Genetic Mechanism Of Marine Hydrocarbons In Western Tazhong

Western Tazhong is located in the southern margin of Awati-Manjiaer transitional zone,with great exploration potentials of Lower-Paleozoic marine hydrocarbons.In recent years,there are some breakthroughs such as tight sandstone oil reservoirs of Silurian in SH9 block,condensate gas reservoirs of Ordovician in Shunxi block and gas reservoirs of Ordovician in ST1 well.However,Western Tazhong is a new exploration area having many problems,and this paper concentrates on distribution and formation mechanism of Lower-Paleozoic marine hydrocarbons in multiple phases,by macrogeological and micro-chemical analyses.On one hand,it analyzes the characteristics of hydrocarbons in physical property,light hydrocarbon,saturated hydrocarbon,aromatic hydrocarbon and carbon isotope,to reveal genetic types and epigenetic changes.On the other hand,by analyses of fluid inclusions and thermal history,it shows the main hydrocarbon accumulation periods of typical reservoirs and analyzes their accumulation processes.The results show that,these high maturity oils and high-over maturity gases have similar parent materials,derived from sapropelic and humic-sapropelic kerogen depositing in strong-reduced environments with high-salinity.According to biomarkers,the oils seem to be mixed,having the Cambrian-Lower Ordovician oils and the Middle-Upper Ordovician oils,and can be divided into three types.According to composition characteristics and carbon isotope,natural gases can be divided into three types.A type of that is estimated to be DLH(dispersed liquid hydrocarbon)-cracking gases,and the other two types give priority to kerogen-cracking gases,with different degrees of oil-cracking gases.The Ordovician gases in Tazhong mostly belong to the latter.In Western Tazhong,Lower-Paleozoic oils basically have no cracking and mostly have gas washing,showing that condensate oils are not residue from oil cracking and gases were mainly filled in Late-Hercynian.Combined with hydrocarbon accumulation periods and relationships between hydrocarbon generation and accumulation,it is inferred that Ordovician-Silurian hydrocarbons probably came from Cambrian-Lower Ordovician source rocks of the Manjiaer depression in Hercynian and possibly came from deep-oils cracking in Shunnan-Gucheng blocks or the Middle-Upper Ordovician source rocks.Considering oil-cracking degrees,genetic types of gases and burial history,deep dolomite reservoirs of Tazhong have good preservation conditions for liquid hydrocarbons and show great exploration potentials of condensate hydrocarbons.