Sediment Source And Sedimentary System Study In Upper Paleozoic In Yanchang Oil Region Of Ordos Basin

Gas reservoir which had been discovered is located in the central and north of Ordos basin. The new discovery has been found in Yanchang region in southern basin these years, and it shows great exploration potential. But the research degree is lower. So the study on stratigraphy, source and sedimentary system has great meaning in this area. Based on the Sedimentology, Petroleum Geology and other theories, according to the data of drilling, logging, gas testing and outcrop, the stratigraphy, sediment source and planar distribution feature of the skeleton sand bodies are analyzed in the study area. The characteristics of sedimentary facies and distribution rules are also analyzied. The sedimentary facies model of skeleton sand bodies is summarized. Additionally, with the present exploration, the gas reservoir-forming rule is discussed. The research provides certain foundation for further gas exploration. The conclusions are as follows:The analysis of the profiles of stratigraphic correlation and isopach maps of stratigraphy, it reveals that the benxi and taiyuan formations are thick on the east and thin on the west. This phenomenon suggested that the transgression direction is from east. The thickness of other formations is stable. Furthermore, with the analysis of paleocurrent direction and heavy minerals, it shows that the shanxi and shihezi formations are located in the junction area affected by north and south source. The source is from northeast, northwest and south, the central part is placed in junction area.The analysis of sedimentary facies suggested that it developed carbonate platform, coast, lake, meandering river delta and braided river delta system. Among them, the carbonate platform and coast sediment were developed in benxi and taiyuan formations. Shanxi, shihezi and shiqianfeng formations developed meandering river delta and lake sediment, he8 and qian5 members developed braided river delta sediment.The sedimentary micro-facies of gas-bearing skeleton sand bodies in the study area is barrier island and deltaic front subaqueous distributary channel. The deposits of benxi formation is sand bodies of barrier island which extented from northeast to southwest. Shanxi, shihezi and bottom of shiqianfeng formations developed four deltaic front lobes which are convergent to the central.The gas reservoir type of Upper Paleozoic in the study area is lithologic gas reservoir. So the source rock, reservoir spreading controlled by favorable facies belt, and beneficial tectonic locations are the main factors of gas reservoir-forming. Based on all of these, four favorable exploration belts are predicted.