Abstract
Our geochemical study on the gases from the Sagara oil field and its surrounding areas is summarized as follows.
1) The gases are of a thermogenic origin partly mixed with a small amount of microbial gas and are slightly biodegraded.
2) The δ13C1-δ13C2 plot of the gases shows that the maturity of the gases is estimated to be about 1.0% equivalent Ro and that the source rock for the gas from the MITI Omaezaki-oki well is different from the others.
3) The maturity of the Sagara oils estimated by light hydrocarbon and methylphenanthrene maturity parameters may be about the same as the maturity of the gases except for the Omaezaki-oki gas, which is more mature.
4) Geochemical characteristics of the oils and gases integrated with the results of the source rock analysis and the model simulation, reported by Ueda et al. (2007), indicates that the Sagara oils were probably generated in the Setogawa Group and migrated upward through the Ooigawa Group along faults. The Omaezaki-oki gas may be derived from the Mikura Group with long migration. The thermogenic gases absorbed in the Lower Miocene and older strata from the METI Nankai Trough well may be a mixture of gases generated in both the Mikura and Setogawa Groups.
