Sedimentary System Study Of Permian Shaanxi Formation In South-eastern Of Ordos Basin

The sediment source and the development of the sand body of Permian Shaanxi formation in southeast of Ordos Basin has not only drawn an attention in hydrocarbon exploration in this area, but also been the controversial topic of the basic geological structure study for the sedimentary and structural evolution of Ordos Basin as well as the related research in Qinling Orogen. On the basis of the field section study and well log interpretation, using sedimentary, geochemistry and zircon U-Pb age analyize methods, the study of sediment resource, sedimentary system and sedimentary facies of Shaanxi formation which were based on the sub-layer division and correlation haven been carried out systematically. The main results and new understanding in this study are concluded as follows:The Shaanxi formation of study area has been divided into Shan1 and Shan2 layers, the Shan2 layer was further divided into 3 sub-layers. Thus the framework for sub-layer division and correlation of Shaanxi formation in study has been set up. The thickness of the sub-layers have shown a depositional characteristic of thin south and thick north.The paleocurrent, light and heavy mineral, cathode luminescence, rare-earth element and zircon U-Pb age study have suggested that the sediment of Shaanxi formation are mixed, the majority of it came from south while the north sediment has played a subsidiary role. The boundary of the source was located along Fuxian and Yichuan. The south sediment came from Early Precambrian metamorphic complex in the south part of the north Qinling is a complicated structural zone and Hercynian period intrusive rock as well as caledonian granite close to Shangdan suture. Therefore, the sedimentary boundary of the south part of the study area are roughly in the north of Shangdan subduction collision and orogeny.The depositional environment and its evolution of sub-layer in Shaanxi formation has been described precisely using trace elements B and Sr/Ba, the sedimentary structure mark and the sandstone grain size analysis. It is considered that the bottom of Shan₂³ belonged to marine sedimentary environment while the rest of Shan2¹⁺² and Shani belonged to terrestrial environment. Hence, the sedimentary system of Shaanxi formation was residual epicontinental seas and neritic lake basin environment.The well log interpretation along with sand thickness ratio analysis have indicated the sedimentary facies and sub-facies of Shaanxi formation. The bottom of Shan₂³ developed epicontinental tidal-flat, lagoon and delta front sub-facies. Shan2¹⁺² and Shan1 developed shallow lake sub facies under neritic lake environment and large scaled delta front sub-facies, the sediment supply of Shan1 is greatly sufficient, the underwater distributary channel from south and north flowed together in Fuxian.The isopach map of sand bodies and the section panels suggest that the sand body of the study area are consist of delta front sandstones which are controlled by the paleocurrents from south and north. The continuity and extension of the Shan1 is the best which has follow by Shan2¹⁺², Shan₂³ has developed small scale delta front sand body controlled by paleocurrent and potato-like shape tidal-flat sand while the property of the sand is worse compared with Shan1 and Shan2¹⁺².The petrographic thin section and scanning electron microscopre have indicated that the main sandstone types of Shaanxi formation are quartz sandstone, lithoclastic quartz sandstone and lithic sandstone with the development of intergranular pore and intercrystalline pore. The physical property data has shown that the property of the Shan1 formation is the best, followed by Shan₂¹⁺² and Shan_<sup>2³. The sedimentary system controlled the sediment source and sedimentary facies which has influenced the property of the reservoir. The characteristic of the parent rock has decided the sand type and the compositional maturity. The sufficiency of sediment supply has an effect on the scale and distribution of the sedimentary facies. Sedimentary facies has a further impact on the reservoir by controlling the distribution of the sand body, the grain size of the sand and diagenesis.