Geochemical Characteristics and Distribution of Mesozoic Hydrocarbon Source Rocks in the Southern Part of East China Sea

In order to study the distribution characteristics of Mesozoic source rocks in the Southern part of East China Sea,the drilling data,seismic data and geochemical data of hydrocarbon source rocks have been used fully.Based on comprehensive analysis of the depositional conditions and geochemical characteristics of Mesozoic hydrocarbon source rock,the effective thickness of Mesozoic source rock and planar distribution characteristics of maturity have been forecasted by using Trinity basin simulation software and constrained by the study results of regional sedimentary facies,geochemical index of source rock and statistical data of dark mudstone percentage content.The results show that influenced by marine (or lake) transgression,the middle-lower Jurassic Fuzhou Formation and the Lower Cretaceous semi-deep to deep lake facies source rock are distributed widely,with large thickness and good hydrocarbon generation conditions,of which Fuzhou Fm.source rocks have been confirmed by drilling,being the main source rocks in the study area.The Fuzhou Fm.source rocks consist of dark mudstones and coal,with high organic matter abundance,and the organic matter is dominated by type Ⅲ,with minor type Ⅱ₂,being mature to highly mature,evaluated as good quality hydrocarbon source rock,distribute mainly in south of Minjiang depression and Jilong depression.The dark mudstones range from 200 to 760 m in thickness,thicker in south and thin in north.The maturity increases from northwest to southeast direction gradually,with vitrinite reflectance values (R₀) ranging from 0.5% to 3.5%.The Lower Cretaceous dark argillaceous source rocks are the secondary Mesozoic source rocks in the study area.This understanding expands a new field for oil and gas exploration in Mesozoic in southern part of East China Sea.The Lower Cretaceous dark argillaceous source rocks are mature,being moderate to bad quality,with low organic matter abundance,and the organic matter is type III.The Lower Cretaceous source rocks distribute widely in Minjiang Depression and Jilong Depression.The dark mudstones range from 50 to 450 m in thickness,thicker in east and thin in west.The maturity increases gradually from west to east direction.