Sedimentary-tectonic Evolution And Gas-potential Exploration Of Late Paleozoic In Southern Ordos Basin

It is disequilibrium for the gas exploration of Paleozoic in Ordos Basin:the large gas fields are currently found at the northern-central basin, which are large lithologic gas pools mainly controlled by sedimentary facies, but there is no important breakthrough in southern Ordos basin. Due to the limitation of the degree of exploration and many discrepancies in geological characteristics of southern basin, there is lack of systematic and in-depth research, especially in southern basin, the sedimentary system and the geological conditions of oil-gas form reservoir do not reach a clear and uniform viewpoint.Under the guidance of the modern theory of sedimentology and structural geology, based on the drilling, logging and core data, combined with the outcrop and seismic sections, and used many petrological and geochemical analysis methods, such as the light and heavy mineral composition analysis and the minerals of clastic rocks cathodoluminescence characteristics analysis, measuring the direction of palaeocurrent, sedimentary rocks geochemical diagrams, detrital zircon dating methods and so on. The authors systematically research the sedimentary system and the direction of provenance of Upper Paleozoic in southern Ordos Basin, together with the late Paleozoic tectonic evolution of the Qinling orogenic belt to discuss its control on the deposition of Late Paleozoic southern basin, reconstructed the originally sedimentary boundaries of Carboniferous and Permian of the basin; while taking advantage of cast thin sections observation, scanning electron microscopy (SEM), cathodoluminescence (CL) and electron microprobe physical property analysis testing methods, take the Upper Paleozoic member 8 of lower Shihezi Formation and member 2 of Shanxi Formation as the main study objects, which are the primary gas-bearing members, to analysis the composition of clastic mineral, pore assemble characteristics and its representatives of main diagenetic events, and to divide diagenetic sequence and diagenetic stage, at last to study the role of different types of diagenesis effecting on the reservoir properties; based on the above research, and combined with the characteristics of Meso-Cenozoic basin evolution and reformation, to analysis gas exploration potential of study area. The primary concludes including:There were five sedimentary systems in the research area, which including lake, meander river delta, braided river delta, shelf and coast sedimentary system. The main gas-bearing formations are Shanxi and member 8 of lower Shihezi developing meander river delta system and braided river delta system respectively. Physical properties of reservoir beds controlled by the distribution of sedimentary facies which controlled by the direction of provenance. There were four directions of provenance including west, southwest, south and southeast in late Paleozoic in southern Ordos basin at last, Every provenance provide different clastic assemble, reflecting the different rock assemblages in southern basin made the various contribution for sediments of the basin. The originally sedimentary boundary in late Paleozoic should have over the WeiHe basin nowadays and reached the region of North-Qinling, there might have a low uplift ancient land in North-Qinling providing the persistence provenances in late Paleozoic to southern basin. North-Qinling and southern Ordos basin might be the same basin in late Paleozoic, the differences of the sedimentary characteristic just because they belonged to the different sendimentary facies, North-Qinling might be nearer to the edge of the basin. A lot of structure phenomenon was caused by Caledonian movements in southern basin and North-Qinling, which showing the sedimentary process was controlled by the orogenesis of north-Qinling in Caledonian. Integrated analysis the distribution of sendimentary tracts and the directions of provenance in Paleozoic in southern basin, combined with the hydrocarbon-generating potential of source rock, the diagenesis types of reservoir beds, the conditions of cap rock and the characteristics of reforming in later period, the author think the research area has some potential for gas exploration, especially in the inner of WeiHe basin.