doi:10.16186/j.cnki.1673-9787.2021120118

Received:2022/12/29

Revised:2023/02/28

Published:2024/03/25

Differences in the development of deep and shallow mudstone fractures in Qigu anticline in the Junggar Basin and their constraints on oil and gas preservation conditions

WU Wei1，ZHOU Yong1，FENG Zhendong2，LU Xuesong3，ZHUO Qingong3，LIU Weiqing1，WANG Guangxu1

1.School of Resources & Environment， Henan Polytechnic University，Jiaozuo 454000，Henan，China；2.College of Safety Science and Engineering，Henan Polytechnic University，Jiaozuo 454000，Henan，China；3.Petro China Research Institute of Petroleum Exploration and Development，Beijing 100083，China

Abstract: The foreland thrust belt is rich in oil and gas resources，but the differences in the sealing capacity of the caprock under the action of complex structures has hindered the process of oil and gas exploration. Objectives Aiming at the current situation of surface oil and gas seriously leakage and insufficient exploration in the thrust belt in southern Junggar basin， Methods the deep and shallow mudstone caprock of Qigu anticline was taken as the research object.Based on the stratigraphic model，combined with logging data inversion and actual measurement data，a numerical model of the mechanical parameters of the study area was constructed，and the finite element method was used to simulate the paleo and present tectonic stress fields in the Qigu anticline.The obtained stress field data was combined with fracture parameter theory to analyze the formation period of fractures in the caprock and calculate the transformation results of the current stress field on the fractures in the caprock. Results The results showed that the paleo and present stress distributions of the Qigu anticline were all controlled by buried depth，tectonic location，fault strike，and fault convergence location；under the current stress field，the shallow caprock of Qigu anticline was not easy to break，and only played a role in reforming the fractures formed by the paleotectonic stress field；the degree of fracture development in deep and shallow caprocks varied greatly，and the development of fractures at high points of the anticline may be the main cause of shallow oil and gas leakage；the fracture zone was more developed near the fault，and the fracture permeability near the fault zone was much higher than that of the normal formation，which may provide a channel for the migration and even loss of deep oil and gas to the shallow layer；the deep caprock was relatively stable，and the caprock condition wass not the key factor to restrict the success of exploration in the lower Junggar Basin.Conclusions The research results can provide some references for deep oil and gas exploration in the thrust belt of the southern margin of the Junggar Basin.

Key words:in-situ stress；fracture prediction；caprock evaluation；numerical simulation；Jurassic；Qigu anticline