Paleostructural evolution of Lower-Middle Ordovician top and its relationship with hydrocarbon in Tahe Oil Field

Ordovician is the main hydrocarbon producing reservoir in the Tahe Oil Field.Studies of tectonic evolution and paleostructures indicated that the Middle—Lower Ordovician top was a nose-like convex to the northeast during the 2nd phase of Caledonian Movement.The highest point located around the present Luntai Fracture.During the early Hercynian period,structure axial changed to NNW.From the late Hercynian to the early Himalayan,the study area was a rectangular convex composed of NW and NE oriented slopes and the structure axial was to the northeast.From the late Himalayan till now,influenced by the western stress effect and the regional warping,the southern axial remained unchanged,yet the northern axial changed from west to east,and the highest point migrated from north to south.Tectonic,karst and diagenesis effects during the middle and late Caledonian provided room for the first hydrocarbon accumulation in the Tahe Oil Field.Tectonic movements during the early Hercynian destroyed the original reservoirs and heavy oil reservoirs were formed in some region,providing reservoir room for oil-and-gas filling during the late Hercynian.Since Indosinian,accumulation system of the study area has been adjusting.