Abstract
The Yabase oil field in the Akita Prefecture is the largest oil field in Japan, and it has produced approximately 35 million barrels of cumulative oil by the end of 1995. Relationship between characteristics of extracted steranes and mineral compositions was studied for the Neogene source rock samples from four wells in the northern part of the field. Gas Chromatography-Mass Spectrometry for saturated hydrocarbons from the rock extracts, X-ray fluorescence and X-ray diffraction analyses were carried out.
Diacholestanes/cholestanes ratio tends to increase westward among the four wells. Detritus content calculated from SiO2 and Al2O3 contents, and the total content of clay minerals also tend to increase westward. Therefore, positive correlation was observed between the detritus content and the diacholestanes/cholestanes ratio, and between the total clay content and the ratio. The diacholestanes/cholestanes ratio relates to thermal maturity and generally increases with increasing maturity. Positive correlation was observed between the ratio and 20S/(20S+20R) ratio of stigmastanes. The diacholestanes/cholestanes ratio is also influenced by the depositional environment of source rocks. The ratio will become more useful to the quantitative estimation of clay content in their source rocks, by taking the environmental influence as well as the maturity influence into account.
