Geochemical Characterization of Oils in Yabase Oil Field and its Geological Significance

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The purpose of this study is to acertain the origin of crude oils accumulated in the Katsurane Formation in the Yabase oil field by geochemical characterization.The characterization of crude oil types is carried out on light hydrocarbon components in the range C4 through C8 and heavy normal alkanes in the range C14 through C34. For the light hydrocarbon components a four-parameter procedure is used. It includes (1) determination of the straight chain, branchedchain and cyclic constituent in C7-compounds; (2) the order of relative concentration in C7-compounds; (3) the change of n-heptane/1 trans 3-dimethylcyclopentane ratio with depth; and (4) the correlation between dimethylpentanes and dimethylcyclopentanes. It has been revealed by this procedure that crude oils in the shallow zone are obviously different from those in the deep zone in the Yabase oil field. The boundary between the shallow and deep zone is located in the middle of the Funakawa Formation, and it coincides with an abnormal pressure zone.In contrast with light hydrocarbons, the distribution of heavy normal alkanes show no difference between the shallow and deep zone, and also fairly maturation.From these results the origin of oils in the shallow zone is concluded to be different from those in the deep zone, and therefore the shallow oils seem to have migrated from the adjacent down-dip area where burial depth is deep enough to originate oil.
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