The Study On Reservoir Characteristics And Formation Mechanism Of The Upper Triassic Bagong Formation In The Qiangtang Basin

The Qiangtang Basin has great potential in oil and gas resources,it is an important strategic reserve area for China's oil and gas resources.While,The Upper Triassic clastic reservoir in the North Qiangtang Depression is the most promising position because of its good preservation conditions and excellent hydrocarbon source conditions.Through analysing the mechanism of the sedimentary and diagenesis effects on the physical properties of the Triassic clastic reservoir,combined with the researching of hydrocarbon accumulation laws,and predicting the distribution of favorable reservoir,we can effectively fill the gaps in petroleum geology research in this region and providing the geological basis for the next exploration work.Under the premise in considering the precision of subdivision and the convenience of use,we take the Bagung Formation representing for the coal-bearing clastic rock strata,which is under the Nadanggang Formation and widely distributed in the central and southern parts of the Northern Qiangtang Depression.On account of the compact characteristics of the reservoir in the Upper Triassic Bagung Formation,based on the outcrop observations,by means of some indoor analysis methods such as sheet identification,X-ray diffraction,cathodoluminescence,scanning electron microscopy,general properties,high pressure mercury injection,and fluid inclusions.Take the perspective of reservoir characteristics and diagenesis characteristics as the starting point,determining the diagenesis stage,dividing the diagenetic facies,quantitatively restored the pore evolution process,discussed the coupling relationship between reservoir densification and reservoir formation,defining the mechanism of reservoir densification and the formation condition of high-quality storage,and establishmenting multi-index reservoir comprehensive classification criteria.Then,we can predict the distribution of favorable reservoir areas.This research indicates that:The provenances of the Bagung Formation is from the central uplift zone,that is in the south of it and within its southern denudation zone.The sedimentary facies of Bagung Formation is delta depositional system,which is characterized by a typical three-step accumulative delta in the longitudinal direction.And in the plane,it is along the higher position with zonal distribution,with a narrow distribution in the west and a wide distribution in the east.The lithology is mainly composed of lithic sandstone,feldspathic lithic sandstone,lithic quartz sandstone,and lithic arkose.The compositional maturity and structural maturity are medium and low.It means that the haulage distance is close,the source of material is abundant,and the deposition rate is fast.The type of reservoir space is mainly dissolved pores,with an average porosity of4.31%.The measured permeability is mainly between 0.001?0.01 m D.The pore structure has the characteristics of small pore throat,poor sorting,and poor connectivity.The main diagenesis of the Bagung Formation includes compaction,cementation,metasomatism and dissolution.The shallow burial zone near the uplift zone is in the B phase of diagenesis.The deep burial zone inside the depression is in the late diagenetic stage.According to the characteristics of diagenesis development,the sandstones in the Bagung Formation are divided into four types of diagenetic facies: strong dissolution diagenesis facies,chlorite cementing diagenetic facies,carbonate cementing diagenetic facies,and strongly compaction pressolution diagenetic facies.In the premise with considering the apparent volume change and compaction segmentation of rock,the quantitative analysis of pore evolution of different diagenetic facies was carried out.The calculation results show that compaction is the primary cause of the decrease of reservoir porosity,followed by cementation.Fluid inclusions studies indicate that there are at least 4 phases thermal fluid activities in the Bagung Formation.In the 2nd and3 rd phase,we detect that the oil-bearing fluid inclusions are in the same period.Combining with the surface natural gas,the sandstones in the Bagong Formation may experienced liquid oil filling during the Late Jurassic and natural gas filling after the late Neogene.So,we believe that the sandstone reservoirs of the Bagung Formation can form tight sandstone oil reservoirs after the previous accumulation,as well as form tight gas reservoirs after the preceding compaction.The densification of the Bagong Formation is the result of a combination of multiple factors such as high plastic debris content,development of coal measures,clay mineral cementation,and low geothermal gradients.Sedimentation controls the basic conditions for the formation of high-quality reservoirs.Sand bodies developed in the microfacies of distributary channels have a coarser grain size,a higher degree of structural maturity,a higher degree of compositional maturity,a larger thickness of the sand body,more developed primary pores and a better initial physical properties due to the strong hydrodynamic conditions and long transport distances.And in the subsequent diagenetic transformation process,it has a stronger ability to resist compaction,then it can relatively preserve more primary pores under the same burial conditions,and the acidic fluids are more accessible inflow.The formation of chlorite lining can inhibit siliceous cementation to some extent,and it has positive contribution to reservoir development.Tectonic fractures also have a positive effect on the improvement of reservoir properties.And the development of coal measures can promote the formation of secondary pores.According to the controlling factors and the data richness of dominant reservoirs,a comprehensive classification criterion for multi-index reservoirs,which is suitable for the tight sandstones of the Bagung Formation was established.The evaluation result indicates that most of Type I and Type II reservoirs were distributed in the Caiduo Caka and Zanagarba section.It shows that the reservoir condition in the central area of the depression is better.Combining with the control effect of the preservation conditions and the source rock conditions,the Tonamu-Jiagen block was eventually selected as the most favorable block of reservoir development in the Bagung Formation in the study area.