ORGANIC GEOCHEMISTRY OF PETROLEUM FROM THE GREEN-TUFF BASIN, JAPAN

Stable carbon isotopic composition and distribution of saturate hydroc arbons were analyzed for oils accumulated in volcanic (VR) and sedimen tary (SR) rocks from the Green Tuff basin of Northeast Japan in order to investigate their origin. The average deltaC-13 values relative to PDB of the whole phase and saturate hydrocarbon fraction are identical for both the VR and SR oils within experimental error. They lie withi n the range of deltaC-13 of kerogen in the sediments from the same bas in. The distributions of saturate hydrocarbons of the VR and SR oils a re similar to each other, and not distinguished from that of a normal marine oil. They include n-alkanes as major components, and acyclic is oprenoid alkanes such as pristane and phytane as the next abundant com ponents. These data indicate that the VR oils are biogenic, i.e. pyrol ysis products of kerogen just like the SR oils. The abiogenic scenario assuming that the VR oils alone were derived from mantle carbon by th e Fischer-Tropsch reaction is not plausible.