富有机质页岩方解石脉成因、成岩模式与地质意义——以四川盆地南部龙马溪组为例

Genesis, diagenetic model and geological significance of calcite veins in organic-rich shale: a case study of the Longmaxi Formation, southern Sichuan basin, China

富有机质页岩中方解石脉体普遍发育,其形成过程和机制对成烃储层和成藏具有重要指示意义。本文以四川盆地南部龙马溪组为例,采用显微岩相学、阴极发光、原位微区电子探针、同位素地球化学及流体包裹体相结合的方法,分析其中的方解石脉成因,并结合地质背景探讨了成岩流体动态演化模式及其勘探意义。结果表明,研究区龙马溪组页岩发育三期方解石脉,第一期(Cal-1)近围岩或独立生长,形成于同生-准同生成岩阶段,是微生物还原作用的产物,具有富Fe、Mg、Al元素、贫Mn元素的特征,其δ13CV-PDB=0.06‰~4.53‰,δ18OV-PDB=-13.21‰^-10.79‰,n(87Sr)/n(86Sr)=0.719366~0.719689;第二期(Cal-2)形成于早成岩阶段,是有机质脱羧作用的产物,以相对富Fe、Mg、Mn、Al元素为特征,其δ13CV-PDB=-6.93‰^-0.08‰,δ18OV-PDB=-13.28‰^-10.05‰,n(87Sr)/n(86Sr)=0.719378~0.720688;第三期(Cal-3)远离围岩,形成于晚成岩阶段,是甲烷热氧化作用的产物,具有富Fe、Mg、Mn元素,不含Al元素的特征,其δ13CV-PDB=-19.00‰^-12.64‰,δ18OV-PDB=-9.08‰^-6.65‰,n(87Sr)/n(86Sr)=0.719855~0.721342。通过对方解石脉成因及流体来源刻画,结合龙马溪组页岩热演化史,三期方解石脉的成岩演化对页岩的储集空间具有改善作用,有利于页岩气的勘探开发。

Objectives: With abundant occurrence in organic-rich shales,calcite veins carry important information about hydrocarbon migration and accumulation. This study aims to elaborate their formation process,reveal the behind mechanism,and further reconstruct the hydrocarbon migration and accumulation history,with a case study in the Lower Silurian Longmaxi Formation in the Southern Sichuan Basin,China.Methods: The origin of calcite veins is analyzed by microlithofacies identification,cathodoluminescence observation,in-situ micro-electron probe analysis,isotope geochemistry characterization and fluid inclusion measurement,accompanied by discussion on dynamic evolution of diagenetic fluids and its geological significance to shale gas exploration and exploitation based on understanding of the overall geological background.Results: There are mainly three generations of calcite veins,denoted as Cal-1,Cal-2 and Cal-3,respectively.Specifically,Cal-1 is featured by its growth in the vicinity of surrounding rocks or independent growth,abundance in Fe,Mg,and Al and deficiency in Mn,its δ13CVPDB range of 0. 06‰ to 4. 53‰,δ18OVPDB range of-13. 21‰ to-10.79‰,and strontium isotope range of 0. 719366 to 0. 719689. In comparison,Cal-2 is characterized by its abundance in Fe,Mg,Mn and Al,δ13CVPDB range of-6. 93‰ to-0. 08‰,δ18 OVPDB range of-13. 28‰ to-10.05‰,and strontium isotope range of 0.719378 to 0.720688. Cal-3 is featured by its growth far away from the surrounding rocks,abundance in Fe,Mg,and Mn and deficiency in Al,δ13CVPDB range of-19.00‰ to-12.64‰,δ18OVPDB range of-9.08‰ to-6.65‰,and strontium isotope range of 0.719855 to 0.721342.Conclusions: Cal-1 is formed in the syngenetic to pene-syngenetic stage as product of microbial reduction;Cal-2 is formed in the eogenetic stage as product of organic matter decarboxylation;and Cal-3 is formed in the telogenetic stage as product of methane thermal oxidation. Through above investigation into origin and fluid source of these three generations of calcite veins,their diagenetic evolution models are built in conjunction with knowledge on the thermal evolution history. In general,presence of these calcite veins improves shale reservoir performance,and thus is conducive to exploration and exploitation of shale gas.