开平凹陷古近系稠油和砂岩储层沥青质成因分析

Genesis analysis of Paleogene heavy oil and asphaltene in sandstone reservoir in Kaiping sag

通常情况下埋深大于2 000 m的油藏中原油受次生改造作用影响较小,但珠江口盆地首次在开平凹陷埋深大于2 000 m的古近系发现了稠油油藏和大量沥青质砂岩层。为揭示该凹陷区稠油和砂岩储层沥青质的形成机制,开展了原油和岩石样品系统取样和地球化学测试分析,结合凹陷埋藏-热演化史、油藏地质特征等分析,对典型稠油油藏和沥青质砂岩层进行了深入解剖。研究表明,10 Ma左右开平主洼文昌组淡水中深湖相烃源岩生成的油气自深部向浅部依次充注到恩平组、珠海组圈闭中聚集成藏,成藏后遭受生物降解、水洗和汽油馏分烃散失等次生改造作用,使得原生成因的中质油藏原油变重变稠,从而形成了现今次生成因的稠油油藏和大量沥青质砂岩层。开平凹陷不具备生成原生成因稠油的石油地质条件,开平主洼文昌组淡水中深湖相烃源岩主要生成原生中质油和轻质油,因此洼陷内深部构造-地层圈闭具有中质原油和轻质原油的勘探潜力。

Usually crude oil is less prone to secondary reconstruction in oil reservoir at buried depth below 2000 m.However,heavy crude oil accumulations and numerous asphalt sandstone layers were discovered for the first time at a buried depth below 2000 m in paleogene in Kaiping sag in Pearl River Mouth Basin.In order to uncover the formation mechanism of heavy oil and asphaltene in sandstone reservoir in this sag,systemically collection and geochemical test and analysis of oil samples and rock samples were carried out.In combination of the analysis on buried-thermal evolution history and reservoir geologic characteristic,typical heavy oil reservoir and asphalt sandstone layers were studied in depth.Studies show that hydrocarbon generated from source rocks of middle and deep lacustrine facies in fresh water of the Wenchang formation in Kaiping sag at about 10 Ma flowed into Enping and Zhuhai formations from down to up to form accumulation.The original medium oils in the accumulations were secondarily altered by biodegradation,water-washing and gasoline fraction dissipation,forming current heavy oil reservoirs and numerous asphalt sandstone layers of secondary origin.The Kaiping sag does not have the geological conditions required for forming primary heavy oil,while source rocks of middle and deep lacustrine facies in fresh water of the Wenchang formation in Kaiping sag mainly generate primary light-medium oil.Therefore,deep structures and stratigraphic traps in Kaiping sag have the exploration potential of light-medium oil.