The Study Of Sources And Senquence-lithofaces Palaeogeography Of Upper Palaeozoic ,northern Ordos

Ordos Basin is the second largest petroleum basin in our country, the Upper Palaeozoic strata are widely distributed with rich gas-bearing and large natural gas resources whose potential is huge. In recent years, natural gas exploration has made important achievements in the north Sue's area, Yulin region, Wushen banner region and Daniudi area. The upper palaeozoic strata have become the main formations of increasing the natural gas'production which lay a solid foundation for the West-East natural gas transmission. The new breakthrough in oil and gas exploration in the north part of the basin could not be achieved without new knowledge, new theories, new ideas, new technologies and the introducing and using of the new method. However, due to the large region, a series of new geological problems start bothering in different parts with the deepening and developing of the exploration, such as sediment sources in the northern basin, distribution and differences in depositional systems and unified layer sequence stratigraphic framework etc. In order to have a fully recognization about the related issues of the northern basin, these scientific problems require to carry out a systematic and holistic research about the northern basin or even the whole basin. The thesis reveals the distribution in the development of sedimentary system and sedimentary evolution process on the basis of the previous research. The following understands are mainly obtained:(1) The clastic compositions of Upper Palaeozoic in the northern basin show different periods of sediments in the west part of the research area are mainly from the northwest of the basin. While in the east and middle research area, the sediments are from the north and northeast of the basin which are under the control of the sources from the west and east ancient land in Yin Mountain. What is more, sediment sources have properties of recycled orogen.(2) The combination of heavy minerals show the western part of northern margin of the basin in Yin Mountain and northwestern margin of the basin in Lang Mountain and Helan Mountaim are the source areas in the Western study area. The Ural Mountain in the central of the northern margin and the western part of the Daqing Mountain in the eastern basin are the source areas of the central basin. ZTR index increased gradually from south to north which indicates the study area is in the north.(3) The debris combination types of Upper Palaeozoic in 8 formations are different from each other. These 8 formations are Benxi Formation, Taiyuan Formation, Shan2 Formation, Shan1 Formation, lower part of member 8 of Shihezi Formation (Lower H8 Formation), upper part of member 8 of Shihezi Formation (Upper H8 Formation), member 7- meber 5 of Shihezi Formation (H7-H5 Formation) and member 4- member 1 of Shihezi Formation (H4-H1 Formation). Each layer of debris combination types can be divided into three regions: the western, the central and the eastern part which indicate that different regions in the Upper Paleozoic northern basin are under the control of three different sources.(4) The cathode luminescence of quartz in Taiyuan Formation and Xiashihezi Formation are mainly brown then followed by blue, bluish violet, amaranth and nonluminous ultimately. It shows its parent rock is the mesomemetamorphic and the low- grade metamorphic rock. Plutonic, volcanic, contact metamorphic, sedimentary rock and sediment sources are from the crystallization metamorphic basement in the northern basin in Cambrian with mixed properties in its tectonic background.(5) The geochemistry characteristic of macroelement shows that the tectonic setting in source areas of Taiyuan Formation and Shangshihezi Formation is an active continental margin with trench-arc-basin systems and a passive continental margin with collisional belt. Taiyuan period, Shanxi period have an obvious nature of island arc and continerntal island, while Shihezi period turns to a single passive continental margin with collisional belt gradually.(6) The geochemistry characteristic of macroelement shows that source areas in northern basin have a complex tectonic setting background which exist at the same time. These are active continental margin, passive continental margin and continental island arc. Compared with basin drilling and REE characteristics of peripheral archicontinent, a conclusion is drawn that the material sources in the west are from Alashan archicontinent and the western archicontinernt in Yinshan, the material sources in the east are from the eastern archicontinent in Yinshan and the material sources in the middle is influenced by the Alashan archicontinent and Yinshan archicontinent. (7) According to the basin evolution, regional structure and sequence interfacial features, combined with previous studies, the Paleozoic of Ordos Basin is divided into one giant cyclical sequence, three super long base- level cyclic sequences and twenty long cyclic sequences and established corresponding sequence stratigraphic framework from outcrop, drilling, logging and seismic data.(8) According to the sedimentary, paleontology, geophysics and geochemical symbols, combined with rock assemblage, depositional fabric and profile sequence characteristics, the northern Upper Paleozoic Ordos Basin is divided into continental facies, paralic deposition and marine facies and recognized alluvial fan, rivers, lake delta, lakes, river control- tide dominated delta, a barrier coast and carbonate platform.(9) The system which makes long term cycles as the mapping unit compiled 20 pieces of the Upper Paleozoic- lithofacies palaeogeography maps. It reveals the differences of the northern basin's development and distribution and the evolution process in the sedimentary systems. The development of Late Paleozoic basins in the north is characterized by carpet delta and depositional systems in different periods have some differences. The sediment in Late Paleozoic experienced marine facies, paralic facies and continental facies and established a corresponding sedimentary evolution mode. It provides geological basis for oil and gas exploration in the Upper Paleozoic.