柴达木盆地北缘中段侏罗系页岩有机质孔隙演化特征

Pore evolution characteristics of organic matter pore for Jurassic shale in the middle part of northern margin of Qaidam Basin

柴北缘侏罗系页岩的有机碳含量高、厚度大、分布广,但有机质成熟度总体处于未成熟—低成熟阶段,制约了研究区页岩气的表征与评价。通过开放体系下的加热实验,系统刻画了不同温度下的页岩有机质孔隙发育特征及演化过程。分析认为,热模拟温度200~400℃是有机质孔隙发育的优势温度区间,孔隙数量多,从微孔到宏孔都非常发育,孔隙间可形成网状连通。之后随着温度进一步升高,孔隙会出现“塌陷”现象,对页岩气的赋存起到抑制作用。与此同时,有机质的孔隙发育也具有不均一性,不同有机质显微组分孔隙发育能力存在差异,以镜质组、壳质组等为主的有机质显微组分孔隙发育较迟缓,有机质转化不彻底,而以沥青质体、藻类体、无定形体等为主的有机质显微组分孔隙发育时间早、强度大、孔隙之间的连通性好,是页岩气储集空间的主要贡献者。另外,结合能谱分析,认为元素的重量百分比及Ca、Mg等碱土元素的含量也是影响有机质孔隙发育的主要因素。

The Jurassic shale in northern margin of Qaidam Basin is characterized by high organic carbon content,large thickness and wide distribution.However,due to immaturity-low maturity of organic matter,the formation and characterization of shale gas in this area are restricted.Through heating experiments under an open system,the development characteristics and evolution process of shale organic matter pores at different temperatures were systematically characterized.Analysis shows that the thermal simulation temperature of 200~400℃is the dominant temperature range for the development of organic matter pores,with a large number of pores,ranging from micropores to macropores,which can form a network connection between the pores.After that,with the further increasing of temperature,the pore will appear"collapse"phenomenon,which will inhibit the occurrence of shale gas.Besides,the pore development of organic matter is also heterogeneous,and the pore development of different organic matter macerals is different.The pore development in vitrinite and exinite is relatively slow,and the transformation of organic matter is not complete.However,the pore development of organic matter macerals,mainly composed of bituminite,alginite and amorphous body,are early and strong,and connectivity between pore is good,which are the major contributors to shale gas reservoir space.In addition,combined with EDS analysis,it is considered that the development of organic matter pore is mainly controlled by the weight percentage of elements and the content of alkaline earth elements such as Ca,Mg.