosθ/γ where Pc is capillary pressure, γ is surface tension, θ is contact angle of mercury and γ is pore-throat radius. We can get specific surface area values from the measurement of capillary pressure simultaneously with the pore-size distribution, by the following expression empirically, S=1/γcosθ∫PdV where S is specific surface area, P is capillary pressure and Vis mercury volume injected in the pore spaces. According to probability plots of pore-size distribution and specific surface area plots against mercury porosity percent, we may estimate properly the reservoir quality of rocks. Micropores approximately smaller than a half micron (5, 000 A) have been considered to be ineffective for the oil and gas storing till now. Recently many mineralogists propose the capability of preservation of oil and gas in such micro spaces. In fact such micropores were measured to be over than a few tens of percent occupied even in porosity of excellent volcanic reservoir rocks and fine- to coarsegrained sandstone reservoir rocks in this study.
Log interpretation on volcanic rocks is usually difficult because of the mineralogical complexity of them, the drastic change of lithology and the lack of information of the type of pore system. Therefore, the method of log interpretation on volcanic rocks has not been standerized as the method used in the intergranular formation yet. This paper presents the example of log interpretation on basaltic rocks in the Yurihara district. At the log interpretation on this area, it is recognized that some relationship between the values of ILD and the texture of groundmass exists, and each flow unit may be discriminated with the log pattern of ILD and GR. Two methods are discussed for the estimation of matrix values, and porosities and water saturations calculated by the use of average values of grain density are relatively well coincided with the core analysis data and production data.
Rhyolite in the Minami Nagaoka gas field is divided into three types of facies, i.e. hyaloclastite, pillow breccia and lava. Basalt and these facies of rhyolite can be identified by geophysical logs; GR, FDC and CNL. Hyaloclastite has a high GR value and is differentiated from lava and basalt which display a low GR value. Pillow breccia usually indicates a low GR value and sometimes a high GR one. The high value or the low one of GR is distinct on GR curves and the boundary is usually 25 to 30 API units. Basalt has the separation like dolomite on FDC and CNL logs and is different from lava and pillow breccia which display sandstone separation. Hyaloclastite is different from pillow breccia because the former indicates dolomite-like separation. Lava and pillow breccia are similar on the GR value and the separation of FDC and CNL, but lava has less porosity calculated from FDC and CNL. The facies identification on FDC and CNL logs is carried out by using ρb-φN crossplot.
The formation process of hydrocarbon pools in the "Green Tuff" reservoirs has been studied from a geochemical point of view for the Mitsuke oil field, the Yoshii-Higashikashiwazaki gas field and the Minaminagaoka-Katagai gas field. The accumulated hydrocarbons originated in the shales of the Nanatani Formation and partly of the Lower Teradomari Fm. The hydrocarbons were primarily expelled in oil phase from the source rocks at the thermal maturity levels of 0.7 to 1% in vitrinite reflectance. In the Mitsuke field, oils have not been thermally cracked throughout the geologic time because of the low thermal levels of reservoir. Thereafter, gas-condensate was secondarily added to the reservoir from the highly matured source rocks which attained to the gas-generating stage. On the other hand, hydrocarbons in the studied gas fields were primarily trapped in oil phase and subsequently cracked into gas in the reservoirs.