Influence of two-phase extension on the fault network and its impact on hydrocarbon migration in the Linnan sag, Bohai Bay Basin, East China
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
References (91)
- et al.
Mesozoic to cenozoic tectonic transition process in zhanhua sag, Bohai Bay Basin, east China
Tectonophysics
(2018) - et al.
Influence of fault reactivation during multiphase rifting: the Oseberg area, northern North Sea rift
Mar. Petrol. Geol.
(2017) - et al.
Fault growth and interactions in a multiphase rift fault network: horda Platform, Norwegian North Sea
J. Struct. Geol.
(2015) - et al.
Fault linkage and relay structures in extensional settings-A review
Earth Sci. Rev.
(2016) Balanced cross-section construction from seismic sections in areas of extensional tectonics
J. Struct. Geol.
(1983)- et al.
Revitalization of a mature oil-bearing basin by a paradigm shift in the exploration concept: a case history of Bohai Bay, Offshore China
Mar. Petrol. Geol.
(2010) - et al.
Polymodal faulting: time for a new angle on shear failure
J. Struct. Geol.
(2015) - et al.
Normal-fault development during two phases of non-coaxial extension: an experimental study
J. Struct. Geol.
(2010) - et al.
How do the properties of a preexisting normal-fault population influence fault development during a subsequent phase of extension?
J. Struct. Geol.
(2011) - et al.
Cenozoic evolution of the tan-Lu fault zone (east China)-Constraints from seismic data
Gondwana Res.
(2015)