Genetic Mechanism Of Conventional-Tight Sandstone Reservoirs And Reservoir Differentiation Characteristics Of The Fuyu Oil Layer In The Southern Songliao Basin

After the shale gas revolution in the United States,the technical and management innovation with the concept of integrated development of multiple types of tight reservoirs as the core realized the second leap of resources.The core of the second revolution was that the integrated exploration and development of various tight rocks made the hard-to-use resources become reserves(He,2019).The tight sandstone reservoirs in China are characterized by low permeability to tight reservoir system,and are rich in oil and gas resources,accounting for about40%of the recoverable oil and gas resources.The national oil and gas resources evaluation results in 2015 show that China has 127.5 billion tons of petroleum geological resources,including 14.7 billion tons of tight oil geological resources.Exploration and development practice shows that oil and gas uneven distribution and enrichment differences characteristics are very obvious,belongs to the conventional-tight reservoir system,among them,the difference of reservoir quality plays an important role in controlling the differential distribution and enrichment of oil in conventional-tight reservoirs.Therefore,the conventional-tight reservoirs of the Fuyu oil layer in the Zhongxin-Qianguonan slope zone of the Southern Songliao Basin has been taken as the research object in this thesis,emphatically from the angle of reservoir quality difference research,reveal the enrichment regularity of oil and gas is uneven distribution and difference.In this thesis,core observation,thin section identification,X-ray diffraction analysis,scanning electron microscopy(SEM),analysis of fluid inclusions and stable carbon isotope analysis analysis method were applied comprehensively.From the perspective of sandstone reservoir,taken the low permeability,ultra-low permeability and tight reservoir as a unified system,intergrated with sequence stratigraphy,sedimentary facies,diagenesis and tectonic movement,illustrates their impact on reservoir qualtiy and the genetic mechanism of the low permeability-tight reservoir system.On this basis,combined with the analysis of the difference characteristics among conventional-tight reservoirs,the distribution and main control factors of accumulation in conventional-tight reservoirs were clarified,and the accumulation model of conventional-tight reservoirs was established.The Fuyu oil layer in the study area corresponds to a complete third-level sequence,which is composed of lowstand system tract,transgressive system tract and highstand system tract,and can be further divided into 5 parasequence sets and 13 parasequences.Based on the study of regional sedimentary background and sedimentary characteristics,combined with the analysis of core observation and particle size distribution characteristics of 12 Wells in the study area,it is clear that the Fuyu oil layer in the study area develops shallow-water delta facies,which is composed of the delta plain and the delta front subfacies.Fine anatomy of the distribution of sedimentary microfacies was carried out in three dense well pattern areas of block Rang-11,block Rang-46 and block Qian-60 in the study area from west to east.The distributary channel distribution pattern gradually changed from the woven network distribution pattern in the lowstand system tract and the transgressive system tract to the branched distribution pattern in the highstand system tract.As a result,the development scale of the channel decreased,the sandstone thickness decreased gradually,and the vertical and horizontal superposition became worse.The Fuyu oil layer in the study area is mainly composed of lithic arkose and feldspathic litharenite with low composition maturity.The lithology of the sandstone reservoir is mainly siltstone and fine sandstone with grain support,medium to good sorting,subrounded to rounded,medium structure maturity.The pore space of the Fuyu oil layer is composed of primary intergranular pores,secondary dissolution pores and intercrystalline micropores of clay minerals.The fuyu oil layer in the study area is a complex reservoir system with low permeability,extra low permeability and tight sandstone reservoirs.On the plane,the reservoir quality has obvious zonation.From west to east,the whole reservoir quality shows a trend of transition from tight to low permeability.Within the sequence stratigraphic framework,the reservoir quality of sandstone reservoirs in different system tracts is different.The reservoir quality of different sedimentary microfacies in the same system tract is also obviously different.The diagenetic events of the Fuyu oil layer in the study area include compaction,cementation(carbonate cementation,clay mineral cementation and quartz cementation)and dissolution,and a series of diagenetic processes caused by local event-type fluid intrusion,namely mantle-magmatic CO₂ influx.The decarboxylation of organic matter from the adjacent mudstone and the overlying source rock in the zone without CO₂ charge provided the required materials for the precipitation of carbonate cement in the sandstone reservoir.The dissolution of feldspar,illitization of kaolinite,illitization of smectite and compressive dissolution between quartz particles are important silica sources in the sandstone reservoir.The acid fluid that causes the dissolution of feldspar is mainly the organic acid fluid discharged from the overlying source rock.In the CO₂ charge zone,mantle-magmatic CO₂ intrusion resulted in the formation of carbonate mineral assemblages of dawsonite and ankerite.The charge of mantle-derived CO2can lead to a secondary dissolution of feldspar in the sandstone reservoir,accompanied by the silica release and precipitation.Based on thin section observation,SEM and fluid inclusions analysis,combined with metasomatic cutting and dissolution filling relationships among authigenic minerals,the diagenetic sequence of the Fuyu oil layer in the study area was established.Sequence stratigraphic framework and sedimentary facies control the sedimentary environment,particle size,sorting,original physical properties and sandstone superimposed style.The trangressive system tract sandstones and highstand system tract