A Study On The Sequence Stratigraphy And The Characteristics Of Reservoir Of Changxing Formation In Fuling Area,Southeastern Sichuan Basin

This study area is located in the Fuling area,southeast Sichuan basin,and the main target layer is the upper Permian Changxing Formation.Based on the knowledge of sedimentary and tectonic settings of Changxing Formation in Fuling area in southeast Sichuan Basin,and under the guidance of the theory of classical sequence stratigraphy by P.R.Vail,combining domestic and international biostratigraphy,sequence stratigraphy,sedimentary facies and reservoir research,the paper makes a division of the single well sequence and the system tract,and makes a comparision of the crosshole profiles by using drilling,well logging and seismic analysis.After that,the framework of the sequence stratigraphy is built up,and a detailed study of sedimentary facies and sequence are carried out.In the paper,with the system tract as the isochronic unit,the plane distributes of the sedimentary facies are researched and the mode of the sedimentary evolution in the study area is determined.Based on the theory of deposition reservoir geology,by utilizing the core and the thin section,the type of the reservoir pore and diagenesis of the Changxing Formation in the Fuling area is studied.On the basis of the analysis of reservoir properties and pore structure characteristics,combining with sea-level changes,sedimentary evolution and the ancient geomorphological features,the paper makes a study on the main controlling factors of the reservoir development,and makes a comprehensive evaluation of the reservoir.By analyzing the reservoir accumulation characteristics of typical wells in the Fuling area and adjacent areas,the paper makes a study on the hydrocarbon reservoir accumulation regularity in the study area,and makes a prediction about the favorable reservoir facies.Fuling region in which structure have a northeast direction distribution and fault is not well developed mainly is located in the Bashansi syncline between the high and steep structure of Huangnitang and the high and steep structure of Dachi dry well.The structure whose southeast wing is steep and northwest Wing is flat is asymmetrical syncline seen from the profile.Current tectonic formed in the early Jurassic and late Yanshan movement,and the area has been in the slope of the ancient uplift before Late Indosinian movement.Based on the theory of classic sequence stratigraphy by P.R.Vail,with well drilling(the sign of the exposure,the lithologic association),well logging and seismic as the recognition method,the Changxing Formation is divided into three sequence boundaries and two system tract interfaces,and then it is further divided into two third-order-sequences.There are only transgression systems tract(TST)and highstand systems tract(HST)in every sequence,and the lowstand systems tract(LST)is not found in it.The sequence boundaries are the lithology conversion surfaces and also are the action surfaces exposed to atmospheric water,and the the system tract interfaces are the maximum flooding surfaces.As the short rapid transgression and the long slow regression,the deposition thickness of the transgression systems tract in the two third-order sequences is significantly smaller than the thickness of sediments of the highstand systems tract(HST)(ie,T<H).Based on the drilling results of the study area and adjacent areas,with the cuttings,core observation,logging facies analysis,seismic data interpretation and other methods,the paper thinks the Changxing Formation in the study area is carbonate deposition platform system which can be divided into four types of sedimentary facies which are the open platform,platform margin and platform margin slope and shelf.And then the sedimentary facies are divided into subfacies and microfacies,and a systematic study is carried out on the deposition facies.The sedimentary facies maps are drawn with the synchronic unit of system tract.The distribution regularity of sedimentary facies are shown distinctly by fine characterizations of seismic facies.Combined with the ancient geomorphological analysis,the distribution and evolution regularity of the sedimentary facies are discussed in the sequence stratigraphic framework.After the studies of cores,slices and casting thin slices of six drilling in Changxing Formation,the paper thinks that the lithology of Changxing Formation in the Fuling area mainly includes reef dolomite and bioclastic limestone.The reservoir space types are mainly corrosion holes.The Reservoir types are mainly II,III class reservoirs which are fractured-porous,low permeability,and medium porosity reserviors.Most rocks in the region are formed in the shallow burial stage in which there are mainly compaction,pressure solution,cementation,dissolution(atmospheric water leaching),recrystallization,and dolomitization and so on.In particular,the atmospheric freshwater leaching near the highstand systems tract interface is the most significant one.Through the above analysis,the paper thinks sedimentary facies control the distribution of reservoirs,and dissolution and dolomitization is the key to the formation of high quality reservoir;so looking for advantages facies and analyzing the diagenesis have important guiding significance for the oil and gas exploration in the study area.