Discovery and geological significance of large landslide bodies using borehole image logs in the Longmaxi Formation of Da'an area, southern Sichuan Basin,China
PAN Feng1,2, JIANG Yuqiang1, JIANG Liwei3,4,5, WANG Yue2
1 School of Geoscience and Technology,Southwest Petroleum University,Chengdu 610500,China; 2 SLB Chengdu,Chengdu 610500,China; 3 State Key Laboratory of Petroleum Resources and Engineering,China University of Petroleum(Beijing),Beijing 102249,China; 4 College of Geosciences,China University of Petroleum(Beijing),Beijing 102249,China; 5 PetroChina Zhejiang Oilfield Company,Hangzhou 310023,China
Abstract：Large landslide bodies are discovered for the first time in the Longmaxi Formation from the Da'an area,southern Sichuan basin,based on high-resolution borehole image well logs. Landslide bodies are distributed in the Long12 Sub-member and Long2 Member. They are both composed of black shales,and thus display similar features to background shale deposits on conventional logs. On borehole image logs,however,the landslide bodies can be identified by slide and slump structures,which have sharp contrasts with normal shale. Multi-layered landslide bodies are found in 6 drilling wells distributed in the Da'an area(~1000 km2). The landslide bodies are layered and separated by normal shales. The number of layers and thickness of landslide bodies in different wells are markedly different,making it challenging to correlate between wells. The landslide bodies are overall thick in the north and thin in the south. A single landslide body varies in thickness from 0.5 to 32.4 m,while composite landslide bodies in a single well can reach a thickness of 41 to 194.5 m. It is believed that landslide bodies are widely distributed,occur at multiple stratigraphic levels with large bed thickness,and have the characteristics of earthquake origin. They were the product of short distance transportation and rapid deposition,which was controlled by the southeast slope of the Leshan-Longnvsi palaeo-uplift. Different areas have different sedimentary responses to the same earthquake event. A single landslide body may be a response to one earthquake event,or it may be the result of the superposition of multiple earthquake events. The discovery of the landslide bodies provides a new perspective on stratigraphic correlation of the Longmaxi Formation in the southern Sichuan Basin. At the same time,the landslide bodies are thick with well-developed fracture networks,and feature favorable reservoir and seal conditions. These landslide bodies may thus represent potentially important prospects for the future shale gas exploration.
PAN Feng,JIANG Yuqiang,JIANG Liwei et al. Discovery and geological significance of large landslide bodies using borehole image logs in the Longmaxi Formation of Da'an area, southern Sichuan Basin,China[J]. JOPC, 2024, 26(4): 972-984.
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