| The Cambrian subsalt in Tarim Basin is a significant strategic succeeding field of natural gas exploration in Tarim Basin.In 2013,well Zhongshen1 which located in Tazhong uplift made a breakthrough in the Cambrian subsalt for the first time.In 2020,well Luntan1 which located in Tabei uplift obtained high-yield oil and gas flow in the Cambrian subsalt in Wusongger Formation,and well Ketan1 which located in Keping fault uplift also achieved a breakthrough of producing million cubic meters of natural gas per day in Wusongger Formation.These series of breakthroughs have confirmed that the Cambrian subsalt in Tarim Basin has the geological conditions for large-scale hydrocarbon accumulation and also has good exploration prospects.Although the Cambrian subsalt exploration in Tarim Basin has made breakthroughs successively in the Xiaoerbulake Formation and Wusongger Formation,there are still many problems in the exploration of Cambrian subsalt dolomite,and the distribution and main controlling factors of large-scale distributed ancient mound shoal in the subsalt are still unclear.The middle-lower Cambrian strata in Tarim Basin are deeply buried.At present,there are only more than 20 wells to reveal this strata,and less wells to completely expose the lower Cambrian.Therefore,the reconstruction of lithofacies paleogeography and the prediction of favorable reservoir facies zones of the middle-lower Cambrian mainly depend on the interpretation of seismic data.This paper mainly studies the interpretation of Lunnan 3D work area ans 2D seismic data which covering the whole basin in Tarim basin.Combining the data of outcrop,drilling data,logging data,core and thin section in the field,and combined with the previous research results,the seismic facies characteristics,formation and evolution of platform margin zone,sedimentary characteristics and lithofacies paleogeography,reservoir characteristics and controlling factors of lower Cambrian Xiaoerbulake Formation and Wusonggeer Formation,sealing ability and favorable area of middle Cambrian cap rock in Tarim Basin are studied.And based on the method of seismic wave attenuation anomaly,the favorable gas bearing area of Lunnan 3D work area is predicted.According to the above research results,the favorable zones of Lunnan 3D work area and the whole basin are predicted with different scales and accuracy.Through drilling and field outcrop as constraints,the seismic facies of 2D and 3D seismic data in the study area are interpreted,the characteristics of the middle-lower Cambrian platform margin in Tarim Basin are analyzed,and its evolution model is established.The study shows that the middle-lower Cambrian in Tarim Basin can be divided into nine typical seismic facies with sedimentary and reservoir significance.The typical seismic facies include plate-shaped progradation reflection of slope,mound-shaped rimmed progradation reflection,blank weak amplitude reflection of dolomite,strong amplitude parallel reflection of shale in Yuertus Formation,low amplitude mound-shaped reflection of mound shole facies in platform,etc.The progradation reflection of platform margin-slope indicates the evolution of platform type and the relationship between microfacies type and reservoir distribution.According to the drilling calibration,thickness of each group and seismic facies,the middle-lower Cambrian has experienced continuous strong reflection shelf deposition of Yuertus Formation,the plate progradation gentle slope of the Xiaoerbulake Formation which has no marginal platform margin,weak rimmed platform margin of mound progradation in Wusonggeer and Shayilike Formations,the platform margin of the Awatage Formation which is strongly margined by mound progradation from bottom to top.And the platform of each stage will migrate eastward relative to the previous stage.The relationship between the lateral accretion of the Cambrian carbonate platform and the evolution of the platform margin types,as well as its control over the distribution of the dolomite reservoir and the evaporate cap rock in the lagoon are clarified.The platform with weak-strong rimmed margin corresponds to the evaporate deposition in the interior of the platform,and the platform margin zone and the inner platform mound shoal zone are favorable areas for dolomite reservoir development.The results of research show that the main rock types of the middle-lower Cambrian in Tarim Basin are crystalline dolomite,oolitic dolomite,algal dolomite,gypsum and limestone.According to the lithofacies types and logging sedimentary facies interpretation,the sedimentary facies of the middle-lower Cambrian in Tarim basin can be divided into five types: