Depositional features and pattern of the fourth member of Xujiahe formation in middle and eastern Yuanba area

Elaborate research of sedimentary system and facies has been taken in this analysis,combined with seismic,geology and well logging data.Tectonic background,sedimentary form,sedimentary scale,sedimentary structure and lithology feature from drilling cores are used to identify the sedimentary system.Well logging curves are tied with drilling cores carefully to build up curve models of different microfacies so as to distinguish facies along wells.Distinction of debris component between the middle area and the eastern research area and the distinct sedimentary facies distribution between middle and eastern part of Yuanba area are analyzed to set up the sedimentary pattern.All the above researches indicate the area is dominated by braided river delta rather than fan delta.Facies distribution and evolution reveal that the sedimentary center transported eastward and overall it displayed a trend of regression.The debris component of the middle area is dominated by carbonates while the eastern area is dominated by silicates.In addition,the middle and eastern part have access to different microfacies distribution.These two evident differences between the two parts prove that the deposition is controlled by two distinct sedimentary sources.Thus a bi-source sedimentary pattern should be set up in this area.Because of the prosperity of the fourth member of Xujiahe formation in Yuanba area,making clear its depositional features and facies is beneficial for gas exploration and develop.