Influence of two-phase extension on the fault network and its impact on hydrocarbon migration in the Linnan sag, Bohai Bay Basin, East China

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Highlights

  • We indicate that the regional stress direction in Linnan area changed from NW-SE (~65-50Ma) to SN (~50-42Ma).

  • The fault network evolution is often controlled by the regional stress, the local stress and the pre-existing fault.

  • We discusse the influence of the fault network formed by two-phase extension on the hydrocarbon migration.

Multiphase rifts produce fault networks formed by the non-coaxial faults, these networks evolve through the formation of new faults and pre-existing faults. Pre-existing faults have a great influence on the formation of new faults under later regional stress, resulting in a complex fault network. Here, abundant 3D seismic and log data are used to reveal the evolution of fault networks in the Linnan sag in southwestern East China, a complex basin that experienced multiple phases of extension during the Cenozoic and developed NE-SW-, ENE-WSW- and E-W-striking faults.

(1) During deposition of the Kongdian formation (65-50 Ma), NE-SW-striking faults formed under regional NW extension. In contrast, the ENE-WSW-striking and E-W-striking faults are younger, as they show no impact on the Ek formation. (2) During deposition of the Shahejie 4 formation (50-42 Ma), faults of all orientations (NE-SW, ENE-WSW and E-W) were active. However, the pre-existing NE-SW-striking faults show dextral strike-slip characteristics. The ENE-WSW-striking faults in the central parts show shear properties. The minor faults controlled by the regional extension and local strike-slip faulting near the pre-existing NE-striking faults strike NE, and the E-W-striking faults are distributed far from the pre-existing fault. These phenomena all indicate that the stress field changed from NW-SE to N-S extension in this stage. (3) During deposition of the Shahejie 3 formation (42-38 Ma), all fault activity was the strongest, and NE-striking faults began to connect and control basin deposition. The ENE-WSW-striking faults became longer. The density of E-W-striking faults increased. (4) During deposition of the Shahejie 2-Dongying formations (38–23.5 Ma), all fault activity weakened, and the Linnan sag received sediment. These observations demonstrate that later extension with a different direction can form local stress near pre-existing faults and that faults with new strikes can enhance the geometric and kinematic complexity in the fault network in the late stage. This study provides a reference for the interpretation of other multiphase rift, where two-phase extension fault networks were controlled by regional and local stresses, the reactivated pre-exist faults and newly-formed faults coexist in non-coaxial extension. Additionally, such fault networks can have important controlling effects on the distribution of hydrocarbon accumulation.

Introduction

The fault systems of petroliferous basins can form complex networks composed of differently oriented faults (Nixon et al., 2014; Duffy et al., 2015; Peacock et al., 2016). Within a single extensional background, a simple normal fault system typically strikes sub-perpendicular to the extension direction (e.g., Gawthorpe and Leeder, 2000). However, most petroliferous basins are known to have experienced multiple phases of tectonic activity, resulting in two or more rift phases of non-coaxial extension; such basins include the Bohai Bay Basin, East China (e.g., Hou et al., 2001; Wu et al., 2003; Li et al., 2010; Gong et al., 2010), the North Sea rift system (e.g., Badley et al., 1988; Whipp et al., 2014; Duffy et al., 2015; Deng et al., 2017), the Thailand rift basins (e.g., Morley, 2017; Pongwapee et al., 2019), and the East African Rift System (e.g., Korme et al., 2004). In such multiphase tectonic settings, the spatial and temporal differences in development characteristics are obvious due to various geological factors, such as the regional geological background (e.g., Duffy et al., 2015; Collanega et al., 2019), local stress conditions (e.g., Peacock, 2002; Destro et al., 2003) and the type of rock formations (e.g., Zhou et al., 2003; Li et al., 2017). Late-formed basins are often not purely extensional or strike-slip basins but are transtensional basins and tend to produce fault networks composed of new faults and reactivated pre-existing faults (e.g., Deng et al., 2017; Deng et al., 2020). The resulting fault networks in multiphase rifts always comprise non-colinear fault sets (e.g., Morley, 2007; 2017: Nixon et al., 2014), and the fault patterns tend to be complex.

The Linnan sag is located in eastern China and is an important hydrocarbon generation sag. In the Linnan sag, two extension phases are recognized during the Cenozoic (e.g., Li et al., 2013; Li et al., 2015). The early consensus was that the boundary faults initiated during deposition of the Kongdian (Ek) formation (~65-50 Ma), while minor faults formed during deposition of the Shahejie 4 (Es4)- Dongying (Ed) formations (~50–23.5 Ma) (Li et al., 2017; Wang et al., 2018). A recent study showed that the boundary faults were not directly formed during deposition of the Ek formation (~65-50 Ma) but experienced a long evolutionary process and became boundary faults. These faults also played an important role during deposition of the Es4-Ed formations (~50–23.5 Ma) (Wang et al., 2018, 2020). However, the fault development characteristics during different extension stages are not clearly understood, and the influence of multi-stage faults on hydrocarbon migration remains unclear.

