Genetic Types And Source Correlation Of Oil And Gas In Baiyun Sag,the Pearl River Mouth Basin

The genesis types and parent material sources of crude oil and natural gas in the Baiyun Depression have been a hot research topic.In the paper,the crude oil was classified into four categories A,B,C,and D using C₂₇R-C₂₈R-C₂₉R and OL-W-T distribution fingerprint differences in pentacyclic triterpenoids.The geochemical characteristics,biogenic composition,formation environment,and origin of the four types of crude oils were analyzed using the rule sterane and hopane distribution characteristics,W,T resin compounds,tricyclic terpene ratio,Pr/Ph,Pr/n C₁₇,Ph/n C₁₈,γ/C₃₀H,content of homohopane,ratio of hopane to sterane（H/S）,CPI,OEP,and the stereoisomer ratio of sterane and hopane.It is believed that crude oil of category A is mainly input from terrestrial higher plants,formed in the lake-phase low-salt freshwater weakly oxidizing environment,and originates from the hydrocarbon source rocks of the eastern or Mainwa North Enping Formation;Crude oil of category B is mainly from lower aquatic sources,supplemented by terrestrial higher plants,formed in the lake-phase low-salt freshwater highly oxidizing environment,and originates from the hydrocarbon source rocks of the Dongwa Enping Formation,with moderate contribution from the Dongwa Zhuhai Formation;Crude oil of category C is input from terrestrial higher plants and lower aquatic organisms together,formed in the low-salt freshwater weakly oxidized environment in the lake phase,from the hydrocarbon source rocks of the Mainwa North Enping Formation;D crude oil is mainly input by planktonic algae,formed in the low-salt freshwater highly oxidized environment in the lake phase,from the hydrocarbon source rocks of the Nanwa Zhuhai Formation,or with contribution from the Wenchang Formation.The natural gas of Baiyun depression was classified into coaliferous gas,petroliferous gas,and coaliferous-petroliferous mixed gas based on the components,carbon isotopic and hydrogen isotopic characteristics of natural gas.Further study according to the degree of thermal evolution concluded that the natural gas in all the areas has the characteristics of kerogen cracking gas except for the gas in Nanwa which is crude oil cracked gas.The Baiyun Xiwa is considered to be coaliferous gas,which comes from type III kerogen and has the characteristics of kerogen pyrolysis gas.Baiyun North Slope is condensate-associated gas and coaliferous gas,from Type II to Type III kerogen.The northeast part of Baiyun is condensate-associated gas and coaliferous gas,with a source biased toward type-III kerogen.Dongwa is coaliferous gas with a source bias toward Type-II kerogen.Dongwa South is mostly condensate-associated gas and coaliferous gas,and a small portion is a crude oil-associated gas,originating from Type II～Type III kerogen.Nanwa’s petroliferous gas is associated gas from crude oil cracking and its source is biased towards Type I kerogen.The distribution of different genetic types of oil and gas in Baiyun depression is controlled by the source and distribution of hydrocarbon parent material,and also by the structural control of the rift basin in Baiyun depression.The distribution of type A and C crude oil is controlled by the lateral spreading of tectonic ridges,while the distribution of type B and D crude oil is controlled by the vertical transport of fractures.Finally the four types of crude oil are gathered in favorable tectonic traps to form reservoirs.