日本の油田地帯におけるノルマルアルカンと石油熟成の関係

抄録

Distribution of normal alkane hydrocarbons in sediments and crude oils of the oilfields in Japan were studied. Carbon preference indices of heavy normal alkanes (CPI) range from 1.41 to 3.00 for recent sediments, 1.01 to 6.41 for ancient sediments and 0.94 to 1.30 for crude oils.
CPI is generally reduced with depth and inversely related to hydrocarbon/organic carbon ratio and heavier gaseous hydrocarbon/methane ratio in the mudstones from drilled wells. The relation between CPI and hydrocarbon/organic carbon ratio is not reciprocally proportional but rather hyperbotal. During early compaction stage which releases quantities of water the ratio is apparently invariable in accordance with the reduction of CPI, while it increases proportionally with CPI reduction during middle compaction stage of mild dewatering.
At the transition point between early and middle compaction stages in MITI Hamayuchi, Hokkaido, distributions of normal alkane change to n-docosane or n-tricosane from n-heptacosane or n-nonacosane in their maximum frequencies. The depth and temperature of this transition point are about two kilometers and sixty centigrades respectively. This transitional point may suggests the initial stage of maturation of petroleum by thermal transformation from insoluble organic matters.