四川盆地下古生界硅质页岩层系中硅质来源及其对有机质保存的影响初探

四川盆地下古生界两套硅质页岩层系(寒武系底部页岩和奥陶系—志留系五峰组—龙马溪组页岩)是其重要的页岩气产层,五峰组—龙马溪组硅质含量与TOC及含气量呈正相关性,暗示硅质矿物对该地区页岩气的生成具有重要意义。但硅质来源复杂,且存在后期硅质流体的注入。因此,本文在总结前人的研究成果基础上,对四川盆地寒武系底部硅质岩系和五峰组—龙马溪组硅质岩系的硅质特征进行了详细的岩石学分析,揭示了3种硅质来源特征:(1)同沉积无机硅质流体;(2)生物硅;(3)后期无机硅质流体。其中寒武系底部以同沉积无机硅质流体和生物硅来源为主,同沉积硅质流体导致有机质快速石化埋藏,有机质内部结构及形态保存较好;五峰组—龙马溪组硅质来源以后期无机硅质流体和生物硅来源为主,有机质在保存过程中受硅质流体的影响很小,有机质腐泥化作用充分,内部结构和形态多数保存较差。该研究初步揭示了两套页岩层系的硅质发育岩石学特征,及其对有机质保存的影响,期望能为四川盆地两套页岩气储层差异性提供新的思路和基础地质数据。

Siliceous source and its influence on organic matter preservation of the two Lower Paleozoic siliceous shale reserviors in Sichuan Basin

Two sets of siliceous shale formations in Lower Paleozoic,the bottom of Cambrian and the Ordovician—Silurian Wufeng—Longmaxi Shales,are the important shale reservoirs in Sichuan Basin.The TOC and shale gas content of the Wufeng—Longmaxi shales are positively related to the siliceous quantity,suggesting that the siliceous materials could be very important for the shale gas formation.However,the origin of siliceous materials is complicated,and there could be some siliceous fluid filled after the rock sedimentation.This study,based on the summary of previous research results,has carefully analyzed the petrographical characteristics of siliceous minerals.Three types of siliceous sources have been detected:(1)syngenetic hydrothermal inorganic siliceous fluid,(2)biosilica,and(3)late-filled inorganic siliceous fluid.Syngenetic hydrothermal inorganic siliceous fluid and bio-silica are the dominated siliceous sources in Lower Cambrian shales.Syngenetic hydrothermal inorganic siliceous fluid made the organic matter rapidly silicified,and the organic matter could be exquisitely preserved.The siliceous source in the Wufeng—Longmaxi Formation was mainly from late-filled inorganic siliceous fluid and bio-silica,and organic matter preservation has little influence by siliceous fluid.Thus,the organic matter has gone through slowly saprofication process,resulting in the morphology of organic matters are poor preserved.The study has initially revealed the petrographical characteristics of siliceous materials and the influences on the organism preservations for comparision the Lower Cambrian and the Wufeng—Longmaxi shale reservoirs,with hoping to provide new geological data and ideas for shale gas exploration in Sichuan Basin.