柴达木盆地东部上石炭统海陆过渡相烃源岩生烃特征:来自天然气地球化学的证据

Hydrocarbon generation characteristics of the Upper Carboniferous marine-continental transition source rocks in the eastern Qaidam Basin:Evidences from natural gas geochemistry

推进柴达木盆地东部(以下简称柴东地区)上石炭统油气勘探进程亟待明确其烃类地球化学特征、赋存状态及其生成过程。为此,通过天然气组分与碳同位素测试,探讨了柴东地区欧南凹陷2口关键井上石炭统天然气的成因与来源,在此基础上厘清了上石炭统海陆过渡相烃源岩生烃过程及烃类赋存状态。研究结果表明:①上石炭统天然气中烃类气平均含量为94.51%,非烃气平均含量为5.49%,干燥系数介于0.28~1.00,干气为主,湿气仅发育于上石炭统浅层,深层天然气干燥系数较高;②浅层气甲烷碳同位素明显偏轻,乙烷碳同位素和二氧化碳碳同位素分布范围大,低碳烷烃以腐泥型干酪根初次热解为主,早期产出的液态烃经微生物降解形成生物甲烷,高碳烷烃与原油裂解作用密切相关,腐殖型干酪根初步热解的煤成气多以吸附态形式赋存;③深层低碳烷烃为裂解反应较充分的原油裂解气混入部分煤型游离气,高碳烷烃可能与腐泥型干酪根二次活化裂解有关;④天然气中二氧化碳不仅存在干酪根热降解有机成因类型,也发育碳酸盐矿物化学反应释放的无机成因类型。结论认为,上石炭统烃源岩混合了海洋、陆地两种环境来源的生物有机质,海相腐泥型有机质因热化学活化能低而优先进入生烃门限生成液态烃及伴生油型湿气,原油裂解和陆相腐殖型干酪根生气在高成熟阶段逐步启动,在过成熟阶段2种生气机制的作用愈发突显,残余腐泥型干酪根也因高温条件发生二次活化裂解并生成少量高碳烷烃。

In order to promote the exploration of Carboniferous oil and gas in the eastern Qaidam Basin,it is in urgent need to clarify the geochemical characteristics,occurrence state and generation process of the hydrocarbons there.In this paper,the origin and genesis of the Upper Carboniferous natural gas produced by two key wells in the Ounan depression of the eastern Qaidam Basin are discussed by testing the components and carbon isotopes of natural gas.On this basis,the hydrocarbon generation process and hydrocarbon occurrence state of the Upper Carboniferous transitional source rocks are clarified.And the following research results are obtained.First,the Upper Carboniferous natural gas has an average hydrocarbon gas content of 94.51%,an average non-hydrocarbon gas content of 5.49%,and drying coefficient ranging from 0.28 to 1.00.Dry gas is predominant,and wet gas is only developed in the Upper Carboniferous shallow layer.The drying coefficient of deep gas is higher.Second,the carbon isotope of shallow methane is much lighter,and that of ethane and carbon dioxide change extensively.Low-carbon alkane is mainly the primary pyrolytic product of sapropelic type kerogen.The early liquid hydrocarbon is biodegraded into biomethane.Highcarbon alkane is closely related to the cracking of crude oil.Moreover,the coal derived gas from the primary pyrolysis of humic type kerogen is mostly in the adsorption state.Third,the deep low-carbon alkane is crude oil cracking gas through sufficient cracking reaction,mixed with some free-phase coal gas,and the high-carbon alkane may be related to the secondary activation cracking of sapropelic type kerogen.Fourth,in the natural gas,there are not only organic types of carbon dioxide from kerogen thermal degradation,but also inorganic carbon dioxide released from carbonate minerals through chemical reaction.In conclusion,the Upper Carboniferous source rocks contain marine and continental biological organic matters.Due to low thermochemical activation energy,marine sapropelic organic matter has the priority of entering the hydrocarbon generation threshold to generate liquid hydrocarbon and associated oil-type wet gas.Gas generation from crude oil cracking and continental humic type kerogen starts gradually in the high mature stage,and plays an increasingly prominent role in the over mature stage.Under high temperature,residual sapropel type kerogen undergoes secondary activation cracking to generate a small amount of high-carbon alkane.