Origin And Paleoenvironment Reconstruction Of Phosphorus In Sandstones Of The Cambrian Xinji Formation,the Southwestern Margin Of Ordos Basin,China

Phosphorus,as a bioelement,has been the focus of geochemical research.As a geological event deposit,phosphorus concentration in sandstone is closely related to the paleoenvironment.Increasing exploration interest in oil and gas hosted by early Cambrian strata has focused research effort on early Cambrian sandstones.Previous studies have shown that the sandstones of Xinji Formation are rich in phosphorus and a superior reservoir of physical properties for oil and gas.However,until now,the research on the enrichment of phosphorus is still insufficient.The sandstones enrich phosphorus are closely related to the paleoenvironment.In this paper,X-ray diffraction(XRD),optical microscopy,grain size analysis,total organic carbon(TOC),and the concentrations of major,trace,and rare earth elements(REEs)are analyzed for using to understand the character of phosphorus-bearing sandstones from a profile through the Xinji Formation at Jingfushan,Longxian,Baoji City,China.The origin of phosphorus and paleoenvironment of deposition are discussed through the combination of the petrology and geochemistry.The sandstones are mostly detrital quartz sandstone,with calcareous cement.According to XRD,the quartz content of the sandstone samples is 39.8–73.9%(average 59.18%),with illite(7.9–27.6%,average 16.35%)and calcite(4.5–29%,average 12.66%).Grain size analysis indicates that the mineral particles of sandstone samples are mainly well sorted with fine-grained size,depositing in strong paleohydrodynamic force such as turbidite.In the major elements results,the value of SiO₂ is 37.69–78.19%(average 61.59%),followed by Al₂O₃(6.11–13.67%,average 9.16%).Compared to the Upper Continental Crust(UCC),P₂O₅and CaO are obviously enriched.The trace elements factors show that the boron is highly enriched,while Sc,Ni,Sr,and Ba are relatively depleted.The chondrite-normalized distribution patterns of rare earth elements reflect that light rare earth elements show more enriched pattern than heavy rare earth elements.According to the petrological and geochemistrical characteristics and the results of TOC,the phosphorus enriched in the calcareous cementation of sandstone is of biological origin.A wide variety geochemistry parameters and diagrams indicated that the Xinji Formation was a near-source at the passive continental margin.The source of Xinji Formation sandstone samples was mainly a mixture of sedimentary rock,calcareous shale,granite and alkali basalt,with the provenance of terrestrial clastic materials.The sandstone deposited under oxic condition in shallow high energy hydrodynamic and warm and humid paleoclimate with saline to brackish feature,which means ocean sedimentary envrionment.The Phosphorus is sensitive to paleoclimate and is more enriched in warm and humid weather.Based on the above research results,the sedimentary model of Xinji Formation in the southwestern margin of the Ordos Basin was established.These insights can provide a sound theoretical basis for oil and gas exploration in early Cambrian rocks of the Ordos Basin.