Study On Sedimentary Facies And Reservoir Characteristics Of The First Member Of Liushagang Formation In A Area Of Fushan Sag

Fushan sag is a Cenozoic sag located in the eastern margin of the southern slope of Beibu Gulf Basin.Area A of Fushan depression is located in the central and western part of Fushan Depression,between the central tectonic belt and the northern fault zone.Due to the complex structure of the first member of Liushagang Formation in the study area,the rapid change of sedimentary facies zone,few typical stratigraphic correlation markers,the characteristics of sedimentary cycles are not obvious,it is difficult to divide and contrast the strata.Affected by volcanic activity,the formation lithology and effective reservoir control factors are complicated.The difference of resistivity between oil and water reservoirs is small,and the distribution of oil reservoirs is influenced by both structural and lithology factors,and the lithology factors account for a large proportion,which leads to the complicated law of oil and gas accumulation.The data of drilling,coring,logging and oil testing were used to study the distribution of sedimentary facies and sand body.The method of establishing the secondary interpretation model of physical property logging is adopted to study the distribution law of physical property,analyze the corresponding relationship between sedimentary facies and physical property,and study the development law of dominant reservoir.The results of this study are as follows:(1)Using high-resolution sequence stratigraphy and the method of controlling sedimentary cycles and secondary cycles for stratigraphic correlation,the stratigraphic correlation relationship of several fault blocks is drawn up.(2)The lithology of the first member of Liushagang Formation is mudstone,siltstone,fine sandstone,medium coarse sandstone,pebbled sandstone and tuffaceous sandstone.The sedimentary rocks are mainly gray,gray and dark gray in color,and sedimentary structures such as scour surface,massive bedding,cross bedding and parallel bedding are developed.(3)The strata of the first member of Liushagang Formation are braided river delta depositional system with provenance from the south and southwest.(4)There are two kinds of sedimentary subfacies,delta front and prodelta,which can be divided into six kinds of sedimentary microfacies,such as underwater distributary channel,mouth bar,distributary bay,sheet sand and far sand bar.(5)The first member of Liushagang Formation is a medium-high porosity and medium-low permeability reservoir.The lithology of the effective reservoir is mainly fine sandstone,followed by sandy conglomerate and siltstone.The lower limit of effective reservoir porosity is 9% and permeability is 1.5md.Longitudinally,the depth of reservoir above3600 m is better.On the plane,underwater distributary channel have good physical properties,but mat sand has poor physical properties.