Abstract:

Based on 3D seismic, core, well logging and dynamic production data of Zitong area in Sichuan Basin, the fault spatial structure and segmented activity patterns were precisely characterized, the fault evolution process was restored, and the mechanism of fault controls reservoir was analyzed.The results show that: （1） Two sets of NW-trending main faults are developed in the northeast part of Zitong area, which extend continuously in the deep layer and are distributed in segmented en-echelon patterns in the shallow layer, showing inhe-rited development and multi-stage activity characteristics. During the depositional period of the third member of Xujiahe Formation,the fault activity intensity of Weicheng No. 1 fault zone is strong in the west and weak in the east. It experienced segmented thrusting first and then strike-slip connection. During the fourth member of Xujiahe Formation, the fault activity attenuated and the fault system achieved complete through-going connection. Laoguanmiao structural belt is dominated by thrust faults, with domino-style and convergent fault assemblages developed. （2） The fault evolution in the study area primarily experienced five stages. Initially, minor pre-existing strike-slip faults were reactivated during the Indosinian Phase Ⅱ. Subsequently, thrust faults formed during Indosinian Phase Ⅲ, followed by the development of dextral en-echelon (left-stepping) strike-slip faults during Indosinian Phase Ⅳ. During Jurassic tectonic quiescent period, the stress attenuated, leading to stable sedimentation. Finally, Himalayan compression induced small-scale faulting and shaped the definitive structural framework of the entire region. （3） The third member of Xujiahe Formation, the primary reservoir in the study area, is a tight sandstone reservoir with low porosity and low permeability, where dissolution pores and filled fractures are well-developed. Fractures significantly enhance the reservoir permeability, and segmented activities of fault and differential folding structures control the reservoir distribution. （4） Hydrocarbon deliverability of the third member of Xujiahe Formation in the study area is co-controlled by the distance to faults, structural positions of folds, fault activity stages. Consequently, three types of thrust-fold reservoir-controlling models are established: broad fold, transitional fold, and tight fold models. The structural forelimbs of folds and zones with well-developed secondary faults constitute favorable areas, whereas the late-stage fault reactivation tends to tri-gger hydrocarbon dissipation. Weicheng No. 1 fault zone and its peripheries are identified as the most promising targets for natural gas exploration in the third member of Xujiahe Formation.

Key words: strike-slip fault, fault evolution, thrust fold, fault-controlled reservoir, fault activity, Xujiahe Formation, Zitong area, Sichuan Basin