晚奥陶世—早志留世之交细粒沉积类型及演化——以川南地区五峰组为例

Shallow-water fine-grained sediments evolution and control of marine black shale in the Late Ordovician-Early Silurian:Case study of the Wufeng Formation in southern Sichuan Basin,China

细粒沉积物类型及演化是周缘板块构造演化的响应,并深刻影响页岩的储层品质。通过川南地区五峰组页岩岩心精细描述、大薄片全尺度成像和氩离子抛光片场发射扫描电镜分析,揭示晚奥陶世—早志留世之交海相页岩沉积类型及演化。结果表明:①川南地区五峰组发育细粒浊流沉积、半远洋沉积和陆棚浅滩沉积3大沉积类型:细粒浊流沉积物质主要来自于陆源,可清晰见到9个典型的沉积序列;半远洋沉积物以陆源和内源混合沉积为主,递变型水平层理发育;陆棚浅滩沉积以陆源成因和原地成因物质混合为主,沉积物主要以悬浮沉降的方式沉降。②细粒浊流沉积主要发育于笔石带WF1—WF2,半远洋沉积主要发育于笔石带WF3,而陆棚浅滩沉积主要发育于笔石带WF4。③细粒沉积类型及演化反映了周缘板块构造演化:笔石带WF1—WF2沉积期,周缘板块构造挤压强烈,陆源供给充分,故细粒浊流沉积发育;笔石带WF3沉积期,周缘板块构造活动减弱,海平面快速上升,故半远洋沉积发育;笔石带WF4沉积期,周缘板块构造活动进一步减弱,全球海平面快速下降,故陆棚浅滩沉积发育。细粒沉积类型直接影响页岩的储层品质,细粒浊流沉积孔隙度和总有机碳含量相对较低,而半远洋沉积孔隙度和总有机碳含量相对较高。

The sediment type and evolution are the response of peripheral plate tectonic activities,and profoundly affect the reservoir quality of shale.The sediment types and evolution of marine shale at the turn of Late Ordovician-Early Silurian were revealed by detailed description of the Wufeng Formation shale core,full-scale imaging of large thin sections and field emission scanning electron microscopy analysis of argon ion polished sections in southern Sichuan.Marine shale mainly develops three shallow-water fine-grained sediments:fine-grained turbidites,source mixing sediments and in-situ mixing sediments.The fine-grained turbidite is dominated by clastic quartz(45.5%-54.5%)and clay minerals(29.0%-38.9%),and nine typical sedimentary sequences can be clearly seen.The source mixing sediment is mainly composed of quartz(36.5%),carbonate(24.3%)and clay minerals(39.3%),and carbonate minerals are mainly exogenous transport deposits.In-situ mixing sediments is dominated by clay minerals(30.5%),ankerite(39.5%)and calcite(26.5%),and carbonate minerals are mainly derived from in-situ.The fine-grained turbidite are mainly developed in the graptolite zone WF1-2,the source mixing sediment is mainly developed in the graptolite zone WF3,and the in-situ mixing sediment is mainly developed in the graptolite zone WF4.The sediment type and distribution reflect the characteristics and evolution of the tectonic activity of the surrounding plate.During the sedimentary period of the graptolite zone WF1-2,fine-grained turbidite developed,indicating that the tectonic compression of the surrounding plate was strong,and the terrestrial supply was sufficient.During the sedimentary period of the graptolite zone WF3,the source mixing sediment developed,indicating that the tectonic activity of the surrounding plate was weakened,and the terrestrial supply was reduced.During the sedimentary period of graptolite zone WF4,the in-situ mixing sediment developed,indicating that the tectonic activity of the surrounding plate was further weakened,and the terrestrial supply was further reduced.The sediment type directly affects the reservoir quality of shale.The porosity and total organic carbon content of fine-grained turbidite sediment are the lowest,while those of in-situ mixing sediment are the highest.