Ordos Block Southwest Of The Early Palaeozoic Rift Continental Margin Structure And Evolution

The southwestern part of the Ordos Block was a rift continental margin opened to the Qinling and Helan trough of the Qinling-Qilian-Helan three-arm rift system in Early Paleozoic. Based on the sea-level changes during that time, the sedimentary sequences deposited on the margin may be divided into two composite sequences by the boundary of large scale marine regression in the Yeli-Liangjiashan stage of Early Ordovician. i. e. the Cambrian composite sequence and the Ordovician composite sequence. The structures of the margin which show by the framework of these two composite sequences have two different types, too: the gentle slope type in the Cambrian, which is characterized by rift side slope-shelf break-platform, and the differential type in Ordovician, by rift side slope-rift shoulder arch-extra arch depression. The former had mainly been formed in Middel Cambrian (about 536-517Ma), and the latter was chiefly constructed in Majiagou stage of Early Ordovician (about 490-469.6Ma). After the Early Paleozoic rifting, the rift continental margin had been undergone three times of different reformations: before the Permian, it was characterized by inherited uplifting and subsiding, from Permian to Early and Middle Trassic, by the uniform isostatic subsiding, and from Late Triassic up to present, by differential subsiding. Then, the resultant structural framework of present has been formed. According to the analysis of the structural framework of the rift continental margin in Early Paleozoic, the features of sedimentary sequences, and the present structures of the area where we study, it is considered that the occurrence of the natural gas in the lower Paleozoic would exist in the dominant form of deep basin gas. And there would be three gas systems with two genetic types, i. e. the Dingbian-Jingbian gas system occurring in the composite syncline of the middel-eastern part of the Ordos Block, and the Huanxian-Jingchuan gas system occurring in the foredeep depression of the western part of the Ordos Block, both are occurred in the form of deep basin gas. The third is the Ningxian-Binxian gas system occurring in the form of normal traps in the southern part of the Ordos Block.