Liu, C; Wang, J; Qiu, X; Zhang, D; Zhao, J; Zhao, H; Deng, Y
The abundant petroleum resources of Ordos Basin mostly come from the hydrocarbon-rich sag of Middle-Late Triassic Yanchang Formation located at the southern of the basin. The shaly high-quality source rock from Chang 7 Member is the greatest contributor,which has the characteristics of rich uranium and multi-layered tuff. During Yanchang Period,the large hydrocarbon-rich sag had the paleogeographic structure of deep in south and shallow in north,and developed in semi-deep and deep lacustrine facies with high-quality source rock,high uranium content and thick tuff interlayer in trinity and mixed. The high-thermal evolution zones with multi-layers from Upper Paleozoic to Middle Jurassic were basically located at the same area,and the continuous subsidence areas of Early-Middle Jurassic were also superimposed on them. Comprehensive investigations have been revealed that during Yanchang Period,especially in the period of Chang 7 Member,various types of diapir structures (such as sandstone dyke or vein,mud phacolith as well as hydrothermal calcareous concretion),syndepositional deformations (such as faults,folds,crumple deformation,slump and landslide etc.) and event deposits (such as turbidite deposit,seismite and sublacustrine fan) occurred due to activity of the deep crust matters and mainly distributed at semi-deep to deep lacustrine facies of the hydrocarbon-rich sag. A variety of minerals and geochemical anomalies are detected in the source rocks which indicate the activity of deep thermal fluid. The deep geophysical survey shows that all the interfaces of crust-mantle above 400km in deep of the sag are arched. All above information indicates that hydrocarbon-rich sag and the high-quality source rock in Yanchang Formation formed under significant tectonic activity and deep process,which reveal the active geodynamic conditions and environments. The hydrocarbon-rich sag was spatially adjacent to the Qinling Orogenic Belt during Indosinian Period. The formation of the sag was simultaneous and synchronous with the development and magmatism,developing states,evolutionary stages and activity level changes of Qinling Orogenic Belt,which was related to the dynamic environment transition from collision to intraplate tectonic of the North China block-Yangtze block. The twice long-distance migration of the deposit-subsidence centers of the sag in Yanchang Period were the response and record to the initial significant environment changing of the Qinling Orogeny. Massive subduction materials were accumulated,melted and converted at the deep part backland of the North China block,which promoted the supersaturated and accumulation of deep matter,and increased abnormally of the thermal energy and pressure in this area during the convergent and collision of the two blocks. Under this circumstance,small scale mantle convection was formed,and material melted and thermal energy arched,caused the shallow superficial rupture and lateral extension in the crust. This process should be the main reasons to lead to the subsidence occurred,and the appeared of the hydrocarbon-rich sag with thermal-tension characteristics in Yanchang Period. This type of basin is named backland basin. The backland basin and the foreland basin are located at the thrusting block side and subduction block side respectively and very close to the collisional orogenic belt. These two types basin differ greatly in tectonic attributes,hydrocarbon occurrence and accumulation characteristics. © 2020 Science Press. All rights reserved.