Search

Widespread exploration for oil and gas in the Niigata oil and gas fields, involving extensive drilling, has shown that Early to Middle Miocene volcanic rocks bearing the petroleum and natural gas are extensively developed, and they have been buried at depths greater than 4,000-6,000m since the Middle Miocene. We determined Sr and Nd isotopic ratios of volcanic rocks recovered by deep drilling at thirteen wells in the fields. The present and our previous data demonstrate that these deep-seated basaltic rocks older than about 16 Ma have lower NdI (initiaI^143Nd/^144Nd ratios) compared to those younger than 16 Ma. The former NdI (0.51264 to 0.51284) are within the Nd isotopic range of an undepleted source mantle shown by NdI of basaltic rocks older than about 15 Ma outcropped at the land surface of the back-arc margin of the NE Japan arc, whereas the latter NdI (0.51285 to 0.50302) within the Nd isotopic range of a depleted source mantle shown by NdI of basaltic rocks younger than about 15 Ma from the back-arc margin of the Japan arc. The NdI 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 areas of the undepleted and depleted source mantle in SrI-NdI diagram. In addition, thirty samples, older than about 16 Ma, including basaltic andesite, andesite and more felsic volcanics, collected from eight drilling wells in the southern part of the fields 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. Consequently, the present results imply 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 older than about 16 Ma collected from three drilling wells in the northern part of the fields show significantly higher SrI values (0.70727 to 0.70914) and lower NdI values (0.51247 to 0.51265) than the recovered basaltic rocks prior to about 16 Ma, indicating a lower crust origin for these rhyolitic rocks.

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.