Abstract:

The main oil-generation region of Jizhong depression,including Gu'an,Baxian,Raoyang,Shenxian and Shulu sags,was a unified oil-generation depression during the Eocene and Early-Middle Oliocene.The basement rocks of the depression were cut into a series of rhombic or semi-rhombic fault blocks with uplifts and depressions by fractures due to multi-tectonic movements.These fractures are normal faults in profiles,but have echelon arrangement,shear displacement,branching fractures and dragging structures on the horizantal plane,showing that most of the faults were formed by shearing stress.This is further proved by anticlines,synclines and even the echelon arrangement of higher-order structures.The compressive structurt in the present area are complicated by tensil or tensil-shearing fractures.This kind of geological phenomena suggests that these structures are characterized by both directions of clockwise and anti-clockwise and compresive and shearing mechanical properties.The study on the history of tectonic development indicates that the extention of the depression took place during the deposition period,thus formed mid-small dextal structures;the contraction of the depression occurred in the discordant period,and formed sinistral structures.This kind of sinistral and dextral shearings have happend alternatively for many times ever since the Tertiary,and finally formed a huge structural complex with buried-hill as its main body.The reversal motions led to multi-migration,extenct distribution and the best enrichment of hydrocarbon.