Research On The Cambrian Lithofacies And Paleogeography In Northwest Sichuan

Based on the data of 8 wells of Cambrian system and 10 Cambrian sections that is exposed to the surface,the previous research results are combined with comprehensive analysis of outcrop,logging,core and microscope slides data in the study area,the correlation theorys such as Lithostratigraphy,Biostratigraphy,Logging geology,Carbonate petrology and sedimentology etc.are used in a comprehensive way,the method of "single factor analysis and multifactor comprehensive mapping" is adopted to lithofacies-paleogeographic map compilation,the sedimentary model of Cambrian system in the study area has been preliminarily established.(1)Combining the Cambrian division scheme in each area,the Cambrian System can be divided into five formations in the study area,such as:Qiongzhusi Formation,Canglangpu Formation,Longwangmiao Formation,Gaotai Formation,Xixiangchi Group,and the upper and lower two member are divided in the Canglangpu Formation,and the specil oolitic limestone(formerly knowed as Xiannvdong Formation)in the area is falled into the bottom of Canglangpu Formation.The denudation line of Xixiangchi Group is re-characterized in this study,and the distribution range of Xixiangchi Formation increases than the previous studies.(2)The stratigraphic division of Cambrian in Northwest Sichuan is reformulated and the stratigraphic and lithological characteristics of Cambrian strata are further defined.The mixed deposition of terrigenous clastics and carbonate occurs in many formations of Cambrian in the area,and the terrigenous clastic shows decreasing trend from north to south and from east to west.The lithological characteristics of five formations from bottom to top are described as follows:?Black mudstone,silty mudstone and pelitic siltstone,and some thin bed of limestone are sandwiched in the formation are the main sediments of Qiongzhusi Formation;?Argillaceous limestone and oolitic limestone are the most developed sediments in lower parts of Canglangpu Formation and conglomerate,sandstone,siltstone and mudstone develop most in the upper part;?Sandy and argillaceous dolomite,unequal thick sandstone,siltstone and other terrigenous clastic rocks are the main sediments of Longwangmiao Formation;?The mixed sediments of Gaotai Formation are particularly prominent,such as:sandstone,siltstone,dolomite and mudstone in different thickness;?thick-massive argillaceous dolomite and fine-crystalline dolomite are the main' sediments of Xixiangchi Group.(3)Based on the comprehensive analysis of lithology,electricity,paleontology and structural features of the field and drilling section,the sedimentary facies and depositional systems of each group of Cambrian System in the study area are deeply carried out:?Shelf facies that contain Terrigenous clastic are the main sedimentary facies of Qiongzhusi period i;?The platform marginal facies is developed in the early stage of Canglangpu period,and influenced on the uplift of the paleo land,the deltaic facies is developed in the later stage of Canglangpu period;?the facies is mixed tidal flat facies of restricted platform is developed in the period of the Longwangmiao;?In the period of the Gaotai,the facies is mixed tidal flat facies of restricted platform too;?Tidal flat facies of restricted platform are the main facies in the period of the Xixiangchi.(4)Guided with "single factor analysis and multifactor comprehensive mapping method",the lithologic palaeogeography characteristics are studied.The rise and fall of the paleo land and the relative variation of the regional sea level are the main controlled factors of the sedimentary pattern in the region.Near the northwest of the study area,the terrigenous clastic increases obviously and the carbonate rock salt decreases on the contrary.At the same time,three transgressions occurred in Cambrian in the Sichuan basin,with regional tectonic movement,which controlled the sedimentary environment in the study area.?The first marine transgression occurs in the Early Cambrian Qiongzhusi stage,maximum flooding surface occurs between the Qiongzhusi stage and Meishuchun stage.With the formation of Xingkai tension groove movement,thick continental shelf facies shale deposits in the studied area in Qiongzhusi formation which became a powerful source of hydrocarbon.The Early Cambrian Canglangpu period occurred regressive.The sedimentary facies of the early Canglangpu period are the platform marginal facies in study area.After that,the Motianling old land and Hannan old land continued uplift,which provided a sufficient supply of terrigenous clastic.The sedimentary facies of the later Canglangpu period became to delta facies.?The second transgression occurred in the Longwangmiao period of the Early Cambrian,and the regional tectonic movement tends to be gentle.The study area developed limited platform carbonate sedimentary.The second regression occurred in Gaotai period.The tidal flat facies of limited platform expansion in the study area.The content of terrigenous clastic increases and the characteristics of the mixing are more significant.?The third transgression occurred in Xixiangchi period.Restricted platform and tidal flat sedimentary development in the study area.Shoal facies should also be developed at high terrain,but it is denuded because of late tectonic uplift.The platform marginal beach that developed in the early Canglangpu period and the tidal flat facies developed in the Longwangmiao period are the favorable facies belts for reservoir development.