Characteristics,Mechanism And Significance Of Sinian/Cambrian Unconformity In And Around Sichuan Basin,SW China

The turning period from Sinian to Cambrian,as a crucial point for the Earth revolution,witnessed a significant change in the global environment.Unconformity surface,an important boundary of the turning period from Sinian to Cambrian,has aroused extensive discussion and attention in the scientific community since its global discovery.And widely-developed and well-exposed Sinian/Cambrian has been detected in and around Sichuan Basin,SW China.But the formation mechanism of the huge unconformity remained to be determined due to the lack of sound identification methods with few deep exploration drillings in the strata.Further,elaborated studies were not conducted on the spatial distribution feature of the Sinian/Cambrian specific stratigraphic boundary(the unconformity).The study on the Sinian/Cambrian unconformity surface is particularly important since breakthroughs have been recently made in oil-gas exploration near the Sinian/Cambrian unconformity surface in Sichuan Basin.In such a context,this paper first discussed the differences in stratigraphic structure and spatial distribution of Sinian/Cambrian unconformity in the Sichuan Basin using a combination of petrology,sedimentology and geochemistry with an equal emphasis on field observation and indoor analysis,so as to clarify the formation mechanism of the Sinian/Cambrian unconformity.Then,the influences of spatial distribution difference of unconformity on the structural unit of the karst reservoir were analyzed.At last,it disclosed that the controlling effect of top unconformity on oil gas in the Sinian Dengying Formation is prodigiously significant for promoting the advancement of oil-gas exploration and enriching the petroleum geological theory.To be specific,10 kinds of rock facies dominated by carbonate and 5 kinds of rock facies dominated by clasolite can be recognized in the Dengying Formation of the Sinian/Cambrian underlying strata in Sichuan Basin Outcrop with described exposure description and slice observation.Additionally,six stratigraphic contact relationships were divided through identifying rock association types in the vertical direction,the contact relationship of the upper-lower strata,the structural types of the unconformity zone and the geometric manifestations of the strata on both sides,including the unconformable contact relationship between the fourth member of the Dengying Formation and the Qiongzhusi Formation,the unconformable contact relationship between the fourth member of the Dengying Formation and the Maidiping Formation,the unconformable contact relationship between the third member of the Dengying Formation and the Maidiping Formation,the unconformable contact relationship between the second and third members of the Dengying Formation,the unconformable contact relationship between the Sinian system and other lower paleozoic strata,and the conformity contact relationship of the Sinian/Cambrian system.It concluded that ancient paleogeomorphy characteristics of the underlying Dengying Formation that is controlled by unconformity surface are varied in different regions,leading to a difference in the spatial distribution of Sinian/Cambrian unconformity,according to the comprehensive analysis using impression and residual thickness methods based on vast of seismic profiles in Sichuan Basin.The whole is presented below.Silt containing shale of the Lower Cambrian Qiongzhusi Formation has an unconformable contact with the dolomite of the fourth member of the Dengying Formation in a large area of the outer edge of the NNW direction Mianyang-Changning intracratonic sag.The phosphorus/siliceous mudstone of the Lower Cambrian Maidiping Formation is in unconformable contact with the clayey dolomite of the third member of the Dengying Formation inside the intracratonic sag.The Maidiping Formation has an unconformable contact with the fourth member of the Dengying Formation in the platform marginal ramp zone.And there is a Sinian/Cambrian conformable contact relationship in the deeper deep-water basin area.The Dengying Formation in the Upper Yangtze area was categorized into 2.5 thirdclass sequences using the transgressive-regressive(T-R)sequence model as per the temporal and spatial distribution of lithofacies and sedimentary environments.Geochemical indicators of Sinian/Cambrian unconformity were compared worldwide in terms of sedimentary geochemical characterization in the study area.At the same time,the global sea-level fluctuation was caused by the two continental glacier events in the Vingerbreek glacial period(ca.547 Ma)and Baikonur glacial period(ca.540 Ma)at the end of the Neoproterozoic,which is combined with the isotope changes of the Late Sinian carbonate strata section in other plates.In this way,the parallel Sinian/Cambrian unconformity surface is found to be dominated by the changes in global sea level.Karst paleogeomorphology of the Dengying formation is refined into Karst highland,slope,platform,depression,and basin after analyzing its features of Late Sinian Dengying Formation were analyzed with the impression method.As is concluded from the comprehensive analysis,the paleogeographic pattern of the underlying Dengying Formation was eventually finalized under the controls of different characteristics of pre-existing paleogeomorphology,the re-transformation effect on different landscapes by weathering Karst and its intensity of multi-geomorphic units.Consequently,the contact relationship and spatial distribution of the weathering Karst unconformity surface are diverse in different regions.The natural gas of Dengying Formation in the Sichuan Basin has differential distributions along the Sinian/Cambrian unconformity,which can be effectively traced by the morphology and spatial distribution characteristics of bitumen in the Dengying Formation.The occurrence state and spatial distribution of bitumen in Dengying Formation of Sichuan Basin at reservoir strata are divided into ring-shaped bitumen,stick-branched bitumen,patchy bitumen and broken granular bitumen based on the summarized morphology and spatial distribution characteristics.Among them,broken granular bitumen with the most complicated spatial distribution can be subdivided into granular bitumen lying in residual pores,granular bitumen accumulated near the small pore throat,and granular bitumen located between or inside crystals of secondary minerals according to its spatial distribution in pores.Occurrence forms of bitumen in Dengying Formation can be classified into in-situ and ex-situ bitumen according to their genesis in combination with characteristics of pore-filling minerals and the evolution history of burial accumulation in Dengying Formation.Based on this,the no-oil gas reservoir,in-situ primary oil-gas accumulation process,adjustment and destruction process of primary oil-gas reservoir,accumulation process of the secondary gas reservoir,and multi-period adjustment process of the secondary gas reservoir in the Dengying Formation were traced in the oil-gas adjustment and evolution process,which can interpret the differential accumulation of natural gas of Dengying Formation along the unconformity.Furthermore,Cangxi-Langzhong(Chuanshen-1 well)-SouthYanting-Shehong-Daying(Pengtan-1 well)-Ziyang-Weiyuan areas are predicted as the areas favorable for adjustment and differential distribution of natural gas in Dengying Formation along the unconformity According to the occurrence form of reservoir bitumen in Pengtan 1 well.