Study On Sedimentary Facies Of Triassic Reservoirs In Lunnan Oilfield Of Tarim Basin

The Lunnan Oilfield is located in the Lunan fault zone in the Tarim uplift of the Tarim Basin and is a key area for oil and gas exploration and development of the Triassic.In recent years,many people have done a lot of research work in the Lunnan area of the Tarim Basin.However,there are still many different opinions on the classification of the stratum and sedimentary facies,which restricts the exploration and research.Therefore,research of the basic geology in Lunnan Oilfield is important.The paper takes the Triassic reservoirs in Lunnan Oilfield as the study focus.Based on analyses of core,well log,mud log,and 3D seismic data and understanding of the geological background,high-resolution sequence stratigraphic framework of the Triassic reservoirs has been constructed and detailed sedimentary facies are identified.Finally,potential hydrocarbon exploration targets are predicted.Total 9 marker beds,including three first-order,three second-order,and three third-order beds,are identified in the study area through analyses of well logging curves.Under the constraints of the marker beds,the Triassic strata in the study area are divided into TI,TII,and TIII oil-bearing beds,through seismic attribute analyses by using the principle of sequence stratigraphy and application of synthetic seismogram.Finally,the three oil-bearing beds have been subdivided into seven sandstone layers,named as TI2,TI3,TII1,TII2,TIII1,TIII2,and TIII3.The Triassic reservoirs in the study area are composed of conglomerates,sandstones and mudstones.The sandstones are mainly brown and gray in color.The mudstones are mainly dark gray and grayish black,formed under a reduce environment.Grain-size cumulation curves are variable,but mainly show a "two-stage" geometry.Sedimentary structures include erosional surface,massive bedding,cross bedding,parallel bedding,and bioturbation.The C-M diagram indicates that traction flow is main depositional force.Well logging curves mainly show box,bell,funnel,and serrated geometry.Seismic reflection features mainly have three types: sub-parallel reflection with strong amplitude,discontinuous intermediate frequency reflection with medium amplitude and discontinuous low frequency reflection with weak amplitude.Based on the aforementioned characteristics,the Triassic reservoirs in the study area are interpreted as braided river delta and shallow lake depositional systems,within which delta front and littoral lacustrine subfacies are dominated.On the basis of provenance analysis,sediment supplies are mainly derived from the north,and minor from the south.The sandstone bodies in the study area are well developed and their thickness are highly variable.Except the TII oil-bearing bed,the lateral connectivity is slightly poor,and the other beds have better connectivity.The sandstone bodies in the plane view are distributed as belts or fans,mostly oriented in the north-south direction.Based on the analysis of physical properties and sedimentary evolution,it is considered that underwater distributary channel microfacies and beach-bar microfacies are favorable reservoir facies.Using the waveform indication inversion technique,potential sandstone reserviors can be recognized on the basis of delineation of sedimentary facies,analyses of hydrocarbon potential from well logs,and understanding of structural configuration.The predicted potential reservoirs provide guidance for the following exploration.