Formation of inter-salt overpressure fractures and their significances to shale oil and gas: A case study of the third member of Paleogene Shahejie Formation in Dongpu sag, Bohai Bay Basin

作     者：LIU Weibin ZHOU Xingui XU Xingyou ZHANG Shiqi LIU Weibin;ZHOU Xingui;XU Xingyou;ZHANG Shiqi

作者机构：Oil&Gas SurveyChina Geological SurveyBeijing 100029China China University of PetroleumQingdao 266580China 

出 版 物：《Petroleum Exploration and Development》 (石油勘探与开发（英文版）)

年 卷 期：2020年第47卷第3期

页      面：560-571页

核心收录：

学科分类：081803[工学-地质工程] 0808[工学-电气工程] 0709[理学-地质学] 0819[工学-矿业工程] 08[工学] 0818[工学-地质资源与地质工程] 0807[工学-动力工程及工程热物理] 0708[理学-地球物理学] 0815[工学-水利工程] 0816[工学-测绘科学与技术] 0813[工学-建筑学] 0814[工学-土木工程] 

基　　金：Supported by the China National Science and Technology Major Project(2011ZX05006-004) 

主　　题：shale reservoir inter-salt overpressure overpressure fracture fractures formation and evolution Bohai Bay Basin Dongpu sag Paleogene Shahejie Formation 

摘      要：Taking the inter-salt organic-rich shales in the third member of Paleogene Shahejie Formation(Es3)of Dongpu sag in Bohai Bay Basin as an example,the origin of overpressure,development characteristics,formation and evolution mechanism,formation stages and geological significance on shale oil and gas of overpressure fractures in the inter-salt shale reservoir were investigated by means of thin section identification,scanning electron microscopy observation,analysis of fluid inclusions,logging data analysis,and formation pressure inversion.The results show that overpressure is universal in the salt-lake basin of Dongpu sag,and under-compaction caused by the sealing of salt-gypsum layer,pressurization due to hydrocarbon generation,transformation and dehydration of clay minerals,and fault sealing are the 4 main factors leading to the occurrence of overpressure.The overpressure fractures are small in scale,with an average length of 356.2μm and an average underground opening of 11.6μm.But they are densely developed,with an average surface density of 0.76 cm/cm2.Moreover,they are often accompanied by oil and gas charging,and thus high in effectiveness.Overpressure fractures were mainly formed during two periods of large-scale oil and gas charging,approximately 25-30 Ma ago and 0-5 Ma ago.Inter-salt overpressure fractures play dual roles as the storage space and migration paths of shale oil and gas.They contribute 22.3%to the porosity of shale reservoir and 51.4%to the permeability.They can connect fracture systems of multiple scales,greatly improving the quality of shale reservoir.During the development of shale oil and gas,inter-salt overpressure fractures can affect the extension and morphology of hydraulic fractures,giving rise to complex and highly permeable volumetric fracture networks,improving hydraulic fracturing effect and enhancing shale oil and gas productivity.