Improved method for quantitative evaluation of fault vertical sealing: A case study from the eastern Pinghu Slope Belt of the Xihu Depression, East China Sea Shelf Basin

• Published in: Marine and petroleum geology Vol. 132; p. 105224
• Main Authors: Wang, Fuwei, Chen, Dongxia, Du, Wenlei, Zeng, Jianhui, Wang, Qiaochu, Tian, Ziye, Chang, Siyuan, Jiang, Mengya
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.10.2021
• Subjects: Fault vertical sealing; Hydrocarbon accumulation; Hydrocarbon migration; Quantitative evaluation; Xihu depression; Fault vertical sealing; Xihu depression; Hydrocarbon migration; Quantitative evaluation; Hydrocarbon accumulation
• ISSN: 0264-8172; 1873-4073
• DOI: 10.1016/j.marpetgeo.2021.105224

Fault vertical sealing evaluation is significantly important for revealing hydrocarbon migration and accumulation processes and reducing exploration risks. Based on the analysis of the stress normal to the fault plane, the pore-fluid pressure and the compressive strength of fault rock, a new parameter, namely, the fault vertical sealing index (FVSI), is proposed for improved quantitative evaluation. In this article, static faults distributed in the eastern Pinghu Slope Belt (EPSB) of the Xihu Depression are selected as examples. The vertical sealing properties of four major fault systems that control hydrocarbon distribution are analyzed and evaluated. The FVSI value is positively correlated with the vertical sealing capacity of the fault zone, and the heterogeneous spatial distribution of the FVSI along the fault planes indicates that a fault cannot be uniformly regarded as a conduit or a barrier. According to the relation between the FVSI and the corresponding natural gas and oil shows, a fault zone with an FVSI value less than 0.4 commonly acts as a vertical conduit and cannot accumulate hydrocarbons, while a fault zone with an FVSI value greater than 0.4 has the ability to seal hydrocarbons. However, whether hydrocarbons can continue to migrate vertically depends on the maximum hydrocarbon column height that the corresponding FVSI can seal. In this case study, fault zones with FVSI values greater than 1 completely act as barriers to vertical hydrocarbon migration and no longer transport hydrocarbons. The changing FVSI of the fault zone controls the process of hydrocarbon migration and accumulation, thereby affecting the distribution of hydrocarbons. In the structurally lower position of the EPSB with sufficient hydrocarbon sources, the weak vertically sealed fault zone commonly causes a vertical multilayered distribution of hydrocarbons. Conversely, in the structurally higher position of the EPSB with insufficient hydrocarbon charging, the strong vertically sealed fault zone results in a more concentrated hydrocarbon distribution in the lower strata. However, there are no natural gas and oil shows in the fault zone that completely act as vertical conduits. Therefore, the FVSI can be an effective quantitative method to analyze vertical hydrocarbon migration pathways and accumulation locations controlled by faults and has great application potential in reducing the risks in petroleum exploration projects. •A new parameter for quantitative evaluation of fault vertical sealing.•The vertical sealing variations make the fault zone not a uniform conduit or barrier.•The hydrocarbon column height that the fault zone can seal controls the vertical migration conditions.•The vertical weakly sealed fault zone is conducive to the multilayered accumulation of hydrocarbons.