The Tokyo Gas Field

Abstract

The Tokyo gas field was found in 1951 by geological-geochemical inference.
The gas dissolved in formational water is accumulated in the upper Pliocene marine formations of gentle structure overlain unconformably by the lower Pleistocene coarse sediments.
The gas is produced with connate water. The gas-water ratio is roughly same with methane solubility into saline water under the corresponding pressure, and temperature.
Where the gas-water ratio is high, the connate water is highly saline. This saline water is fossil sea-water under reduced condition, rich in NH₄⁺, NCO₃^- and KMnO, consumption, and scarece of oxides, such as NO₂^-, NO₃^-, SO₄⁼, and dissolved oxygen. Where the chlorosity of the connate water becomes low (generally 5 g/l>), the gas-water ratio often takes under saturation value. When the chlorosity is smaller than 3g/l, the connate water becomes brown-coloured by dissolved humic acid.
Thus, the gas potentiality correlates with the chlorosity of connate water, and shows no direct connection with stratigraphic horizon, geologic structure nor depth from the surface.
Where the gas-water ratio is high, free gas always contains methane in 97-98 % and a litlle of nitrogen and carbon dioxide, however, methane content decreases by relative increase of nitrogen in the case of undersaturated gas-water ratio.
The gas reservoirs are sands intercalated in muddy formations. The main reservoir is named “Kõtõ sands” which consists of fine sand with intercalation of conglomeratic sand and mudstone. Although the MI6 sands contain abundant fragments of molluscan fossils of (upper) neritic habitants and foraminiferal assemblage of Pseudononion japonicum ASANO (a recent representative for neritic fauna off the coast of Pacific side of south-west Japan), the mudstone facies under G or H marker (Fig. 1) shows bathyal microbiofacies represented by Bulimina cf. indata d'ORBIGNY assemblage. And the lithofacies and thicknes of the Kõtõ sands is remarkably variable being thined and merged into alternation facies towards east. The KeitO sands might be formed by transportation of neritic coarse sediments into deep water.
In the main producing area, Kõtõ district, the productivity indices of wells for the Kõtõ sands are larger in west and south (over 1, 000 kl/d/kg/cm²) and become smaller towards east (few hundred kl/d/kg/cm²). The reservoir pressure has been lowered with increase of cumulativewater production, and the lowest part coincides with the area where the daily production has been large.
The fact that the average gas-water ratio is nearly constant despite of gradual lowering of reservoir pressure, indicates that the Tokyo gas field may belong to a ordinary gravity flow type. And this gas field merely occupies a western end of the South Kanto gas field which is one of the largest gas field of Jasan.