四川盆地及邻区上奥陶统五峰组—下志留统龙马溪组底部笔石带及沉积特征

Graptolites zone and sedimentary characteristics of Upper Ordovician Wufeng FormationLower Silurian Longmaxi Formation in Sichuan Basin and its adjacent areas

根据四川盆地五峰组—龙马溪组主要页岩气井和典型剖面的资料,分析了不同笔石带页岩的沉积环境、厚度等特征,明确了五峰组下部WF2—WF3笔石页岩段(奥陶纪凯迪阶晚期447.62~445.16 Ma)、五峰组观音桥层WF4灰岩/泥质灰岩/灰质泥页岩段(奥陶纪赫南特阶早期445.16~444.43 Ma)和龙马溪组底部LM1—LM4笔石页岩段(奥陶纪赫南特阶晚期444.43~443.83 Ma和志留纪鲁丹阶早中期443.83~441.57 Ma)3个层段的沉积环境、岩性和厚度特征,分析了页岩平面展布特征。研究表明:1五峰组下部WF2—WF3笔石页岩段沉积期,研究区主体上受广西运动的影响较小,沉积环境为广海深水环境,主要沉积了黑色页岩,沉积中心发育在黔东北—川东北和川南2个区域,沉积中心最大厚度为4~6 m。2五峰组观音桥层WF4笔石页岩段沉积期,受全球海平面下降和广西运动增强的影响,研究区沉积环境发生重大变化,川中古隆起、黔中古隆起和江南—雪峰古隆起进一步扩大,沉积盆地面积减小,沉积环境变为以浅海碳酸盐生屑滩沉积为主,少量深水沉积环境为辅(仅分布在川东—川东北地区的石柱—涪陵—巫溪一带和川南地区的长宁—永川一带),浅水沉积物以富含赫南特贝的灰岩、泥质灰岩等沉积为主,深水沉积物主体为含赫南特贝的灰质泥岩、页岩沉积。3龙马溪组底部LM1—LM4笔石页岩段沉积期,受全球海平面上升和广西运动的控制沉积范围比观音桥层沉积期大,主体为滞留的深水沉积环境,沉积了厚度较大的LM1—LM4笔石页岩段黑色页岩,最大厚度超过20 m。进一步分析了川中古隆起、黔中古隆起及江南雪峰古隆起对黑色页岩的控制作用,探讨了自流井水下高地/隆起、华蓥山水下高地、丁山水下高地和湘鄂西水下高地/隆起对黑色页岩沉积与保存的控制作用。

According to the data of main shale gas wells and the type sections of Wufeng Formation-Longmaxi Formation in Sichuan Basin, this study analyzes the sedimentary environment, thickness and other characteristics of shale in various graptolite zones, de-termines the sedimentary environment, lithology and thickness characteristics of the graptolite shale interval WF2-WF3 in the lower part of Wufeng Formation （Late Ordovician Katian, 447. 62-445. 16 Ma）,Guanyinqiao Member WF4 limestone/ argillaceous lime-stone/ calcareous shale interval in Wufeng Formation （Early Ordovician Hirnantian, 445. 16-444. 43 Ma） and the graptolite shale in-terval LM1-LM4 in the bottom of Longmaxi Formation （Late Ordovician Hirnantian, 444. 43-443. 83 Ma and Early-Middle Silurian Rhuddanian, 443. 83-441. 57 Ma） , and also explores the plane distribution characteristics of shale. The study results shows： （1 ） During the deposition period of WF2-WF3 in the lower part of Wufeng Formation, the main study area was less affected by Guangxi movement. The sedimentary environment was characterized by open-sea deep water, where black shale was mainly deposited. The thickness center was developed in Northeast Guizhou-Northeast Sichuan and South Sichuan. The maximum center thickness is 4-6 m.（2） During the deposition period of WF4 in Guanyinqiao Member, Wufeng Formation, the sedimentary environment in the study area had significant changes as influenced by global sea level fall and enhanced Guangxi movement; palaeouplifts in central Sichuan and central Guizhou and Jiangnan-Xuefeng palaeouplift further expanded, and the area of the sedimentary basin decreased; the sedimenta-ry environment changed to be dominated by neritic carbonate clastic shoal and shallow water sedimentary environment, with supple-ment of deepwater sedimentary environment （only distributed in Shizhu-Fuling-Wuxi area in East Sichuan-Northeast Sichuan and Changning-Yongchuan area in South Sichuan）. Shallow water sediments were dominated by limestone abundant in Hirnantia, argilla-ceous limestone and other lithologic deposits. Deepwater sediments were dominated by Hirnantia-bearing calcareous mudstone and shale deposit. （3） During the deposition period of LM1-LM4 in the bottom of Longmaxi Formation, controlled by global sea level rise and Guangxi movement, the sedimentary range was larger than that in the deposition period of Guanyinqiao Member and the main body was hydropexic deepwater sedimentary environment with thick black shale from LM1-LM4. The maximum thickness is over 20 m. Moreover, the control effect of palaeouplifts in central Sichuan and central Guizhou and Jiangnan-Xuefeng palaeouplift on black shale was further analyzed, and that of underwater highland/uplift in Zhiliujing and western Hubei and Hunan as well as un-derwater highland in Huaying Mountain and Dingshan on shale deposition and preservation was also explored.