restricted platform,open platform,platform margin,slope and basin.These five sedimentary facies can be subdivided into nine sedimentary subfacies,which are lagoon,tidal flat,intra platform depression,depression margin,intra platform mound,intra platform shoal,inter mound(shoal)sea,platform margin mound and platform margin beach.At the same time,with drilling and field outcrop as constraints,the seismic facies interpretation of 2D and 3D seismic data in the study area is carried out,and the distribution of each seismic facies on the survey line is tracked to determine the distribution range of sedimentary facies corresponding to each seismic facies.Combined with the above results,the lithofacies paleogeographic characteristics of middle-lower Cambrian in Tarim Basin are studied.The Yuertus Formation is characterized by shelf sedimentary model as a whole;the Xiaoerbulake Formation is characterized by gentle slope platform depositional model without rimmed margin,which mainly develops restricted platform,open platform,platform margin and slope basin facies,with the maximum thickness near Manxi low uplift and Keping outcrop in the northwest;the Wusongger and Shayilike Formations are dominated by weakly rimmed platform marginal sediments;and the paleogeographic pattern of the Awatage Formation is basically inherited the characteristics of the early Cambrian,which is mainly restricted platform facies.The reservoir space types of lower Cambrian in the study area mainly include pores,caves and fractures.The primary pores include intercrystalline pores,algal lattice pores and residual intergranular pores.The secondary pores include intercrystalline,intergranular and intragranular dissolved pores.The caves are mainly secondary dissolved caves,and the fissure can be divided into structural,diagenetic and dissolved fractures.The reservoir can be divided into three types: intra platform mound beach,intra platform depression margin mound beachand platform margin mound beach.It is mainly distributed in the periphery of depression,Tazhong-Badong area,Keping-Bachu area and Lunnan-Tazhong line.The lithology with sealing ability of middle Cambrian in the platform basin area of Tarim Basin is mainly gypsum-salt rock,gypsum mudstone and gypsum dolomite,among which gypsum salt rock has the best sealing ability,but the structural fractures in the stratum and the burial conditions of gypsum have a great influence on its ability of sealing.The middle Cambrian gypsum salt rock caprock in Tarim Basin is mainly distributed in Awati sag,northern Tazhong uplift,southern Manxi low uplift and northern central Bachu uplift.With the northern Bachu uplift as the center,the thickness of gypsum salt rock decreases to the surrounding and presents a circular distribution,and the distribution of gypsum-salt rock has good continuity and stability.At the same time,the vertical distribution of gypsum salt rock,gypsum dolomite and gypsum mudstone strengthens the sealing ability of caprock,so that this large area distribution of caprock can give good conditions of sealing for the accumulation of hydrocarbon from Cambrian subsalt in Tarim Basin.Combined with the lithofacies paleogeographic characteristics of middle Cambrian,it is considered that Awati sag,the west and south of Manxi low uplift,the north and west of Tazhong uplift,and the middle of Tabei uplift are favorable areas for cap rock development.S-transform is the best method to predict the gas bearing capacity of lower Cambrian in Lunnan 3D work area.The first tuning frequency is 15 Hz.According to the results of gas content detection and verified by the method of frequency division energy attenuation gas content detection,the detected energy attenuation distribution trend is in good agreement with the gas content condition revealed by drilling.At the same time,gas detection indicates that there are three gas bearing areas in the study area of wusonggeer formation.In addition to the area where Luntan-1 well is located,there are also large areas of attenuation anomalies in the slope zone and basin facies area in the north of 3D area.The three anomaly areas are all covered with dolostone or marlstone caprock,so these three areas may be favorable gas bearing areas.According to the above research results,and combined with the idea of beach controlled by ancient uplift,circle controlled by ancient fault and near-source stability protection,4favorable zones of Cambrian subsalt system are selected,including northern Tazhong uplift,southern Keping fault uplift and peripheral area of Wensu low uplift,southern Luntai-Shaya area and Lunnan-Tazhong platform margin. |
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