Mesozoic And Cenozoic Tectonic Evolution Of The Transverse Structure In The Middle Of West Margin Of Ordos Basin And Its Significance To Oil And Gas Accumulation

The transverse structure in middle western margin of the Erdos Basin is located between Fort Ciyaopu and Majiatan. As the western extension part of Dingbian-Jingbian east-west fault in central part of the basin, the fault cut through basin basement, presumably which developed in the Archean, still active in the Paleozoic and Mesozoic. Its formation and evolution have caused some banding effect in the north and south sides of the basin topography, stratigraphy, gravity and magnetic anomalies, the distribution of coal seams, Moho changes, geochemistry, oil and gas distribution, etc. The transverse structure belt in the northern boundary of Qingtongxia-Wuzhong fracture, in the southern boundary Zhongning-Zhongwei fracture. The internal structure is composed of face mountain syncline, gravel well anticline, four ear mountain syncline, the Yujia beam (black bumps) anticline, Niubu beam syncline, and Shaba well anticline.The thrust fault in Hengshanpu diped to east and rushed to west, in Majiatan it is opposite. The transverse structure play a regulatory role to the north and south on both sides of the tectonic style.Through the sample fission track analysis, Ciyaopu area uplifting time earlier, started from the early Jurassic interim; Shigouyi-Majiatan inside transverse structure tectonic slightly later than the north,which started in the middle Jurassic; and south of the belt uplifting time later, began in the late Jurassic. Through thermal history modeling of apatite in the study area found that in the late Triassic to Middle Jurassic mid rapid warming, the maximum temperature can reach110-120℃, in the subsidence stage; to the mid of late Cretaceous the study area is in the cooling phase,the minimum temperature dropped to65℃,in the uplifting stage; to Miocene, the temperature has been steady, no big tectonic movement occured in the study area. Since the Miocene, the temperature dropped to about20℃, the study area were uplifted to erode, the samples were lifted to the surface. Tectonic evolution of the study area is divided into6stage: Early-Middle Triassic, Late Triassic, Early-Middle Jurassic, Late Jurassic-Early Cretaceous, Late Cretaceous to Oligocene and since Miocene. Ciyaopu in the north of the transverse structure was uplifted in the mid-Early Jurassic, fission track age was190Ma; the transverse structure was uplifted at the beginning of the Middle Jurassic, fission track age was168to158Ma; Early Late Jurassic to Early Cretaceous, the study area began thrusting largely, fission track time is152to122Ma. The oil and gas fields are mainly concentrated in Hengshanpu and Majiatan,in the western margin of the basin. Transverse structure affected the source rocks of the thickness and distribution in the north and south sides.Different thermal evolution leaded to different maturity and expulsion time of source rocks.Different thrusting systems formed different types of traps in the north and south sides, thus forming oil in the north and gas in the south.