Large Scale Echelon Faults on the Shaleitian Uplift in Bohai Oilfield, Offshore Eastern China
Lixin, Tian; Zhang, Rucai; Jianhua, Fan; Zhou, Xinhuai; Xu, Changgui
The Shaleitian Uplift lies in the western Bohai Bay and strikes EW. Hydrocarbon accumulations on Shaleitian Uplift are tied to large scale echelon faults in the Neogene strata. The strike direction of the echelon faults are mainly NNE. Based on high resolution 3-D seismic data visualization, each group of the echelon faults are composed of two pairs of echelon faults with different dip directions. Along the strike direction the echelon faults have different characteristics at different bands. The topographic map of the buried hill shows that some north-northeast fractures are revealed in the buried hills. The overall trend of strike-slip and down slip of the three groups of echelon faults have changed significantly on the seismic variance slices. This probably shows that there is a north-west broken zone in the middle of the uplift. According to the Seismic profile, the Neogene faults combine into various "Y" shape configurations with dip over 70°. The vertical throw of the echelon fault is minor in the Paleogene but it was increased constantly in the Neogene and then reduced constantly from the late Neogene to present. The Seismic profiles parallel to the echelon fault show that Palaeogene sequences filled between the buried hills and formed an onlap unconformity, which proved that the NNE grooves in the buried hills have been pre-existed. Going with the grooves filled the amplitude of local sinks reduced gradually in the Neogene. The Neogene strata curved upward between the echelon faults. The deepest place is attached to the downthrown section of faults, which show late compresso-shear effects. This is significantly different from the strike-slip pull-apart regime. The en echelon faults in the south of the Shanan depression dip toward SE and trend NEE, similar to the echelon faults on the Shaleitian Uplift. The density of the echelon fault is bigger and with no paired echelon faults. The echelon faults on the uplift terminate at the boundary of the depression, and have not been intersected with those faults in the depression. This shows that the formation and evolution of faults in the depression and the uplift are different. The buried hill basement had dextral slip with horizontal displacement about 2.0 km in the Neogene, and eventually formed a group of left-stepping echelon faults in the shallow strata. The echelon fractures are extensional and can act as excellent conduits for vertical fluid flow.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013