THE INVERTED NW-TRENDING CHENGBEI FAULT AND ITS SIGNIFICANCE IN PETROLEUM GEOLOGY

The Bohai Bay basin is characterized by the NE or EW-trending alternating uplifted zones or depressions due to the NW-trending oceanic subduction. Consequently, rare research focused on the Cenozoic NW-trending faults due to their inconspicuous structural feather. The inverted NW-trending Chengbei fault of the Bohai Bay basin could record the episodic deformation, and it is a practicable approach for understanding the tectonic evolution.

Based on the integration of new-released borehole data and seismic data, the spatial-temporal variation of the fault is investigated. We reconstruct its evolution by means of the depositional fault distribution maps, fault activity intensity, balanced cross sections analyses and visualization technology of Geoframe.

This study reveals that the fault experienced alternating phases of shortening, stretching and shearing deformation. It initiated as a thrust fault due to the north-south convergence between the North China block and the South China block during the late Triassic. During the early Cretaceous, the fault reversed as a normal fault that controlled the development of Chengbei sag, which is synchronous with the destruction of North China craton. The succeeding late Cretaceous is a stage of uplift and denudation without magmatism, which may result from the flat subduction of the Pacific Plate. During the early Paleogene (65-40Ma), the reactivation of Chengbei fault due to the roll back of the oceanic slab caused intensive rifting of the basin. During the late Paleogene (40-23Ma), it transformed to transtensional fault with an anti-“S” shape due to the north-south oblique stretching, meanwhile, a serious of EW-trending newborn faults occurred at the same time. During the Neogene and Quaternary, it was rebuilt by the strike-slip movement deeply, and it consists of a serious of EW-trending secondary faults. Furthermore, the conjugate strike-slip developed due to the east-west compression.

Two distinct belts can be distinguished along the sigmoid sinistral Chengbei fault. At the restrain belt, the steep fault could inhibit the vertical migration of petroleum and the hydrocarbon accumulated within the hanging wall. While at the releasing belt, the extensional fracture induces the long-distance migration of petroleum and it causes the reservoirs develop in shallow structure or buried hill.

Clear the evolution of the inverted fault is new perspective in revealing the tectonic evolution of the East Asia, and also significate for petroleum exploration in the region.