川南地区五峰组—龙马溪组沉积期古地貌及含气页岩特征

Paleogeomorphology and oil-bearing shale characteristics of the Wufeng-Longmaxi shale in southern Sichuan Basin,China

晚奥陶世—早志留世川南地区古地貌特征及其对上覆页岩的控制,目前尚无系统认识。通过地球物理解释、年代地层划分对比、页岩等厚图编制及页岩物质组成分析,明确了五峰组—龙马溪组沉积期川南地区古地貌及其对含气页岩的控制。结果表明:(1)五峰组—龙马溪组沉积时期,川南地区位于乐山—龙女寺古隆起东南斜坡之上,该斜坡为同沉积水下斜坡,由威远地区向东南延伸至泸州地区,延伸距离达85 km,地形坡降0~1 m/km;(2)乐山—龙女寺古隆起东南斜坡发育三大地形坡折,其将东南斜坡分割成水下高地、水下斜坡、水下平原和水下洼地4个地貌单元;(3)五峰组—龙马溪组由东南向西北逐层超覆于乐山—龙女寺古隆起东南斜坡之上,斜坡下部位置五峰组和龙马溪组发育完整,而斜坡上部位置由于地势高,故常缺失笔石带LM1-4;(4)乐山—龙女寺古隆起东南斜坡不同地貌单元页岩的粒度、矿物组成和TOC含量差异,由水下高地至水下洼地,页岩粒度逐渐变细,TOC含量逐渐降低,碳酸盐矿物含量逐渐降低,硅质含量逐渐升高,黏土矿物含量先降低再升高。

The paleogeomorphology and its impact on overlying Wufeng-Longmaxi shale in southern Sichuan Basin of China are still controversial.Based on geophysical interpretation,chronostratigraphic division and transection correlation,shale isometric map compilation,and shale mineral composition analysis,the depositional paleogeomorphology and its impact on the overlying gas-bearing shale were clarified.The results show that:(1)During Late Ordovician and Early Silurian,the southern Sichuan Basin was located on the southeastern slope of the Leshan-Longnüsi paleo-uplift.The paleo-uplift,which was a syn-depositional one and had span distance reaching up to 85 km and topographical gradient ranging from 0 to 1 m/km,extended from Weiyuan area to Luzhou area southeastward.(2)During the depositional period of the Wufeng-Longmaxi shale,there existed three significant slope breaks which divided the southeastern slope into four geomorphological units,namely,sub-aqueous high,subaqueous slope,subaqueous plain,and subaqueous sag.(3)The Wufeng-Longmaxi shale,which onlaps the southeastern slope from southeast to northwest,is wholly developed in subaqueous plain and subaqueous sag but commonly losses the graptolite zones LM1-4 in subaqueous high and subaqueous slope.(4)There exist significant differences in shale grain size,mineral composition,and TOC content of shale upon different geomorphological units.Commonly,from the subaqueous high to the subaqueous sag,the shale grain size gradually becomes finer,the TOC content and carbonate mineral content gradually decrease,the silica content gradually increases,and the clay mineral content first decreases and then increases.