The Kitakanbara Plain, the northern part of Niigata Prefecture, has recently become active in oil and/or gas exploration. In this district, five gas fields were discovered and simultaneously, five well penetrated into the basement complex. Additionally, geophysical prospectings have covered almost entirely this district. Kanbayashi SK-1, an exploratory well of JAPER, reached to the basement granite at the depth of 1, 067m. Surface of the basement matches to the seismic reflections which are flat or gently inclining toward east opposing to the shallower, westward inclining reflections. According to these evidences and referring to the surface geologic data of surrounding hilly lands, the writer went over to scrutinize the preceding seismic records and he made up a structure map of the basement surface. In this region, a fault system which is composed of faults trending north to south and northwest to southeast (or west to east) is remarkable. In the central part of this plain, a basement up-lift was formed by normal faults on both sides of it and is ranging from north to south, but it is intersected by northwest-southwest (or west-east) faults. This fault system may have influenced on the sedimentation of the Tsugawa formation. Volcanic activity of Tsugwa and Nanatani steges and structural development of the Neogene formations may have been controlled by this basement fault system too. The writer tried to explain briefly the geologic develoment of this district.
Characteristics of natural gas found in Chuo-yutai, Niigata, Japan are contorolled mainly by geologic factors; particularly it was shown that the ratios of i-C4H10/n-C4H10 from the definite geologic horizons are almost similar in values. Judging from the data on Table 2, the effects of sampling pressure and sampling tempereture on the composition of natural gas, especially on the ratios of i-C4H10/n-C4H10, seem to be insignificant. The writer supposes that the primary migration and accumulation of natural gas out of its source sediments have occured at a relatively early stage of compaction after the deposition of the sediments.