The natural gases collected during production testing from nine oil and gas fields in the Niigata basin were characterized using parameters such as C2+, C2/C3, iC4/nC4, iC5/nC5, N2 and CO2. The chemical compositions of gases are related to the temperatures of the reservoirs from which the gases are produced. Chemical composition together with isotopic composition of methane suggests that the natural gases in the area are of thermogenic and mixed thermogenic/bacterial origins. The iC4/nC4 ratio is considered to reflect both the type of source rock and its degree of thermal maturation. In the Hirakida gas field the gases from the Nishiyama and Shiiya Formations may have undergone bacterial alteration.
The stratigraphical study of the Mio-Pliocene sedimentary rocks distributed in the Higashikubiki district, Niigata Prefecture, is important to make clear the geohistory of the northern Fossa Magna. These rocks are divided into the Sugawa, the Tamugigawa, the Higashigawa Formations and the Uonuma Group in ascending order. The Sugawa Formation consists of massive siltstone. The Tamugigawa Formation is composed of sandstone, mudstone and flysh type alternation of sandstone and mudstone. The Tamugigawa Formation is further subdivided into four members, that is, the Lower, the Middle, the Upper and the Shobu mudstone Members. The Higashigawa Formation is composed of sandy siltstone. These formations conformably superpose each other. The distribution of tephra marker beds in the Tamugigawa Formation indicates that the flysh type alternating beds of the Lower, the Middle and the Upper Members change laterally into the mudstone of the Shobu Member. Nine tephrozones are discriminated on the basis of the lithology and frequency of marker tephra beds. All tephra beds range in composition from rhyolite to dacite, except for the tephrozone VII, which is composed of andesitic tephras and especially effective to the correlation of Mio-Pliocene strata in this area. From the result of the F. T. dating of tuff layers and biostratigraphical data, each formation is correlative with the strata in the district of the Nishiyama Hill as follows; the Sugawa Formation with the Shiiya Formation, the Tamugigawa Formation with the Hamatsuda Formation, the Higashigawa Formation with the Nishiyama Formation.
The cause of sharp boundary between sand layer and mud layer amomg sand and mud alternation is explained from the hydrodynamical view point. In nature, gravel, sand, and mud are being separated by the difference of settling velosity and threshold velosity. This process causes the sharp boundary between sand and mud layers among sediment. Furthermore, ocean floor spreading theory, plate tectonics theory, accretionary wedge, the discovery of Oyashio ancient landmass beneath the deep sea terrace of japan Thrench and their significance are introduced.