Sedimentary Environments And Mechanism Of Organic Matter Enrichment Of The Meso-Neoproterozoic Era In The Southern Margin Of Ordos Basin

Due to the low oxygen content in the atmosphere,low organisms,no terrestrial organic matter,and the long time the stratigraphic experienced of the Meso-Neoproterozoic,there are a series of research difficulties for the Proterozoic petroleum exploration.For example,did hydrocarbon organisms developed in the Proterozoic? What was the abundance of organisms that represent paleoproductivity? Whether the organic matter preserved after sedimentation.In recent years,Proterozoic petroleum systems,oil seeps,bitumen,and organic-rich rocks have been found in many parts of the world,which shows the vast potential of petroleum systems in the Meso-Neoproterozoic.The distributions of the Proterozoic source rocks were mostly controlled by the ancient Craton rift.There were several Proterozoic rifts in North China Craton.Although no large-scale Proterozoic hydrocarbon reservoir has been found in North China Craton,there are several sets of Proterozoic source rocks in the Yanliao area in the northern margin Craton,such as the Chuanlingou Fm.,the Hongshuizhuang Fm.,and the Ximaling Fm.It is worth paying attention to whether there were source rocks in the southern rift trough.Owning to the high degree of thermal evolution,less drilling,and failure of conventional source rock evaluation parameters,it is difficult to answer the question whether there developed Proterozoic source rocks in the Ordos Basin.In view of this,taking the Meso-Neoproterozoic in the southern margin of Ordos Basin as the target strata,this paper analyzed the sedimentary environment and organic matter enrichment mechanism though the combination of the organic-inorganic methods.The major and trace elements and the biological fossils were used to analyze the sedimentary environment,including tectonic setting,sedimentary provenance,climate,wethering,and hydrocarbon-generating organisms.The organic matter enrichment mechanism was revealed by the synergistic relationships between the proxy indexed of productivity,redox condition and TOC.The In-situ Fe and S isotopes of strawberry pyrite were studied to reveal the mechanism of organic matter oxidative degradation.Finally,the formation modes of the dark shales were summarized.The main results are as follows:(1)Proterozoic rifting was developed in the southern margin of the Ordos Basin under the background of the breakup of the Columbia supercontinent.In the depression stage,there were deposited Baicaoping Formation(1817±22Ma)under the coastal facies and Cuizhuang Formation(1648.4±6.5Ma)under the shallow sea shelf facies.The sand and mud interbedded of the Sinian Luoquan Formation(?543Ma)were deposited under the background of Rodinia supercontinent cracking.The dark shales in the three layers are the potential source rock series.(2)The sedimentary provenances of Baicaoping Fm.and Cuizhuang Fm.were felsic mainly.The climate during the sedimentary period of Baicaoping Fm.was arid.The hydrocarbon-generating organisms mainly consisted of prokaryotes and acritarchs.The conditions for the formation of source rocks were poor.The climate during the sedimentary period of Cuizhuang Fm.was warm and humid.And the shales were rich in hydrocarbon-generating organisms(e.g.cyanobacteria,brown algae).All the conditions satisfied the requirements of source rock formation.The sedimentary provenance of Luoquan Fm.was continental laban basalt mainly.The climate was warm and humid.There also developed hydrothermal activity.The hydrocarbon-generating organism was Microplanktonic algae mainly.All the conditions satisfied the requirements of good source rock formation.(3)The TOC in Cuizhuang Fm.has a mean value of 0.59%.The biological Ba content of Cuizhuang Fm.was less than 400 ppm,indicating low productivity.And there was no obvious correlation between the productivity proxy index and TOC.The enrichment of V elements is obvious,and the V content is positively correlated with TOC,indicating an anoxic-ferruginous condition and the organic matter enrichment was mainly controlled by the redox conditions of the bottom water.(4)The TOC of Baicaoping Fm.and Luoquan Fm.are all low,basically <0.4%.The productivity of Luoquan Fm.is the highest,indicating enrichment of nutrient elements.The bottom water belonged to anoxic-sulfurized condition,implying good preservation condition.The organic matter was oxidized and degraded by BSR,leading to a low residual TOC.The productivity of Baicao Fm.was similar to that of the Cuizhuang Fm.,both of which were lower.The enrichment of the V element is not obvious.The frequent rise and fall of the sea level led to poor organic matter preservation conditions.So the potential of the source rock formation was not obvious.