| Ordos basin is one of the most prospecting basins for natural gas exploration in China.For a long time,North-Central region has been the exploration focus.In recent years,gas accumulation with large scale was also found in Southeast basin in Upper Paleozoic.Studying the regional provenance and depositional system is of great importance for the main gas productive interval,namely from the 2nd member of Shanxi Formation to 8th member of Shihezi formation.Based on the predecessors' comprehensive research,this paper use large quantities of data,including well log,core,sandstone composition,heavy mineral,REE,and Zircon U-Pb dating results,and multiple related disciplines,such as sequence stratigraphy,sedimentology,petroleum geology,to finish the systematic analysis of depositional distribution characteristic on the foundation of regional geological background.Finally,the evolution characteristics of the provenance and sedimentation are summarized.5 main conclusions of this paper are shown as follows:1.Stratigraphy:based on high-resolution sequence stratigraphic analysis,the strata from 2nd member of Shanxi Formation to 8th member of Shihezi Formation have been divided into 8 long-term cycles,6 mid-term cycles,which boundaries in the study area can be traced well.A Second Order sequence boundary is recognized between Shanxi formation and Shihezi Formation.2.Provenance division:through Zircon U-Pb dating results,sandstone component,heavy mineral and the REE analysis,and the combination with the contrast for the source rock features between the study area and the southern-northern basin edge,this paper conclude that the target area is influenced by three parts of the provenance mainly:respectively from North North East,North North West and South.The NNE and NNW provenance come from Alashan-Yinshan ancient land,and the southern provenance come from the Old Qinling ancient land,which is in the southern edge of the basin.3.Provenance evolution:the provenance from 2nd member of Shanxi formation to 8th member of the Shihezi Formation is distributed with inheritance.Northern source supply more than southern source generally.At the end of the 1st member of Shanxi Formation,north and south provenance begin to interchange,in the middle area namely to the north of the Yanan-Yanchang,south of the Yongping.At 8th member of the Shihezi Formation,northern source supply becomes stronger,and north and south provenance completely merge together.After these analyses,this paper determines the three dominated sphere by different provenance in the three sedimentary epic individually.4.Depositional system:Shanxi Formation mainly contains transitional facies,including shallow marine and delta front these subfacies.8th member of Shihezi Formation mainly contains braided delta depositional system,dominated by braided delta front subfacies,and subaqueous distributary channels,subaqueous interdistributary bay,subaqueous overbank sand are the main microfacies.The braided delta plain is in the northwest in the small scale.5.Sedimentary distribution:this paper mapped the subtle microfacies controlled by provenance,and determined the depositional features in south-east Ordos basin.Sand body belts in North-south direction perfectly match with the extending area of the provenance.The favourable microfices for gas-rich accumulation is the subaqueous front delta,because source merge and strong supply provide a good foundation for the reconstrution of the massive and extensive subaqueous distributary channels sedimentary in this area. |
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