Abstract
Isotopic studies for 20 gas samples and 17 water samples from three water-dissolved gas fields, the Nichinan, Miyazaki and Sadowara fields, in Miyazaki Prefecture, were carried out. Their results are summarized as follows.
1) Hydrocarbons are the major components in all gas samples. These hydrocarbons are chiefly composed of methane. The Kitago gas contains some carbon dioxide whoseδ13CCO2 value suggests a magmatic origin.
2) The δ13C values of hydrocarbons indicate that the origin of the gases in the Nichinan and Miyazaki fields is thermogenic. Because their maturity of the gases is estimated to be about 0.8% Ro equivalent based on the δ13C2-δ13C3 plot, they were probably generated in the Paleogene Nichinan Group.
3) Most of the hydrocarbon gases in the Sadowara field are microbial and mixed with a small amount of low maturity thermogenic gases which were supposed to be generated within the late Miocene to Pliocene Miyazaki Group.
4) Hydrocarbons in the Nichinan sample (Kitago) and one sample (Oyodo) of the Miyazaki field are biodegraded.
5) Formation waters in the Sadowara field are altered sea water trapped during deposition, whereas those in the Nichinan and Miyazaki fields are interpreted to be a mixture of meteoric water and fossil sea water.
6) Formation waters in the Sadowara field have relatively homogeneous 87Sr/86Sr ratios which are slightly lower than the ratio of sea water in the period of deposition.
