鄂尔多斯盆地杭锦旗地区山西组-太原组烃源岩地球化学特征对比研究及意义

Comparative study and its significance of geochemical characteristics of source rocks from Shanxi Formation and Taiyuan Formation in Hangjinqi area of Ordos Basin

以鄂尔多斯盆地杭锦旗地区山西组太原组煤系烃源岩为研究对象,通过总有机碳测定、岩石热解分析、碳同位素测定及GC-MS分析等多种技术手段对两套烃源岩的地球化学特征进行了系统研究。结果表明,研究区山西组太原组烃源岩有机质含量均较高,有机质类型以Ⅲ型为主,生烃潜力好。其中山西组烃源岩整体生烃潜力较太原组强,而太原组烃源岩成熟度则较山西组高。通过将两套烃源岩生物标志化合物特征进行对比分析发现,山西组烃源岩具有高姥植比,高(C_(19)三环萜烷+C_(20)三环萜烷)/C_(21)三环萜烷值,高C_(24)四环萜烷、C_(29)规则甾烷、氧芴相对含量和低伽马蜡烷相对丰度的特征,反映其可能形成于氧化偏氧化的淡水环境,有机质来源以高等陆生植物为主;太原组烃源岩则具有低姥植比,低(C_(19)三环萜烷+C_(20)三环萜烷)/C_(21)三环萜烷值,低C_(24)四环萜烷、C_(29)规则甾烷相对含量,低伽马蜡烷相对丰度和高硫芴相对含量的特征,说明其形成环境可能为相对偏还原的淡水环境,有机质来源中低等水生生物的贡献较多。基于两套烃源岩生物标志化合物组成特征上的差异,选取合适的生标参数建立了相应的有机地化判识指标将两套烃源岩区分开,这对研究区后续的油源对比及油气勘探工作有着较为重要的意义。

The geochemical characteristics of the coal-measure source rocks of Shanxi Formation and Taiyuan Formation in Hangjinqi area of Ordos Basin were systematically studied in this paper by means of total organic carbon analysis,Rock-Eval pyrolysis,stable carbon isotope analysis and GC-MS analysis.The results show that source rocks of Shanxi Formation and Taiyuan Formation in the study area have excellent hydrocarbon generation potential with high contents of organic matter,and the organic matter type of them is mainly typeⅢ.The hydrocarbon generation potential of source rocks in Shanxi Formation is better than that in Taiyuan Formation,while the maturity of source rocks in Taiyuan Formation is generally higher than that in Shanxi Formation.Through comparative analysis of the two sets of molecular biomarker compositions,it is found that the source rocks in Shanxi Formation are characterized by high values of Pr/Ph,high(C_(19) tricyclic terpane+C_(20) tricyclic terpane)/C_(21) tricyclic terpane,high relative contents of C_(24) tetracyclic terpanes,C_(29) regular steranes and OF,and low relative abundances of gammaceranes,which reflects that these source rocks may be formed in freshwater environment under oxic to sub-oxic conditions,with organic matter input predominantly from higher terrestrial plants.Differently,the source rocks in Taiyuan Formation are characterized by low values of Pr/Ph and(C_(19) tricyclic terpane+C_(20) tricyclic terpane)/C_(21) tricyclic terpane,low relative contents of C_(24) tetracyclic terpanes and C_(29) regular steranes,low relative abundances of gammaceranes and high relative abundances of SF.It is suggested that they may be deposited in relative anoxic environment with low salinity,and the organic matter input of them comes more from lower aquatic organisms.Based on the differences in the compositional characteristics of biomarker compositions between the two sets of source rocks,appropriate biomarker parameters have been selected to establish the corresponding organic geochemical identification indexes to distinguish the two sets of source rocks,which is of great significance for the subsequent oil-source correlation and oil and gas exploration in the study area.