The subsurface structure in the northern part of the Yurihara oil and gas field is composed of east and west blocks by a high angle fault running north and south. Stable carbon isotope ratios of whole oils and biomarker compositions show no differences among crude oils from the various formations (the Lower-Tentokuji, Onnagawa and Nishikurosawa formations). This suggests that the all oils in the Yurihara field have same origin. Carbon isotopes of saturate and aromatic fractions indicate that the oils of east block are generated at higher maturity level than the oils of west block. The results of methane and ethane carbon isotopes of DST gases also suggest that the gases of east block is generated at higher maturity level than those of west block.
On average, the Onnagawa Formation has the highest potential for hydrocarbon generation based on the results of Rock-Eval and TOC analyses. However, within the Onnagawa Formation, the source rock potential varies considerably with depth, and only a part of the Onnagawa Formation has high potential for oil generation. Correlation of oil to source rock by carbon isotope ratios also suggests that the limited parts of the Onnagawa and Funakawa formations are the probable source for oils. Based on depth variation of the isotope, Rock-Eval and TOC data in each well, source rocks from the lowest part of the Funakawa Formation and the upper part of the Onnagawa Formation are best correlated to the oils in the Yurihara field.
From the study of maturity and subsurface structural history, we assume that the hydrocarbons were generated in the large syncline between the Yurihara and Innai structures and migrated to the Yurihara field across a growth reverse fault.
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