川西南地区荥经坳陷早中奥陶世地震事件记录及意义*

川西南荥经坳陷开展了多口页岩气调查井及参数井(CYY1,YD1和YD2)的钻探。通过大量岩心观察和沉积构造特征分析,在中下奥陶统识别出大量软沉积变形构造及脆性变形构造,包括重荷模、火焰构造、球-枕构造等负载相关构造,液化砂岩脉、液化泥砾等液化构造,以及布丁构造、卷曲变形、震裂缝、震裂角砾、同沉积断层等,具有典型地震导致的同生及准同生构造变形特征。记录显示至少发生了2次地震事件,并引发海啸以及多次余震。该区震积岩的发现表明早中奥陶世曾发生明显的构造活动,是加里东早期都匀运动一次构造运动幕的响应,这对川中隆起具有重要意义。构造运动导致荥经坳陷周缘断裂活动并引发地震。地震对沉积地层的改造极大地改善了研究区油气储集层条件,更利于油气的运移。

Earthquake event records in Yingjing depression of southwestern Sichuan during the Early-Middle Ordovician

The drilling of shale gas wells(CYY1,YD1 and YD2)was carried out in Yingjing depression. On the basis of core observations and sedimentary structures,a large number of soft-sediment deformation structures(SSDS)and brittle deformation structures were identified within the Middle-Lower Ordovician sandstone and shales,including load structures(e.g., ball-pillow structures,flame structures,load casts,etc.),liquefaction structures(e.g., sand dykes,liquefied breccia),boudinage,convolute lamination,shatter fissures,shatter breccia and syn-sedimentary faults,etc. These contemporaneous and penecontemporaneous deformation structures suggest that they were most likely triggered by earthquakes. Our results suggest that at least two strong seismic events happened,while the last one led to a tsunami,which was accompanied by several aftershocks. The tectonic movement occurred during Early-Middle Ordovician led to faulting,and can be related to an early episode of Douyun movement during Caledonian,which,in turn,was significant for Chuanzhong uplift. The earthquake greatly improved the quality of oil and gas reservoirs.