北部湾盆地稠油地球化学特征及成因分析

北部湾盆地涠西南凹陷、乌石凹陷已发现多个稠油油田。为研究稠油特征和成因,开展了稠油油藏原油物性、组分、饱和烃特征、生物标志物、油气来源和盆地模拟分析。结果表明:(1)北部湾盆地稠油主要分布在凸起、斜坡带和近洼带,呈高密度、高黏度特征。(2)北部湾盆地存在3类稠油,第一类为凹陷中央流二段下部烃源岩生成的原油运移至圈闭成藏,遭后期抬升剥蚀,埋深小于2000 m,上覆盖层薄导致油藏遭受生物降解,此类原油成熟度高,C30-4-甲基甾烷含量高,组分遭受不同程度的破坏,饱和烃和芳烃成分有序缺失;第二类稠油主要分布在近洼带,为本地低热演化油页岩、页岩生成的原油,其成熟度低,Ts/Tm值较低,C30-4-甲基甾烷含量低,在近洼就近成藏,埋深处于生烃门限附近,为早期原生稠油;第三类稠油主要分布在斜坡带,主要为深洼流二段下部烃源岩生成的成熟原油和本地流二段上部烃源岩生成的成熟度较低的稠油混合而成,同时受运移、扩散、吸附等因素的影响,原油变稠,C30-4-甲基甾烷含量中等,此类油藏埋深大于3000 m,是未受到生物降解的混合型稠油。该研究成果对北部湾盆地优化勘探开发部署、指导油区勘探具有重要意义。

Geochemical characteristics and genesis of heavy oil in Beibuwan Basin

Several heavy oil fields have been found in Weixinan Sag and Wushi Sag of Beibuwan Basin. In order to study the characteristics and genesis of heavy oil,the physical properties, components, saturated hydrocarbon characteristics, biomarkers, oil and gas source and basin simulation analysis were carried out. The results show that:(1)The heavy oil in Beibuwan Basin is mainly distributed in uplift, slope zone and near depression zone, with high density and high viscosity.(2)There are three types of heavy oil in Beibuwan Basin. The first type is the crude oil generated by the source rocks in the lower part of the second member of Liushagang Formation in the center of the sag and transported to the trap for reservoir formation.Due to later uplift and denudation, the burial depth is less than 2 000 m. The thin overburden causes the reservoir to be biodegraded. This kind of crude oil has high maturity and high content of c30-4-methylsterane. The components were destroyed to different degrees and the saturated hydrocarbon and aromatic hydrocarbon components were lost in order. The second type of heavy oil is mainly distributed in the near-depression zone, which is the crude oil generated from the local lowthermal evolution oil shale and shale. It is characterized by low maturity, low Ts/Tm value and low contentof C30-4-methylsterane. The reservoir is accumulated in the near depression and buried in the near hydrocarbon generation threshold,and it is the early primary heavy oil. The third type ofheavy oil is mainly distributed in the slope belt, it is mainly composed of the mature crude oil generated from the lower source rock of the second member of Liushagang Formation in the deep depression and the low mature heavy oil generated from the upper source rock in the local area. Meanwhile,influenced by migration, diffusion, adsorption and other factors,the crude oil thickens. The content of C30-4-methylsterane is medium,and the buried depth of reservoir is more than 3 000 m. The mixed heavy oil is not biodegraded. The research results are of great significance for optimizing exploration and development deployment and guiding oil exploration in Beibuwan Basin.