Geochemical Characteristics And Distribution Prediction Of Precambrian Source Rocks In The Southern Margin Of Ordos Area

With the discovery of the Mesoproterozoic natural gas in the Yimeng uplift of the northern margin of the Ordos Basin and the discovery of the Proterozoic solid asphalt in the southwestern margin of the basin,it is revealed that the Precambrian ancient source rocks have certain hydrocarbon generation potential,Although it may be damaged due to the high thermal evolution and the tectonic evolution in the later period.,but by clarifying the intrinsic organic matter enrichment mechanism and overall distribution law of the source rock,it can provide a new idea and theoretical basis for identifying potential effective ancient source rocks.In this paper,the source and environment of source rocks are comprehensively studied through tectonics and sedimentary backgrounds,combined with lithology,sedimentary facies and trace elements.The initial evaluation of source rocks is carried out by organic geochemical methods,and different hydrocarbon generations are evaluated in combination with microscopic biocombination type identification.Biomass hydrocarbon source rock hydrocarbon generation potential,parallel comparison of outcrop profile differences,thus establishing an effective ancient source rock identification mark.Finally,based on the evaluation of source rock and organic matter enrichment mechanism,combined with lightfaces palaeogeographic data,the distribution of Precambrian source rocks is predicted.The main conclusions are as follows:(1)The TOC of the Changchengian System of the Yongji area,the Baicaoping Formation,Cuizhuang Formation and the Gaoshanhe Formation of the Changchengian System of the Luonan area are higher than the lower limit of the organic carbon content of the source rocks;while the Sinian Luohuan Formation and the Zhangxia Formation of Precambrian system the TOC did not reach the lower limit of the organic carbon content of the source rock.(2)The overall sedimentary environment of the Gaoshanhe Formation of the Changchengian System is the most conducive to the accumulation and preservation of organic matter particles,followed by the BaicaopingFormation and Cuizhuang Formation,and finally the Jixianian System Wanquankou Formation.(3)Regarding the hydrocarbon-forming biological combination,a large number of benthic algae + planktonic algae were found in the Baicaoping Formation and Cuizhuang Formation of the Changchengian System of the Yongji area,and only a large number of benthic algae were found in the Gaoshanhe Formation of the Changchengian System of Luonan.No hydrocarbon-forming organisms were found in the Wangquankou Formation of the Jixianian System.The TOC of the Gaoshanhe Formation dominated by a large number of benthic algae was significantly higher than that of the Baicaoping Formation and the Cuizhuang Formation with planktonic algae in the sample;(4)The comprehensive comparative analysis indicated that the hydrocarbon potential of the ancient source rocks was the Gaoshanhe Formation of the Changchengian System,the Baicaoping Formation and the Cuizhuang Formation of Changchengian System,and the Jixian system Wangquankou Formation;(5)In the area of Yongji,the black mudstone of Baicaoping Formation,Cuizhuang Formation and the Gaoshanhe Formation of Luonan area are presumed to be widely distributed in the southern margin of Ordos,while in the area of Pingliang,the source rock of Wangquankou Formation may be widely developed in the direction of the southern margin and the western margin.The black mudstone of Sinian Luoquan Formation and the limestone of Cambrian Zhangxia Formation are both judged as non-hydrocarbon source rocks,so the potential ancient hydrocarbon source rocks that may develop in Sinian and early Cambrian should be further exploration of the continental surface sea and shallow sea facies in the southwestern margin.