| The western margin of Ordos spreads across Inner Mongolia, Ningxia Province and Gansu Province. During Caledonian the Paleozoic sedimentary environment of the basin was controlled by the combined action of Helan aulacogen, corridor transitional zone and Qin-Qi passive continental margin, and then formed a unique depositional system which was distinct from open platform facies of Ordos Basin as well as from Abyssal sedimentary facies of Qin-Qi ocean basin depositional system. After that, it experienced multiple transformations by Caledonian. Variscan, Yanshanian and Himalayan tectonic movements, which made the western margin of Ordos basin become the most complex structural unit with combination of lithology, lithofacies and tectonic characteristics.There has been considerable controversy with the structural mode of western Ordos basin for a long time, in the past the mainstream view is that the western margin of Ordos is mainly controlled by overthrust structure, and thus performances as anticline structure controlled by fracture; secondly, it is considered that a large slip plane is developed in Carboniferous coal-bearing strata, resulting in repeated and thickened strata in and above Carboniferous in vertical direction. Therefore, there are great hopes for oil and gas prospects of the western margin of Ordos basin. However, through the system analysis on field observation and geophysical data, the results show that Jixian and lower Paleozoic strata exposure on the surface as belt distribution with an dip angle of60-80°, and these facts can’t be explained by thrust nappe tectonic pattern of slip plane developed in Carboniferous. In addition, based on the stratum structure analysis of Lower Paleozoic and the upper and lower formation, it is concluded that large-scale thrust movements occur in Yanshanian of Late Jurassic, and then is transformed by right-lateral strike-slipping of Himalayan and finally forms the present high-angle monoclinal structural style. |
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