Controls on Compositional Heterogeneity of Oils in Jiuxi Basin
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摘要: 运用先进的分析测试手段对来自酒西盆地六个油田的 2 0多个油样进行了全油碳同位素、饱和烃气相色谱与质量色谱及正构烷烃分子碳同位素等较为全面的分析与研究。确定源岩相、水洗、生物降解作用与成熟度不是酒西盆地石油非均质性的控制因素。最后选取 1 5个油样进行中性吡咯氮化合物分析,用中性吡咯氮中的咔唑类化合物作为运移参数,较为详细地研究原油的运移分馏作用。此类化合物作为运移参数的基本原理是石油运移过程中,不同结构的咔唑类化合物与水及岩石中矿物发生吸咐作用的强烈程度不同,导致不同的运移分馏作用。根据咔唑类化合物的变化得出控制酒西盆地石油组成非均质性的主要因素是油气运移分馏作用,证实酒西盆地石油运移方向主要是从西至东,青西凹陷是其油源区。Abstract: The physical and chemical nature of petroleum changes with source rock and maturity,although the feature of oil and gas can also be influenced by local biodegadration or water washing.During the process of reservoir filling,some polar compound in crude oils,such as carbazoles which can interact relatively strongly with water,rock minerals and each other,and thus induce hetrogenous petroleum. Meanwhile, heterogenous migration pathways also influence the characteristics of oils. In the paper,authors discuss in detail the chemical nature of oil samples,which come from six oil fields in Jiuxi Basin.Among twelve oil samples,except one sample's carbon isotope(δ13Coil‰) of the whole liquid hydrocarbon (crude oil) is -30.5‰,the other's is about -31‰,whose absolute error is less than 0.2‰. Distillation mode of carbon isotope of N-alkane element series is also similar to each other.Based on GC and GC-MS analysis,seventeen conventional geochemical parameters are selected to calculate correlation coefficients between twelve oil samples,correlation coefficients of all samples are more than 0.86,most of them is up to 0.96.All of these parameters show that twelve oil samples come from the same source rock. Two star diagrams (multivariable plots in polar coordinates) have been developed to assess subtle chemical differences among oils.The diagrams are based on analysis of C7 hydrocarbon of crude oils separated by gas chromatography.The first C7 diagrams is used for distinguishing variation in oils caused by transformation,e.g. water washing,biodegradation and evaporation. The second C7-based diagram consists of four ratios composed of compounds that are very resistant to the effects of transformation.Two star diagrams of six oils representing different oil fields show water washing,biodegradation and other transformations are not the main factors to control heterogenous petroleum.Either is maturity,based on the molecular isomerization parameters of C29ααα sterane 20S/(20S+20R) as maturity parameter. At the end of the paper,pyrrolic nitrogen compound distrbutions of crude oil are studied.Pyrrolic nitrogen compounds in oil have been shoun to be very useful geotracers in predicting the direction and even the distance of oil secondary migration in the reservoirs.Alkylcarbazole series in the neutral pyrrolic nitrogen are just discussed,1,8-dimethylcarbazole/3,4-dimethylcarbazole,1,8-dimethylcarbazole/1,3-dimethylcarbzole,carbazole/methylcbazole and methylcarbazole/dimethylcarbzole are used as migration parameters.These parametes of different crude oils with similar other parameters have some changes.It is concluded that migration fractionation is the main factor to control heterogeneity of petroleum in Jiuxi Basin.At the same time,migration parameters verify that oils come from Qingxi sag and the direction of migration is from west to east.
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