Judging from the complicated structures of the pre-Miocene rocks, they are treated as the basement rocks. The Yorii conglomerate, which has been generally considered to be an Aquitanian formation, must be correlated with the Cretaceous Atokura formation, and then, it is a member of the basement rocks.
The Miocene formations of this area are time-stratigraphically divided into four stages, that is, M₁, M₂, M₃ and M, from lower to upper. The M_1a stage is subdivided into two substages, M_1a (lower) and M_1b (upper). The rocks of M_1a substage are characterized by molluscan and foraminiferal faunas, which contain proper Miocene and survived Oligocene elements, and Aniai flora. On the other hand, the rocks of M_1b substage are characterized by the first occurrence of the Lepidocyclina-Miogypsina fauna and Daijima flora, and the Oligocene elemens of Mollusca and Foraminifera are rare in these rocks. The rocks of M₂ stage are characterized by the Kadonosawa and Shirikonai molluscan faunas -of proper Micene type, and some of these contain Lepidocyclina, Miogypsina, Desnzostylus and Cornwallius. The rocks of M₃ stage are also characterized by the Kadonosawa molluscan fauna, but Lepidocyclina has not been found in these rocks. The lowermost horizon of this stage is marked by the Miogypsina-Operculina fauna and pyroclastic rocks. The rocks of M₄ stage are characterized by the Higashi-Tanagura and Zushi molluscan faunas. These faunas contain both Miocene and Pliocene elements. Globigerina nepenthes has been found in some rocks of this stage.
Judging from Kasukabe GS-1, a stratigraphic well, the Kazusa group in the middle part of the Kwanto plain area contains two sedimentary cycles corresponding to lower and middle parts of this group in the type area. In the northern part of the Kwanto plain area, the Kazusa group contains only one cycle, that is, the upper cycle in Kasukabe GS-1. In this well, the Pleistocene formation between Kazusa group and Alluvium is very thick, but this formation is considered to represent only one inter-glacial age, that is, Mindel-Riss.
In the Kwanto area, six diastrophic phases since middle Tertiary have been recognized by the writers. Of these phases, the Kadono, Fuji-Yoshida, Higashi-Matsuyama and Kurotaki phases correspond respectively to Savian, old Styrian, new Styrian and Rhodanic phases in Europe. The ages of remaining two phases, that is, Minami-Tama and Kasukabe, are middle Pliocene and Mindel-Riss inter-glacial age respectively. The distribution of sea and land in this area had been largely changed by the crustal deformation of the Higashi-Matsuyama phase.
In the Kwanto area, all developed gas reservoirs except those of Mobara type belong to normal dissolved-in-water type. Permeability of reservoirs is generally lower than that in the Niigata gas-field, and then, the wells of large diameter are not always favourable for the southern Kwanto gas-producing region.
In the Mobara type gas deposits, gas-water ratio rapidly increases with the progress of devel-opment. Development methods suitable to natural condition are especially important in case of Mobara type gas deposits. It is also very important to clarify the extension of reservoirs of this type. On the other hand, detailed gravimetric survey and facies studies are effective for exploration of the normal dissolved-in-water type gas deposits. As to Miocene formations, we cannot expect too much of free gas pools in the region except the area of thick lower and middle Miocene deposits.

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According to Dr. Ishiwada and present writers (1962), the Uvigerina akitaensis assemblage zone which makes practically the top of the Umegase formation disappears in the west side area of Chiba City and is replaced by sediments characterized by the cold shallow-water fauna.
In the Funabashi FR-18, the top of Umegase formation lies at 665 m below the surface and the shallow-water fauna appears at the Otadai and upper formations. From these evidences, the Kazusa group changes lithofacies and biofacies from southeast to northwest.
Judging from lithofacies, biofacies and sandstone percentage of the Kazusa group, two types, that is, primary and secondary, are recognized in its mode of sedimentation of sand. The secondary transported sands stand in some relation to gas potentiality because of their equigranular texture.

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Here we discuss the basic conception of development of the so-called “Disolved Gas in Water” field in Chiba Gas Field, using the data of Yotsukaido, Naruto gas fields and other exploratory wells, although they are essentially the same as the common gas field in other districts.
The pressure drop of gas reservoir is a very serious problem and may make it difficult to pump out water economically, though theoretically possible. The rise of G/W which is the characteristic phenomenon only seen in the southern part of Kujukurihama district is very interesting.

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In the Mobara gas field, production and gas water ratio histories of the Umegase and Otadai formations are highly indigenous, as compared with the neighbouring fields. The initial gas water ratio starts at 3-4, but after considerable cumulative water production, the gas water ratio rapidly increases up to 10, or, in some cases, higher than 300.
The cause of this peculiar performance may be derived from the following reservoir conditions.
1) Low permeability.
2) Initial gas water ratio is higher than solubility at reservoir pressure and temperature.
3) Shallow depths of reservoirs.
4) Frequent alternation of sand stone and mudstone.
5) Abundant faults.
In this paper, concerning above mentioned five conditions, the areal limitation of the reservoirs with this peculiar performance is discussed.

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以上述べたことを要約すると次の通りである。白里・白子ガス田は,昭和31年より開発の緒についた。当地域では深層(特に黄和田層)も有力な水溶型鉱床であることを明かにした。また,浅層(梅ケ瀬層,大田代層)が,開発の進展に伴い“高ガス水比型”に移化して行くことを確めた。
開発は,第1期(深層開発-海岸地区),第2期(深・浅層混合開発-虎橋地区),第3期(浅層開発-牛込地区)に分けて行つた。生産ガスは,房総半島を横断し東京湾に達する長距離,高圧パイプラインにより輸送することに初成功した。
次に,深・浅坑井混合開発地区に発生した異状現象-浅坑井(高ガス水比型)においてガスの激減,ガス水比の低下,水温の上昇-につき言及した。その原因を追求するために各坑井の坑底圧, 坑内温度を測定し考察を進めた。
すなわち,生産の進展に伴い,深層および浅層の層圧が変化を起し,互に圧力差を生じ,高圧の深層より,浅層にかん水が侵入したものと判断した。
深・浅坑井の開発地域においては,坑井の仕上げを入念に行なう必要がある。

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