2016 Volume 81 Issue 2 Pages 159-168
Three linear zones of oil fields are distributed in the Nisiyama Oil Belt, Niigata Prefecture: (1) the Gomoto, Amaze and Ishiji fields in the western part of the belt, (2) the Miyakawa field in the central part, (3) the Betsuyama, Nagamine and Takamachi fields (the Nishiyama field as a general term) in the eastern part.
Results of head space gas (HSG) analyses in the Nozumi exploratory well (Nozumi SK-1D), Teradomari area, indicate the presence of two horizons of oil showings that were not detected at the wellsite, and this indicates that two compartments exist in the lower part of the Teradomari Formation, and two in the upper part of the Nanatani Formation. The gas maturity of two oil showings is estimated to be about 0.8% and 0.9% Ro equivalent as estimated from a δ13C2-δ13C3 plot.
Oil and gas at the surface and at shallow depth in the Nishiyama Oil Belt, including Teradomari area, are partly biodegraded, as evident in ethane that has C2<C3 and δ13C2>δ13C3. The geochemical maturity parameters of oils from oil fields in the belt indicate that the maturity of the Nagamine and Takamachi oils is about 0.7%, while that of the Amaze oils is slightly higher (0.8% and more), similar to the gas maturity in the Nozumi well.
Although vitrinite reflectance (Ro) of the lower part of the Teradomari Formation in the well, which is affected by a local heat source, is locally>1%, the lateral migration of oil and gas is dominant due to the presence of vertical compartments in the well. A comprehensive interpretation of the vertical distribution of oil and the cumulative production volumes in oil fields, combined with the geological history of the Nishiyama Oil Belt, shows that oil and gas in the belt were generated in and around the deeper part of the Nagamine and Miyakawa oil fields, and migrated vertically along faults before accumulation.
