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

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.