Direct control of normal fault in hydrocarbon migration and accumulation in northwestern Bozhong subbasin, Bohai Bay Basin, China

Highlights

• The investigated normal fault and is interpreted to have resulted from the physical-link of several segments along dip and strike.

• Both ‘isolated-fault model’ and ‘coherent-fault model’ may have contributed to the evolution of the investigated normal fault.

• Petroleum migration and accumulation processes has been revealed to be controlled by multiple motions of fault activity.

Abstract

Fault growth and reactivation activities have been interpreted as potential controlling factors in vertical hydrocarbon migration, but the coupling and direct comparison of fault growth/reactivation history and hydrocarbon distribution patterns remain poorly understood. The spatial distribution of oil fields in the Western subsag of the Bozhong subbasin in the Bohai Bay Basin is closely related to fault geometry and presents a unique opportunity to investigate the coupling mechanism of the evolution of normal faults and petroleum migration and accumulation. The evolution history of two major normal faults, Faults F1 and F2, was investigated via three-dimensional seismic reflection data, and Fault F2 was associated with the spatial distribution of oil. Quantitative throw analyses, including throw versus distance plots, throw versus depth (T-z) plots, throw contour projections, and expansion indexes (EI) were employed. Fault F2 exhibited a large displacement between the footwall and hanging wall along most of the fault plane and thickened hanging walls of the upper fault surface. Laterally, the fault comprised three segments that linked along the strike at local throw minima. Vertically, the fault formed through the coalescence of two parts via dip linkage. Both the isolated- and coherent-fault models may have contributed to the evolution of Fault F2 during different periods and can be summarized in several steps, including blind fault propagation as individual segments, free surface breaching, lateral and dip linkage, and subsequent frequent reactivation as a coherent structure. A key result of this study was that petroleum migration and accumulation could be directly controlled by multiple growth and reactivation activities of faults.

Introduction

Research on normal faults is crucial in basin analysis and hydrocarbon exploration, as faults act as vertical migration conduits, and their growth history provide information on fluid migration and accumulation (Allen, 1989; Cartwright et al., 1998; Ainsworth, 2006; Zhang et al., 2010; Omosanya et al., 2015; Xu et al., 2016; Mattos and Alves, 2018). Various approaches have been used to investigate the growth history of normal faults, including outcrop analogs (Muraoka and Kamata, 1983; Cartwright et al., 1998), seismic interpretation (Baudon and Cartwright, 2008a, 2008b; 2008c; Omosanya et al., 2015; Xu et al., 2016), model-based experiments (Bellahsen and Daniel, 2005), and numerical simulations (Osagiede et al., 2014), which mainly focused on constraining fault nucleation, segmentation, linkage, and propagation. Two end-member models for fault growth have been put forward: (1) the segment growth and linkage model (Trudgill and Cartwright, 1994; Mansfield and Cartwright, 2001) or isolated fault growth model (Walsh et al., 2003; Fossen and Rotevatn, 2016) via fault tip propagation and linkage of pre-existing isolated segments, and (2) the coherent fault model (Walsh et al., 2003; Giba et al., 2012; Jackson and Rotevatn, 2013) via rapid establishment of fault length and growth of individual segments belonging to a single geometrically and kinematically coherent structure without further lateral propagation.

Fault growth and reactivation activities have been considered potential controlling processes in vertical hydrocarbon migration and the final distribution of oil and gas traps (Holdsworth et al., 1997; Cartwright et al., 1998; Tvedt et al., 2013; Mattos et al., 2016; Reilly et al., 2016; Xu et al., 2016, 2018). Using graphical techniques, such as throw versus distance (T-x; Cartwright and Mansfield, 1998; Jackson and Rotevatn, 2013), throw versus depth (T-z; Baudon and Cartwright, 2008a, 2008b, 2008c; Mattos et al., 2016, 2018; Omosanya et al., 2015; Tao and Alves, 2016), contoured throw projections (Barnett et al., 1987; Baudon and Cartwright, 2008a, 2008b, 2008c), and expansion indexes (EIs; Thorsen, 1963; Pochat et al., 2009; Tvedt et al., 2013), to reconstruct the polycyclic motion history of faults, two fault reactivation mechanisms have been recognized: upward propagation of the tip-lines of existing faults (Nicol et al., 2005; Baudon and Cartwright, 2008c) and dip linkage of newly formed fault segments to a pre-existing fault (Mansfield and Cartwright, 1996; Baudon and Cartwright, 2008c). Despite the concept of fault growth and reactivation, the coupling and direct comparison of fault growth/reactivation history and hydrocarbon distribution patterns remain poor, with only a few case studies directly linking hydrocarbon distribution patterns to fault growth/reactivation history (Reilly et al., 2016; Xu et al., 2016).

The Bohai Bay Basin is one of the most petroliferous basins in China, accounting for nearly one-third of the total oil production of the country (Hao et al., 2007, 2009a; 2009b). The western subsag of the Bozhong subbasin is located between the Shaleitian and Shijiutuo uplifts (Fig. 1), and hydrocarbon exploration in this area has recently made major breakthroughs through the discovery of a series of oil fields (Yao et al., 2017; Xu et al., 2018, Fig. 1). Previous studies in the subsag of Bozhong subbasin emphasized the controlling role of active faults in petroleum accumulation (Yao et al., 2017; Zhang et al., 2018). Particularly, the evolution and reactivation history of the Zhangjiakou–Penglai strike–slip fault zone during the Cenozoic, a NWW–SEE large-scale sinistral strike–slip fault running through the central Bohai Bay Basin (Fig. 1), was suggested to have directly controlled the geomorphological shape, trap configuration, and structural characteristics of the study region (Li et al., 2016; Peng et al., 2018; Xu et al., 2019).

However, detailed investigations into the controlling mechanism of the current spatial distribution of oil fields are rare. Current oil fields in the study area are mainly distributed around two major normal faults (Faults F1 and F2), providing a unique opportunity to investigate the coupling mechanism of fault growth/reactivation history and petroleum migration and accumulation. The displacement history of Fault F1 was briefly analyzed by Xu et al. (2018), who did not find compelling evidence of a close relationship with petroleum migration and accumulation, Furthermore, the detailed kinematic history of both faults and their potential link with hydrocarbon accumulation remains unclear.

In this paper, we aimed to describe the nucleation, vertical and lateral segmentation, linkage, and reactivation history of these two major normal faults to explore the influence of the cyclical motion of normal faults on hydrocarbon migration and accumulation and to propose a model for hydrocarbon accumulation in settings with episodic fault activities. Understanding the coupling of the kinematic history of faults and oil migration and distribution has great significance for future exploration, reserve evaluation, and well planning.