Structural Style In Xihu Depression, The East China Sea Basin And It Control On The Oil-Gas Accumulation

Xihu depression is a cretaceous-tertiary basin undergoing multi-episodic tectonism. Multi-episodic rifting and inverted tectonism caused complicated structural styles and geology structure. Integrated structural styles analysis, combined with stress field analysis and sedimentation fill, revealed the control effects of the evolution of the basin and the process of sediment filling to trap development and process of oil and gas accumulation.Xihu depression is a compound half-graben of"fault subsidence in the east and overlap in the west", the faults of synrift phases control the principal structure. There are three group different trends of syndepositional faults, such as NE, NNE and EW. These faults make the basin"zoning in EW and blocking in SN", Syndepositional faults control the depocenters. There are two inversions in depression stage, the inversion structures in the basin can be classified into several types based on their differences in geometory: the simple back-thrusting inversion, complex Y-shaped inversion, H-shaped inversion, flower-like inversion, graben and horst inversion.Through tectonic subsidence analysis of the basin, Xihu depression go through fast subsidence in synrift phases, slow subsidence in depression stage, and then fast re-subsidence after inversion stage. Synrift phases include 3 rifting episodes, depression phases go through 2 inversion. The non-linear subsidence history has important effect in sedimentation filling, sedimentary buring and organic material evolvement.There are four hydrocarbon source rocks, Palaeocene, Eocene, Oligocene and Miocene. Distributary channel sand of river depositional system and delta front sand of delta depositional system in Pinghu and Huagang group are the primary reservoir. Compression and inversion action make series of traps, tectonic traps and stratigraphy traps are the primary trap types, Tectonic traps can be classified into anticlinal traps (compressional anticlinal trap, drape anticlinal trap, faulted anticlinal trap) and fault block trap (scalariform fault block trap, arcuate fault block trap, crossed fault block trap). Stratigraphy traps can be classified into stratigraphic onlap trap and stratigraphic erosion unconformity trap. The western slope belt and the central anticlinal belt are the most favorable zone of oil and gas accumulating.