Origin of oil and gas-bearing volcanic rocks recovered by deep drilling in the Niigata oil and gas fields, northeast Japan : with special reference to Sr and Nd isotopic research

One determination of Sr and Nd isotopic ratios of Early to Middle Miocene volcanic rocks recovered by deep drilling at twenty-six wells in the Niigata oil and gas regions demonstrates that the NdI (initial^143Nd/^144Nd ratios) of basaltic rocks collected from each drilling well vary only slightly, whereas the SrI (initial^87Sr/^86Sr ratios) of these rocks range widely and deviate systematically from the fields of the source mantle in SrI-NdI diagram.In addition, thirty samples (basaltic andesite, andesite and more felsic volcanics) collected from eight drilling wells in the southern part of the regions have nearly constant NdI of around 0.51275, but these rocks have SrI ranging widely from 0.70554 to 0.70851, indicating changes of SrI due to alteration by seawater for these volcanic rocks which were produced from basaltic magma by fractional crystallization. The present results indicate that the alteration of underground oil and gas-bearing volcanic rocks by seawater caused significant increases of ^87Sr/^86Sr ratios, whereas the ^143Nd/^144Nd ratios remained virtually unchanged. Rhyolitic rocks from three drilling wells in the northern part of the regions show significantly higher SrI values (0.70727 to 0.70914) and lower NdI values (0.51247 to 0.51265) than the recovered basaltic rocks, indicating a lower crust origin for these rhyolitic rocks