sandstones are close to the overlying hydrocarbon source rock,and during period of the trangressive systems tract and highstand system tract,the accommondation space is larger,mudstone interlayer is more developed,compared to the lowstand system tract,so the carbonate cements content of the trangressive system tract sandstones and highstand system tract sandstones are relatively higher.For a single sedimentary microfacies sand body within the system tract,compared with sandstone near the contact surface of top and bottom of sandstone-mudstone,the content of carbonate cement in the middle of the sand body is relatively low,and the corresponding porosity and permeability are relatively high.The lowstand system tract distributary channel sandstone is coarse particle size,and good sorting,so the compaction resistance is strong,which can preserve the more primary intergranular pore after the compaction,and thus facilitated the organic acid fluid flow in the distributary channel sandstone of the lowstand system tract and the reaction between the acid fluid and reservoir sandstone,more conducive to the formation of secondary porosity.From the lowstand system tract to the highstand system tract,accompanied by the rise of the sedimentary base level,the sedimentary facies belt continuous retreat,and the hydrodynamic power reduced,resulting in higher clay minerals(mainly for smectite)content in the transgressive system tract and the highstand system tract sandstones,which leads to the higher content of mixed-layer I/S and illite during the burial diagenetic process,and resulted in the worse reservoir quality.The tectonic activity track of Songliao Basin resulted in the difference of buried compaction degree,quartz cement content and clay mineral type in the geological history of the Zhongxin-Qianguonan slope zones,and makes a positive contribution to the difference of reservoir quality of the Fuyu oil layer in the Zhognxin slope zone and the Qianguonan slope zone.The fault dense zones of T₂ reflectance zone formed by multi-stage tectonic movement also has an important influence on the formation of reservoir quality differences of the Fuyu oil layer in the study area.Within a certain range(500m)of the fault,the carbonate cement content in the sandstone reservoir is inversely proportional to the distance from the fault,and the secondary dissolution porosity is positively proportional to the distance from the fault.The influx of mantle-derived CO₂ changes the types and contents of carbonate cement and authigenic clay minerals in the sandstone reservoirs of the“CO₂ charged zone”,and leads to the overall reduction of reservoir quality in the“CO₂ charged zone”(within 5-8km of gudian fault).The carbonate cements in the sandstone reservoirs of the“CO₂ charged zone”are dawsonite and ankerite,and the carbonate cement content is higher than that of the sandstone reservoirs in the“Ordinary zone”.The dominant types of authigenic clay minerals are mixed-layer I/S and illite,while kaolinite and chlorite are rare.The content of quartz cement is higher than that of the sandstone reservoir in the“Ordinay zone”.On the basis of the study on the controlling effect of such factors as tectonic activity,sequence,sedimentation and diagenesis on reservoir quality difference,the genetic mechanism of the low permeability to tight sandstone reservoir system in the study area is clarified.The tectonic activity determined the ordely distribution of the reservoir quality of the sandstone reservoirs in the Zhongxin-Qianguonan area from west to east(tight in the west and low permeability in the east).Within the same tectonic unit,sequence stratigraphic framework,sedimentary facies and diagenesis control the formation of reservoir quality differences at different scales,and finally a low permeability to tight sandstone reservoir system formed.Sequence stratigraphy and sedimentary facies are the genes decided the trend of low permeability or tight of the sandstone reservoirs,burial compaction is the downward force for the low permeability to tight sandstone reservoir,the development degree of carbonate cements is the key point to the sandstone resrvoirs of being low permeability or tight,and the development of authigenic clay mineral is catalyst of the process of the sandstone reservoirs of being low permeability or tight.The tight sandstone reservoirs in the study area is a kind of quasi-continuous reservoir,which is characterized by"overall oil-bearing and difference in productivity between adjacent Wells".The"sweet spot"reservoir determines the enrichment area,the fault or fault intensive zone affects the productivity of the reservoir as the vertical transport conductor,the pressure difference between the source rock and the sandstone reservoir controls the vertical enrichment horizon as the migration power,and the coupling of the three together controls the oil differential distribution and enrichment in the tight reservoir area.In the conventional reservoir area,the first and second sand groups at the top are dominated by fault-lithologic reservoirs and lithologic reservoirs,while the third and fourth sand groups at the bottom are dominated by fault-lithologic reservoirs.Affected by the tectonic background of the slope belt(the south is low,the north is high,and the slope is gentle),the oil-water transition zone in the study area has a wide range,and the north high structural part of the slope is relatively rich in petroleum.The structural background and fault-sand matching relation control the oil differential distribution and enrichment in conventional reservoir area.When the strike of fault dense zone is perpendicular to the updip direction of stratum and slanting at a small angle to the extension direction of sandstone,the fault dense zone plays a role of transverse shielding,and the reverse fault barrier,fault-nose and reverse fault step on the concave side of fault dense zone are favorable locations for oil and gas accumulation.When the strike of fault dense belt is nearly parallel to the updip direction of the strata,and is inclined or nearly parallel to the extension direction of the sandstone,the fault dense belt plays the role of lateral transmission,and the tectonic-lithologic traps formed along the updip direction of the strata are favorable locations for petroleum enrichment.