Marine Basin Prototype Of Early Paleozoic In The Southwestern Ordos Block And Its Restriction On Oil And Gas Geological Conditions

The Southwest of the Ordos block is part of the steady North China craton, where the Early Paleozoic marine strata have completely developed and the phenomena of all kinds of marine sedimentary are rich, adjacent to the Qilian-North Qinling tectonic belt. In recent years, the Natural gas exploration in the Early Paleozoic of the southeastern Ordos basin continuously has new progress. Research on the characteristics of prototype Basin and lithofacies-paleogeographic evolution, and also controlled factors of the primary oil and gas geological conditions in the southwest Ordos block, has become a hot issue. This dissertation, based on methods of the field geology investigation and the samples analysis, as well as the paleostructure and lithofacies-paleogeography maps, synthetically analyzes basin and mountain structure topography, and Early Paleozoic marine sedimentary-tectonic environment and characteristics of prototype basin, as well as the influence factors of the primary oil and gas geological conditions in the southwest of the Ordos block. The main results achieved in this dissertation can be summarized as follows:Field geological survey of structural section combined with regional geological structure research, system analysis reveals the basin and mountain structure of southwestern Ordos block and its controlled fracture tectonic system. we think that the western Ordos basin is bounded by Qinglongshan-Pingliang fault and connected with Laolongshan-Shengrenqiao fault in south of Ordos basin if going east, which together constitute as structural margin between the Ordos stable block and activities orogenic belt adjacent to the southwestern basin; the fault belt of the borderline in the northern of Qilian-North Qinling zone is west of the Haiyuan fault or Xinanhuashan fault in the north of the northern Qilian, connected with Luonan-Luanchuan fault in the north of the northern Qinling if going east, constitutes a boundary between the shallow thrust fold fault belt in the southern margin of the ordos (north China) block and the deeply metamorphic deformation zone in the Qilian-North Qinling.On the basis of the field geology investigation and zircon U-Pb dating and petrologic and geochemical analysis, establish a unified comparable Early Paleozoic marine sedimentary stratigraphic division and correlation scheme in the southwestern margin of the Ordos Block, especially for the several important marine strata units era is controversial proposed by new quantitative constraints chronology in the study area for a long time. The result show that detrital zircon U-Pb age spectrum is basic comparable, which is sedimentary sandstone of the activity type Marine flysch, including the xiangshan group in the margin of western basin and the Miboshan Formationg in the west as well as the Pingliang Formation in the south. The younger age groups of the tuff and tuffaceous sandstone zircon U-Pb age spectrum are mainly concentrated in 462-454Ma of the Southern margin Pingliang Formation, indicating that they should be the main part of the Late Ordovician sedimentary meanwhile different stratigraphic units. Detrital zircon U-Pb age spectrum of the Tangwangling conglomerate in the southern margin of the basin and Sinian Zhengmuguan Formation in the western is very similar, but with detrital zircon U-Pb age spectrum have a significant difference in Pingliagn Formation and it’s quite a stratigraphic unit. The younger age group mainly concentrated in 744-963 Ma, their stratigraphic age belong to the Late Proterozoic Sinian is appropriate.The results showed that in field and drilling profiles sedimentary facies types and its environmental evolution, in Early Paleozoic, there are mainly four different sedimentary facies types, including that coast facies, platform facies, platform edges and deep slope-trough facies in the southwestern Ordos block. The space-time evolution of sedimentary facies types and sedimentary facies mapping in different period, which is divided into two transgressive-regressive sedimentary cycles and the corresponding four evolution stage: Early-Middle Cambrian transgressive stage and Late Cambrian-Early Ordovician regressive stage, Middle Ordovician transgression stage Late Ordovician regressive stage.Through the sedimentary phase, sedimentary source environmental and petrochemical-geochemical combined with regional tectonic paleogeographic map, on the basis of which is divided into three evolution stage about the marine basin in the Early Paleozoic southwestern Ordos block:The prototypes basin of the Cambrian is passive continental margin-stable cratonic epicontinental seas basin, the overall presented platform within the shallow depression, low amplitude platform margin slope and slope break off the outside of the structure of gentle slopes; Middle Ordovician is mainly in post-arc rift basin marginal sea environment, showing the internal structure of the units within distinct differentiation depression, L-shaped rift platform margin slope withers and its outside for deepwater rift trough slopes, the overall display as continental margin-extension arc rift trough marginal and cratonic epicontinental seas basin of the difference structure with the inner shelf downwarped basin; Late Ordovician may be controlled by back-arc basin related activities marginal residual marginal sea with collision back-arc basin.According to the tuff and (tuffaceous) sandstone samples zircon U-Pb dating results in Late Ordovician Pingliang Formation show that not only zircon age spectrum distribution and also zircon trace element determination of the original rock types, the provenance of the tuff and (tuffaceous) sandstone samples is a two-way reflected mainly from the island arc complex zone and collision type island-arc volcanic rocks in the Qilian-North Qinling, and aslo from the Ordos block (North China block); The tuff and (tuffaceous) sandstone samples geochemical analysis show passive continental margin, active continental margin arc complex deposition and related-tectonic setting. Zircon U-Pb age from the tuff sample displayed a mainly age of 462-444Ma, which clearly identifies Ordos (North China) block and Qilian-North Qinling island arc collision time occurs mainly in the late Ordovician.Early Paleozoic Marine strata tectonic lithofacies paleogeographic and prototype basin characteristics study show that Cambrian source rocks is mainly controlled by L-shaped uplift the outside edge deepwater-slope environment, oolitic dolomite reservoir is distributed mainly L-shaped platform margin slope break-off and flank uplift; Middle Ordovician Majiagou Formation of hydrocarbon source rock is mainly controlled by the L-shaped uplift the outside edge and the inside depression area, dolomite reservoirs mainly along paleouplift inside and outside of flank slope environment; Late Ordovician Pingliang Formation source rocks are mainly controlled by the Ordos paleo-landmass outside of the edge deepwater rift-slope facies environment, and the reservoir rocks are mainly paleo-landmass steep-gentle slope transformation belt with the Carbonate reef and its leading edge with a gravity flow deposits.