Characteristics And Dynamic Settings Of Yanchang Period Hydrocarbon-rich Depression In Ordos Basin, China

Ordos basin locate at the overlapping part influenced by activities of various large structural domains consisting of the peri-Pacific, Siberia, Qinling-Qilian and Tethys, and spans in the west the famous N-S trending structural belt of China. The complicated evolution-reformation history is formed as a result of the combination, superposition and continuous changes different structural domain and multiple geodynamic setting. The spatio-temporal relationship of forming middle-late Triassic Yanchang period hydrocarbon -rich depression in middle-southern Ordos basin and the process of Qinling orogenic evolution are intimately related. Formational environment and sedimentary records of hydrocarbon-rich depression in depth study could explore the coupling relationship of orogenic belt and reveal the formational dynamic settings of hydrocarbon-rich depression.The thesis hold the three principles that research thinking"Integrity, Dynamics and Synthesis"of basin analysis as guidance. Based on lots of field profiles investigation, core observation, rock-minerals analysis, geochemical and geophysical data, synthesis research results and data of sedimentology, structural geology, oil-gas geology and exploring, reconstructing the bottom palaeo-topography framework of hydrocarbon-rich depression, analysis distribution characteristics and formational mechanism of excellent hydrocarbon source rocks, accompanies event deposits and fossil beds, then disscuss the major characteristics and formational dynamic settings of middle-late Triassic Yanchang period hydrocarbon-rich depression in Ordos basin.Many analyses of lithology-minerlogy and geochemistry indicate that Yanchang formation Chang7 excellent hydrocarbon source rocks containing much pyrite and forming reduction environment of deep lacustrine to semi-deep lacustrine facies in Ordos basin. The excellent hydrocarbon source rocks present two remarkbly characteristics, the first one is enriching uranium element. The uranium-rich source rock distribute in the center of hydrocarbon-rich depression with NW-SE trending. Uranium concentration induced by the action together with anoxia environment, abundant uranium element, high content organic matter and collophane. The uranium may be mainly from the volcanic ash sediments and deep hydrothermal liquid substances. The second distinguished characteristic of excellent hydrocarbon source rocks is interlay with much volcanic ash sediments. Water and air transport approches are the primary formational mechanism of volcanic ash sediment which depositing in NW-SE trending with the tendency that being gradually thinner, less and smaller from SW to NE. From the discrimination diagram of tectonic environment, we conclude that it came from calc-alkali magma area, the rhyolite of Tianshui area in western Qinling Mountain is one of original areas of volcanic ash sediments. The nutritive substance brought by volcanic ash may lead to blooming of alga and raised the primary productive of the palaeo-lake. It is benefit for building the organic-rich beds and it is important for the development of hydrocarbon-rich depression in Yanchang period.Based on lots of field profiles investigation, core observation, inegrate the latest sedimentary facies data, analysis the evolution characteristics of lake strandlines and deep lake zones, showing twice important depocenter migration in Yanchang period due to the orogenic process of Qinling. Using the relationship of U/Th value which is redox discrimination index and GR valute of log curve, caculated the palaeobathymetric, the deepest region loacating in Huachi-Baibao areas reached at 100m, and recovering the lacustrine basin bottom palaeo-topography frameworks indicate the lacustrine basin with flatter and steep slope in Northeastern and Southwestern, respectively. Express NW-SE trending distribution and opening to the east. Basin filling types analysis indicates that Chang73 deep lacustine sub-assemblage filling type develop excellent hydrocarbon source rocks which have the highest abundance organic matter. It may be closely relating with volcanic ash deposit and deep hydrothermal substance input except the bigger accommodation space due to the bigger amplitude of tectonic subsidence.Many syndepositional deformation structures and their activity imprints in outcrop area and core at hydrocarbon-rich depression can be observed. Through detail analysis on characteristics, distributions, formational mechanisms of seismites, turbidite and soft -sediment deformation sturtures, we conclude that the tectonic setting is obvious activities during the formation of hydrocarbon-rich depression periods, which are controlled by the Qinling orogeny belt. The spatio-temporal distrutbion of excellent hydrocarbon source rocks, volcanic ash sediments, seismites, turbidite are consistent and interactional, and forming the uniform dynamic setting. Combining with the petrological and mineralogical, elements geochemistry, isotopes geochemistry, platinum group elements of interbeds or existence with excellent hydrocarbon source rocks and trace elements in petroleum demonstrate the hydrocarbon-rich depression keeping many evidences about deep hydrothermal fluids activity, which are influenced by the process of Qinling orogeny belt and related with the intermittence rupture-close activity of fundamental faults. The subsidence amplitude is very large of hydrocarbon-rich depression, lacating in deep lake area, and consistenting with abnormal thermal and crust high conductivity area, which are closely related coupling with the the process of Qinling orogeny on spatio-tempal relationship. The deep hydrothermal fluids active areas distribute in 38°N structural zone and the biggest subsidence amplitude with anomalous thermal area in the basin, which shows that the hydrocarbon-rich depression developed in the background of active deep processes. Throngh detail investigation of outcrop, integrating a large number analysis of geochemistry, lithology-mineralogy and sedimentology, combining regional geological characteristics, which indicate that structural action is obvious and active deeply dynamic background settings are the mainly reasons of bivales blooming and death, which is benefit for revealing the formational settings of hydrocarbon-rich depression.After analyzing abnormal magnetic field and lithosperic data, combining many special geological appearances preserved in Yanchang period hydrocarbon-rich depression. So based on the analysis of prototype basin visage and basin-mountain coupling relationship in Yanchang period, the author believe that although its famous for stable, Yanchang period hydrocarbon-rich depression formed in the setting of structural action is obvious and actively dynamic background which response to the process of Qinling orogenic form deep to shallow.