Strike-Slip Duplex Structures: A Case Study from the Bohai Oilfield Offshore Eastern China

Xu, Changgui; Zhang, Rucai; Kui, Wu; Zhou, Xinhuai

The Shabei structural zone of the Bohai oilfield lies in the western Bohai Sea, east to the Bozhong depression and south to the Shleitian uplift and west to the Nanpu depression and north to the Shijiutuo uplift. The structural zone lies in a saddle between the uplifts and depressions. The Shabei structural zone is close to the hydrocarbon-rich depression, a favorable oil and gas accumulation zone with high quality reservoir and cap rocks in the Dongying group and Minghuazhen group and favourable migration conduits. Several oil fields with OOIP of a billion bbl have been discovered in the area including the Caofeidian 11-1/2 Oilfield, Nanpu Oilfield and the Qinghuangdao 32-6 Oilfield.

Strike-slip duplex structures developed in the Cenozoic strata because of the restrain from two major uplifts to the north and south. The strike direction of the main faults are nearly EW and the associated faults are trending NE, with the two sets of faults intersecting each other at nearly 45°angle in the planar view, indicating that faults originated in hard basement rocks. The slices in different time-depth show that the main faults in the EW direction are a single large fault during the Paleogene and developed to right-hand step-over en echelon faults during the Neogene. The main faults and their associated faults formed negative flower structures displaying an inverse Y shape on seismic profiles. In one of the restraining bends of the Tan-Lu strike-slip fault close to the Penglai19-3 giant oilfield in the eastern Bohai Sea, strike-slip duplex structures are developed resembling to those of Shabei structural zone. In the Bohai Sea the regional oblique extensional stress direction was north-west to south-east during the Paleogene. in contrast the Neogene strike-slip movement is dextral according to the drag characteristics in seismic slices. In the Shabei structural zone the duplex structures are formed because of the regional stress with dextral strike-slip and the restrain of the two major uplifts to the north and south during the Paleogene. Many fault-block structures were formed between the main faults and the associated faults with the fault-block traps primarily distributing on structural highs forming good prospects.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013