The Genetic Type and Geochemical Characters and Their Significance of the Condensates in China
摘要: 近十几年的勘探实际表明，在我国一些含油气盆地中，许多天然气藏均伴生一定量的凝析油（或轻质油）。按母质类型可将凝析油分为海相腐泥型有机质生成的凝析油、煤系地层腐殖质有机质生成的凝析油、陆相混合型有机质生成的凝析油。本文较详细地从成因机理上讨论了这三种不同成因凝析油的形成特点。海相腐泥型有机质一般在有机质热演化达到高成熟－过成熟阶段，由干酪根或早期形成的高分子液态烃热裂解才形成凝析油。煤系地层中腐殖型有机质从低成熟到过成熟各个阶段所生成的一定量的液态烃一般主要以凝析油的状态与天然气相伴生。陆相混合型有机质从低成熟到过成熟阶段也都可能形成凝析油。利用凝析油单体烃的组份特征，石蜡指数和庚烷值对我国主要含油气盆地凝析油的成熟度进行了计算，所得结果，大都与地质实际相吻合。研究表明我国凝析油成熟度的分布范围从低成熟到过成熟均有。三种不同成因的凝析油其地球化学特征具有一定差异，煤系地层腐殖型有机质形成的凝析油从组份来讲相对富含芳烃，其芳烃含量一般为１６.２～２３.５%，饱和烃/芳烃值为３.２～５.２，同时其姥鲛烷/植烷值相对较高，通常均大于３；而海相腐泥型有机质生成的凝析油则相对富含饱和烃，其饱/芳比值为１１.８～１８.２，同时姥蛟烷/植烷值较低，一般小于1。陆相混合型有机质生成的凝析油则介于二者之间。三类不同成因的凝析油它们的芳烃组份(蔡系、联苯系和药系)和低分子生物标记物(菇类化合物)以及烷烃和芳烃组份的碳同位素组成都具有明显的差异。利用凝析油的这些地球化学特征可以有效地判识凝析油的成熟度和成因类型，从而确定与之相伴生的天然气的成因或来源。Abstract: During recent decades exploration, it has been proved that a cerium amount of condensate (or light oil) exists in many natural gas reservoires from most of the oil/gas bearing bssins.The condensate (or light oil)can be divided into three types according to their organic matter in source rock, i.e.,the condensate formed by sapropel organic matter of marine factes, the condensate originated from humic organic matter in coal-bearing strain, and the condensate generated by mixed organic matter of terrestrial sedimentary rock.Detailed discussion is presented in this Paper on the foraging mechanic characteristics of the three types of condensates which originated from different ways.The condensate generated from marine sapropel organic matter is generally formed by the splitting of kerogcn or the high molecular liquid hydrocarbon formed formerly, only the thermal evolution reaches high maturation to over maturation stage (the range of R0 is about 1.3% to 2.0% ).In coal-bearing strata, from immaturation (R00.6%) to over mauration stages, the liquid hydrocarbons formed by the humic organic matter generally are in the from of condensates (ot lightoil) and accompany with natural gases.The condensate might also be formed low to high maturation stage by the mixed organic matter of terrestrial source rock.In order to understand the maturation of the condensates from different bssins in China.The paraffin index and heptane values are calculated by the compositions of condensates.The maturation of the condensate which are estimated by paraffin indexI and heptan values are almost in line with the real geological settings, which shows that the thermal evolution of source rock for the condensates in China ranges from immature to over mature.Some differences are found among the three types of condensates on the aspect of geochemicalcharacteristics.As for their composition,the condensate formed by the humic organic matter of coalbearing strata is relatively rich in aromatic hydrocarbons which is generally 16.2% to 23.5%,and saturation/aromatic ratios range from 3.2 to 5.2, morever, its Pr/Ph ratios are comparatively high (generally higher than 3).The condensalte formed by the marine sapropel organic matter is relatively lower than 1.The mentioned ratios of condensate formed by terrestrial mixed organic matter range between the former two types.The composition of aromatic fraction (naphthanten series, bi-benzene series, and fluoreneseries),low molecular bimarkers (terpane), carbon isotopic composition of alkane and aromatic hydrocarbons are obviously different among the three type of condensates.Based on the above geochemical characteristics the maturity and genetic type of condensate can be identified effectively,and finally to confirm the origin and source of the natural gase associating with them.
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