A part from the natural gas of the oil fields, there are methane rich natural gas in the Pleistocene and Pliocene marine formations in the Niigata and Chiba districts, practically, being used for gas industry. The gas occurrs associating with the formation water in the reservoirs, and is called the suiyosei gas, which might be originated in the formations soon after their deposition. On the other hands, however there are also the methane rich natural gas pools of the Miocene marine formations, the Shiiya and Teradomari, non-associated with oil in the Niigata district, which also being used for gas industry. Most of the methane gas of these gas pools non-associated with oil, might, hypothetically, be originated in the Miocene formations, soon after their deposition, to form the suiyosei gas pools, in the early geological time of deposition of the formations as methane rich gas pools shown in the younger formations as those of the Pleistocene and Pliocene, and during geologic historic time, which might be transformed to form the gas pools non associated with oil at present.
This paper deals mainly with diffusion of methane from the oil-free dissolved-in-water type gas reservoirs in the Kazusa group which is widely distributed in the Southern Kanto region, Japan. Diffusion of methane in water-saturated sediments is ruled by Fick's laws as expressed by the equations [2.1] and [2.2]. Several solutions of the differential equation [2.2] under some simple boundary conditions, that is, simplified geologic conditions, are shown in the equations [4.1], [4.2], [4.3] and [5.1]. Concerning diffusion coefficient of methane in water-saturated sedimentary rocks, (Nernst-) Einstein's law (expressed by the equation [2.3]) is valid. Moreover, assuming that adsorption effect of rook-forming minerals is negligible, Klinkenberg's formula (equation [2.4]) is applicable. Consequently diffusion coefficient of methane in water-saturated clean loose sand of porosity 30% at 40°C, is estimated at 8.2×10-6cm2/sec at most. Diffusion coefficient of methane in water-saturated muddy sand or sandy mud can be assumed to be 1×10-6cm2/sec; in water-saturated compact mudstone, 1×10-7cm2/sec. Calculations by putting these values of diffusion coefficient of methane and a value of sodium chloride into the equations [4.1], [4.2], [4.3] and [5.1], were carried out, and the results were tabulated. Through these calculations, the writer has reached the following conclusions: (1) Migration of methane by diffusion in water-saturated formations in several million years is slight; therefore, natural gas deposits of some scale and some depth cannot be largely deteriorated by diffusion. (2) Although diffusion of sodium chloride is less than that of methane, the difference between these two is not large. Hence, positive correlation between chlorine and methane in formation water, a remarkable tendency found in the Southern Kanto region, cannot be largely disturbed by diffusion. (3) In general, migration of methane caused by the pressure difference of formation fluids- migration caused by ground-water flow or by buoyancy of gas bubbles-, is larger than that by diffusion. (4) In the marine Kazusa group, areas of formation water of very low salinity and very low gas potentiality are widely developed. This fact is ascribed to the percolation of meteoric water into the formations; moreover, this shows that the velocity of meteoric water percolation has been much larger than the diffusion velocities of methane and salts. (5) In case thick sediments, generating and containing enough methane, are overlying a gas reservoir under transgressive condition without meteoric water percolation, the quantity of methane which diffuses out of the gas reservoir may be negligibly small.
The natural gas pool is made by the accumulation of natural gas originated from the decomposition of a large amount of dispersed organic mater in the sedimentary rocks. Generally the natural gas from a large gas pool has high percentage of CH4, C2, … and H2S. Ar in natural gas from Neogene Tertiary is mostly derived from atomosphere. In older sedimentary rocks a large amount of He is generated by radioactive decay from U and Th in rocks. It is presumed that the reserve of a gas pool is large in case the ratioes of He/CH4 and He/Ar in natural gas are relatively small in connection with the absolute age of the gas producing sedimentary formations. The writer supposes that the content of heavy hydrocarbon in a gas pool largely depends on the environment of deposition of the source rocks, and partly depends on the physical condition (such as temperatuae and pressure) given to those rocks. Generally speaking, we can expect the existence of large natural gas deposits in the region which has been protected against the strong effect of the atomospheric circulation.
The occurrences of natural gas are considered to be very similar to those of oil. Herein the writer discusses them from the viewpoint of geology, geochemistry and geophysics. 1) The distribution of the oil and gas fields can be illustrated closely related to the isopach, lithofacies and restored section maps. 2) The chemical analysis of the organic matters in the source rocks are introduced. They can be the valuable indices to the origin of the natural gas. 3) The chemical properties of the natural gas in the various fields are considerably different and the mature evolution of the natural gas is the very interesting problem. 4) The geothermal distribution in the geologic structures are investigated. 5) The reservoir pressure distribution or the pressure gradient are examined. The causes of the abnormal reservoir pressure are considered.