Study Of Sequence Strata And Sedimentary Distribution Of Longwangmiao Formation In The Central Sichuan Basin

In recent years,the Cambrian giant gas field was found in the paleo uplift which is in the central of the Sichuan Basin area,the geological reserves exceeds more than a trillion cubic meters.The Longwangmiao Formation has great research significance as the main gas exploration area.In order to understand the detailed reservoir characteristics of Longwangmiao formation for reservoir prediction,we studied the sequence stratigraphy and sedimentary distribution characteristics of the Longwangmiao formation in detail.Based on sedimentology,sequence stratigraphy and previous studies,combined with core,logging,seismic and comprehensive analysis of indoor test data,this paper systemically studied to divide the sequence and find the characteristics of the sedimentary facies of the Longwangmiao Formation of Lower Cambrian in Central Sichuan Basin and the following conclusions are made:(1)To build the Sequence stratigraphic correlation and framework of Longwangmiao formationBased on the cores,logging and seismic data interpretation,analysis of conventional logging data and observation of cores and combined with the results of the GR and KTH,we carried out the research on the sequence stratigraphy of the Longwangmiao Formation.We consider the whole formation as a third-order sequence,divided into two sequence sets which called Sq1 and Sq2 of which transgressive systems tract(TST)and highstand systems tract(HST)were developed.Then we established the sequence stratigraphic framework of Sichuan Longwangmiao formation by using the correlation map.(2)The types of lithofacies of the Longwangmiao FormationThrough the core observation and thin section analysis,combined with the color of rocks,rock types,sedimentary structure,and its mineral composition and structure characteristics,we can summarizes the types of lithofacies of Longwangmizo formation in the lower Cambrian,Sichuan Basin are grain dolomite,micritic dolomite and crystalline dolomite.Grain dolomite can be divided into gravel dolomite,dolarenite and oolitic dolostone;micrite dolomite is divided into muddy micrite dolomite,micritic dolomite and argillaceous micrite dolomite;crystalline dolomite is divided into grannophyric micritic dolomite and finely crystal dolomite.(3)The sedimentary system and sedimentary facies types of the Longwangmiao formation in the central Sichuan BasinAccording to the observation of cores and the analysis description of thin section,combined with petrology and logging data,the sedimentation of Longwangmiao Formation is the restricted platform.The restricted platform can be divided into platform flat,intraplatform shoal,interbank flat and lagoon.(4)The distribution of sedimentary facies in the sequence frameworkThe deposition of Sq1 of Longwangmiao formation distributed large scale in North Beach,and significantly reduced in the south of the intraplatform.About the East and West,the depositions are mainly interbank flat and lagoon.During the deposition of Sq2,the depositions of the northern Moxi area and southern Gaoshiti area are intraplatform and deposited widely,it shows a NE-SW trending.To the southeast and northwest,the depositions are mainly interbank flat and lagoon.Compared with Sq1,the range of intraplatform shoal distribution shows the trend of expansion to the East,the interbank is further reduced and the lagoon migrated to the eastward.According to the study of sequence stratigraphy and sedimentary system,summarizes the process and rules of sedimentary evolution of the study area: on the whole,the Sq2 deposition period of Longwangmiao formation inherited the sedimentary pattern in the period of Sq1,and then,the water went to southeast of the area slowly during the period of the sedimentary of Sq2,and the micro phase showed a migration to the southeast.