四川盆地及周缘五峰组—龙马溪组富有机质页岩沉积演化模式

Sedimentary Evolution Model of Upper Ordovician Wufeng-Lower Silurian Longmaxi Organic-rich Shale in the Sichuan Basin and Its Surrounding Area

四川盆地及周缘地区上奥陶统五峰组-下志留统龙马溪组富有机质页岩是我国页岩气勘探开发的主要层位,厚度20~60 m,该套富有机质页岩的时空展布差异明显。通过对重点剖面及钻井的岩芯观察、笔石生物地层划分、地化及矿物分析,明确了四川盆地及周缘不同地区富有机质页岩的时空展布特征,建立了3种富有机质页岩沉积演化模式:1)长宁-威远"三隆夹二洼"沉积演化模式:长宁地区富有机质页岩主要沉积于WF2-LM5,威远地区为LM1-LM8,隆起区域富有机质页岩沉积受到不利影响。2)黔渝地区的"渐进式"沉积演化模式:该区域呈现越靠近盆地内部,富有机质页岩沉积结束的时间越晚的趋势,焦石坝区域是富有机质页岩的沉积中心,其他区域厚度相对较薄。3)川东北地区的"稳定抬升型"沉积演化模式:位于盆地中心的巫溪2井区域的富有机质页岩厚度大,沉积持续时间长(WF2-LM9),盆地边缘区域富有机质页岩厚度明显变薄,区域呈现整体抬升趋势,随时间推移,富有机质页岩沉积范围越来越小。总体而言,富有机质页岩的沉积主要受广西运动的影响,富有机质页岩沉积中心有逐渐向北迁移的趋势,埃隆阶后仅威远和巫溪区域沉积富有机质页岩。研究成果对该地区页岩地层的精细对比具有重要的指导意义。

The Upper Ordovician Wufeng-Lower Silurian Longmaxi organic-rich shale in the Sichuan Basin and its peripheral regions,whose thickness is 20-60 m,is the main focus for the exploration and development of shale gas in China. According to our research in recent years,the temporal and spatial distribution of the organic-rich shale varies greatly. Based on the observation of key sections,core drilling,lithosphere biostratigraphy,geochemistry and mineral analysis,the spatial-temporal distribution characteristics of the organic-rich shale in the Sichuan Basin and its surrounding areas were clarified. Three types of organic-rich shale depositional evolutionary models were established,which are described as follows:(1)The"two sags sandwiched between three uplifts"model in the Changning-Weiyuan area:the organic-rich shale deposits at WF2-LM5 in Changning and at LM1-LM8 in Weiyuan. The thickness of the organic-rich shale in the uplift is much thinner than that in the sags.(2)The"stage-progressive distribution pattern"model in the Chongqing and Qianjiang area:The closer the area is to the interior of the basin,the later the sedimentation of the organic-rich shale tends to end. The Jiaoshiba area is the depositional center of the shale,while the thickness of other areas is relatively thin.(3)The "stable uplifting" model in northeastern Sichuan:The organic-rich shale in Well Wuxi 2 in the center of the basin has a large thickness and a long depositional duration(WF2-LM9).The thickness of the shale in the marginal area of the basin is significantly thinner,and the region presents an overall upward trend. With the passage of time,the organic shale depositional area becomes smaller and smaller. In general,the deposition of these shales is mainly influenced by the Guangxi movement. The organic-rich shale sedimentary centers tend to move northward gradually,and the shale only deposits in the Weiyuan and Wuxi areas after the Aeronian stage in the Sichuan Basin. The research results have an important guiding significance for the micro-layer division and fine reservoirs contrast of the Wufeng-Longmaxi shale in this area.