In this article, we use 3D seismic data and well-logging data to investigate the geometry and evolution of the Linnan sag. The Linnan sag is a Cenozoic graben that formed in the overlap zone between two boundary faults, the boundary faults formed in the first-phase extension process, and as the stress rotateds, the boundary faults were subjected to oblique extension during the second-phase extension, showing strike-slip characteristics (e.g., Qi et al., 1995; Wan et al., 1996; Zhu et al., 2004; Guo et al., 2009) (Fig. 1b). The structural evolution of the Linnan sag has been discussed by many geologists (Zhao et al., 2000; Gao et al., 2003; Zheng et al., 2004; Ni et al., 2011; Jia et al., 2013; Zhao and Li, 2017), but because of restrictions related to the resolution of available seismic data, there is no detailed description of its development process and how it controlled and influenced the development of faults within the sag. This article uses the latest high-quality 3D seismic data and, through the analysis of geometry and kinematics, examines important seismic profiles using the balanced cross-section technique for restoration, making it possible to study the fault network within this two-phase extensional background. The main goal is to understand the fault development characteristics and to discuss the factors controllings fault evolution in the Linnan sag, and the results have some broader relevance for the fault evolution of multiphase basins in general.

Section snippets

Geological setting

The Bohai Bay Basin is an intracontinental basin developed on the North China Craton (Qi and Yang, 2010). From the late Mesozoic to the Paleogene, the Bohai Bay Basin was subjected to the subduction of the Pacific Plate and the collision between the Indian Plate and the Eurasian Plate (e.g., Hou et al., 2001; Xia et al., 2006; Li et al., 2013; Zhang et al., 2019), resulting in large-scale mantle upwelling (e.g., Wang et al., 2013; Li et al., 2015; Cheng et al., 2018) and lithospheric

Database

This study is based on the interpretation of 3D seismic reflection data (Fig. 1b). The 3D seismic data cover the entire research area, approximately 1300 km2, with a line spacing of 25 m. The data images extend down to 5.5 s two-way travel time (TWT). In addition, more than four hundred wells contribute to the seismic data and help identify strata; most wells penetrate the Es3 Group, and a few wells reach the top of the Cretaceous strata. The 3D seismic data and well data were provided by the

Seismic interpretation

We can see from map view (Fig. 1b) that the faults can be divided into three groups: (1) major faults (Linshang fault (LSF) and Xiakou fault (XKF)) that are >30 km in length and NE striking, (2) secondary faults (Yuhuangmiao fault, Mengsi fault and Yingzijie fault) that are >15 km in length and ENE striking, and (3) minor faults that are short and approximately E-W striking.

Fault activity during basin evolution

According to the above analysis and combined seismic profile interpretation (Fig. 3, Fig. 4), it can be concluded that the fault properties varied across different geological stages and that the faults mainly included normal faults and strike-slip faults. The results show the following: (1) During the Cenozoic, the strikes and types of syn-depositional faults changed from NE-striking normal faults to NE-striking right-lateral strike-slip faults, ENE-striking shear faults, and E-W-striking

Conclusions

The analysis of the non-coaxial fault network evolution during the two-phase extension of the Linnan sag, East China, improved the understanding of fault growth under the regional and local stresses in different periods. The main results are as follows:

  • 1.

    Three populations of faults developed within the Linnan sag, comprising NE-, ENE-, and E-W-striking faults. The NE-striking faults were composed of NE-trending and E-W-trending segments. The ENE-striking faults became active as shear faults

CRediT authorship contribution statement

Di Wang: Conceptualization, Methodology, Formal analysis, Investigation, Software, Writing - original draft. Zhiping Wu: Supervision, Project administration, Funding acquisition. Linlong Yang: Validation, Investigation, Writing - review & editing. Wei Li: Investigation, Resources. Chuan He: Investigation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was partly funded by the National Natural Science Foundation of China (Project number 42072169) and the China Scholarship Council (Project number 201906450071). We thank the Shengli Oil Field Company, SINOPEC, CNOOC and the individuals who contributed the seismic data, well data and hydrocarbon distribution for this work. Furthermore, the HalliBurton company provided access to LandMark software. We also thank the University of Bergen for providing access to Petrel 2019 software

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