Sedimentary Structural Pattern And Its Significance Of Petroleum Geology From Sinian To Early Cambrian In Northern Margin Of Upper Yangtze

The Sinian(equivalent to Edicaran)to Lower Cambrian in the northern margin of the upper Yangtze is the key area to understand the evolution of the late Neoproterozoic sedimentary structural pattern of the Yangtze plate,and also the hot areas for future oil and gas exploration in Sichuan Basin.However,at present,there are still different understandings on the stratigraphic correlation,sedimentary facies distribution,sedimentary structural evolution and dynamic mechanism of Sinian to Early Cambrian in this area,which restricts the analysis of oil and gas geological conditions and the next exploration work in this area.Therefore,this paper selects the Sinian to Lower Cambrian in the northern margin of upper Yangtze,and uses field outcrop and drilling data,seismic data,geochemical data and isotopic chronology data.Guided by sedimentology,petrology,geochemistry and lithofacies paleogeography.Through systematic stratigraphic correlation,and the analysis of petrological characteristics,sedimentary characteristics,lithofacies paleogeography and paleostructure,from point to surface.Combined with the previous research results,this paper discusses the Sinian to Early Cambrian sedimentary structural evolution and its oil and gas geological significance in the northern margin of the upper Yangtze.Based on the analysis of cross-section and plane stratigraphic correlation,it is considered that the difference of stratigraphic thickness of Sinian to Lower Cambrian in the northern margin of upper Yangtze is obvious.The thickness of Lower Sinian Doushantuo Formation is thicker in Mianyang-Changning,Ningqiang,Ziyang-Maliu,Wanyuan-Dazhu,Chengkou Mingyue and Jiulongwan areas,but thinner in Micang mountain,Dazhou-Kaijiang,Chengkou Dong'an and Yichang areas.The thickness of Upper Sinian Dengying Formation is thin in Mianyang-Changning,Dongxihe,Wanyuan-Dazhu,Chengkou Mingyue and Wuxi Gaozhu,but thicker in Ningqiang,Micang mountain,Dazhou-Kaijiang and Chengkou Dong'an areas.In the Early Cambrian,the thickness of Madiping formation and Qiongzhusi formation is thicker in Dongxihe,Kuanchuanpu,Mingyue and Liulindian areas,but thiner in the Micang mountain,Dazhou-Kaijiang and Dongan areas.Therefore,the thickness of Sinian to Lower Cambrian in the northern margin of the upper Yangtze is characterized by uplift depression alternation.There are three different palaeogeographic environments from Sinian to Early Cambrian in the northern margin of the Upper Yangtze: shallow water platform facies,platform margin facies and slope-basin facies.From Doushantuo Formation of Sinian to Qiongzhusi formation of Early Cambrian,the shallow water platform facies,platform margin facies and slope-basin facies developed alternately along the northern margin of Upper Yangtze,showing a structural pattern of uplift depression alternation.There are obvious rifting activities from Sinian to Early Cambrian in the northern margin of upper Yangtze.In the early Sinian,the southern Qinling area experienced strong rifting,and gradually formed a depositional structural pattern of East-Westtrending horst-graben alternation,which led to the formation of deep-water basin facies in the southern margin of the southern Qinling near the northern margin of the Yangtze platform.The northern margin of the Yangtze platform experienced obvious extension,and led to the development of the extensional basin in the Chengkou areaw hich might extend into the interior of the Yangtze Platform during the early Sinian.The continuous extension from Sinian to Early Cambrian resulted in the development of typical siliceous hot spring activity on the top of Dengying Formation.In the northern margin of the upper Yangtze River,the Sinian to Early Cambrian is characterized by multiply “uplift-depression” cycles in vertical,and is typified by uplift depression alternation sedimentary structural pattern in the horizontal.During the Sinian,from west to East,the northern margin of Upper Yangtze successively developed Mianyang-Changning rift?Hannan-Micang Moutain uplift?WanyuanTongjiang rifting(depression)?Dazhou-Kaijiang uplift?Chengkou-Kaixian rifting(depression)?Dong'an-Wuxi uplift?Badong-Enshi rifting?Shennongjia Huangling uplift.In the early stage of Early Cambrian(including Maidiping and Qiongzhusi Formations),from west to East,Mianyang-Changning rifting?Micang MoutainDazhou-Kaijiang uplift?Chengkou-Kaixian rifting(depression)?Dong'an-Wuxi uplift?Badong-Enshi rifting?Shennongjia-Huangling uplift developed successively.In the late Early Cambrian(including Canglangpu and Longwangmiao Formations),with the gradual uplift of the ancient land on the western of the Upper Yangtze,it gradually presented a gentle and stable sedimentary model of high in the northwest and low in the southeast.In the northern margin of the upper Yangtze,there are vertical episodic upliftingrifting sedimentary cycles and horizontal uplifting depression alternation depositional structural patterns.They may be caused by the late Neoproterozoic uplift and underplating of mantle in the South Qinling,the plate drag in the western margin of the Yangtze plate and the plate retreat in the South Qinling,on the basis of the pre-Sinian inherited rift zone.The sedimentary pattern of uplift depression alternation from Sinian to Early Cambrian makes the high-quality reservoir of Dengying Formation closely connected with the effective source rocks of the underlying Doushantuo Formation and the overlying Qiongzhusi formation in the horizontal direction,and forms a sandwich structure in the vertical direction,which can form a high-efficiency petroleum accumulation model with sedimentary faults and unconformity surfaces.Therefore,it is suggested that exploration work should be carried out along the high energy sedimentary belt or slope belt on both sides of the depression.