四川盆地五峰组-龙马溪组页岩岩相类型与沉积环境

沉积物的搬运过程和环境条件控制着富有机质页岩的形成。根据矿物学、岩石组构、生物组成及沉积构造的不同,五峰组-龙马溪组页岩识别出7种岩相类型:硅质页岩、粉砂质页岩、黏土质页岩、灰质页岩、含介壳灰岩泥质/泥灰岩、粉砂岩-细砂岩和斑脱岩。五峰组-龙马溪组页岩并不是一直在低能的水动力条件下以悬浮的形式沉积,大量硅质生物化石、具粒序特征的生物碎屑、粉砂纹层、透镜状层理及沙纹交错层理等沉积构造表明,还存在生物沉积、风暴沉积和底流沉积作用。微量元素分析表明五峰组和龙马溪组下部硅质页岩形成于贫氧/缺氧环境,富含有机质;随后海平面逐渐下降,同时受到底流的影响,岩相类型向粉砂质页岩和黏土质页岩转变,贫氧/缺氧环境受到破坏,有机质的保存条件逐渐变差。

Lithofacies types and sedimentary environment of shale in Wufeng-Longmaxi Formation,Sichuan Basin

The formation of organic-rich shale was controlled by sediment transport and deposition processes as well as sedimentary environment conditions. Based on mineralogy as well as different rock fabrics, biotic components and sedimentary textures, seven types of shale lithofacies were identified in Wufeng-Longmaxi Formation; siliceous shale, silty shale, argillaceous shale, calcareous shale, shell-bearing lime mudstone/marl, siltstone-fine sandstone and bentonite. The shale in Wufeng-Longmaxi Formation was not always able to deposit in a suspending form under the low-energy hydrodynamic conditions. A large number of siliceous biological fossils, biological detritus with grading characteristics, silt laminations, lenticular bedding, ripple cross-bedding and other sedimentary textures indicate that biogenic deposition, storm deposition and bottom current deposition also existed. Trace element analysis suggests that the siliceous shale in the lower part of Wufeng-Longmaxi Formation was formed in the dysoxic/anoxic environment, which was rich in organic matters. Then, sea level gradually declined. Meanwhile, influenced by bottom current, lithofacies types were changed and shown as silty shale and argillaceous shale, while dysoxic/anoxic environment was damaged, resulting in the gradually worsening preservation conditions of organic matter.