Study On Sedimentary Facies Of Huagang Formation In Xihu Sag

X gas field is located on the Central Inversion Structural Belt of the Xihu sag in the East China Sea Shelf Basin.The objective layer of the study area includes Group H3,H4,H5 of Huagang Formation.Gas reservoir property characteristics are mainly low porosity and low permeability even tight.Due to low degree of research,large area and few wells in the study area,the direction of provenance and type of sedimentary facies are uncertainty until now,and the development rule of "sweet spot" in low permeability reservoir is not clear yet,which seriously restricts the next exploration and development of gas reservoirs.To solve the above problems,the paper carries out research on the direction of provenance and type of sedimentary facies by the core and thin section observation and the description,logging data and seismic data.On the basis of the result,the paper conducts a study on the characteristics in the low permeability reservoirs and "sweet spot",so as to provide an important reference for the next exploration and development of Huagang Formation in the Xihu Sag.The paper makes full use of the existing data through the research on the characteristics of petrologic,characteristics of heavy minerals,characteristics of dip logging,characteristics of geophysical.The study thinks the direction of provenance on the H3~H5 Huagang Formation in the research area are determined by northern Hupi reef uplift and eastern Diaoyu island folding zone.Among the provenance the research area takes Hupi reef uplift as the dominant,Diaoyu island folding zone as the auxiliary.Through the detailed observation and description of cores and thin slices,combined with logging and seismic data,the paper identifies sedimentary facies marks,so as to determine sedimentary facies types in the study area.On the basis of single well sedimentary facies,according to the seismic inversion and attribute characteristics of the study area,the plane distribution of the sedimentary facies in the study area is portrayed through the combination of wells and seismic.In the period of H3~H5 of Huagang Formation,the sedimentary environment of the whole area is similar.The study area is on the braided river delta front zone,the main sedimentary microfacies is the underwater distributary channel microfacies with visible underwater distributary interchannel,river mouth bar and sheet sand microfacies.The underwater interdistributary bay microfacies are mainly distributed in the West of the study area,river mouth bar and sheet sand microfacies are mainly distributed in the south of the study area.The channel sand with large thickness and good continuity is a north-south distribution in the study area.Based on the analysis of low permeability reservoir characteristics,according to the standard of production,properties and irreducible water saturation,the paper divides low permeability reservoir "sweet spot" into "sweet spot" class I,class II,class III and non "sweet spot" four categories.Among them "sweet spot" class I and class II are the main type of "sweet spot" in the study area.From the point of view of sedimentary facies,the "sweet spot" class I and class II are mainly developed in the river mouth bar and the underwater distributary channel microfacies.In the same sedimentary microfacies,the development of "sweet spot" is mainly controlled by three factors: lithology,plastic mineral content and bedding type.The "sweet spot" class I and II are mainly distributed in the medium sandstone,less plastic mineral content and the massive bedding.