Study On Evaluation Of Chang-6 Tight Sandstone Reservoir Characteristics In Huangling Block,Ordos Basin

Chang 6 reservoir group of Huangling block,located in the south of Yishan slope in Ordos Basin,is in the early stage of tight oil exploration.The study of reservoir characteristics is largerly underexplored and the development law of high-quality reservoir is not identical,which are difficult to meet the requirements of exploration and development.In our work,we made full use of the information obtained from cores,geology,well logging and geochemistry to comprehensively evaluate reservoir characteristics of tight oil,clarify the distribution rules and invertigate main controling factors for different types of tight oil reservoirs.Our work provides a practical point of view to make a drilling decision or well locations deployment in the further exploration and development.Through this study,the following understandings are obtained:Chang 6 reservoir group of Huangling block located in the south of Yishan slope in Ordos Basin is in the early stage of tight oil exploration.Due to the largerly underexplored reservoir characteristics and the development law,it is difficult to satisfiy the requirements of exploration and development.In our work,we made full use of the information obtained from cores,geology,well logging and geochemistry to comprehensively evaluate reservoir characteristics of tight oil,clarify the distribution rules and invertigate main controling factors for different types of tight oil reservoirs.Our work provides a practical point of view to make a drilling decision or well locations deployment in the further exploration and development.Through this study,we obtained several conslusions:(1)Chang 6 reservoir group of Huangling block belongs to gravity flow-lake sedimentary system where the mainly direction of content source is the northeast.The primarty reason is that the slump deformation structure and incomplete BaoMa sequence are visible that is chariterized as a typical gravity flow deposits including sandy debris flow and turbidite sedimentary type,sandy debris flow tongue shape body and turbidite body of muddy water channel sand body.All these geological features are favorable conditionos for development of reservoirs.(2)The reservoir lithologes of chang 6 reservoir formation are mainly characterized as dark grey feldspar sandstone and lithic feldspar sandstone.The lithic debris are middle-acid magmatic rock debris and metamorphic rock debris.These debris are highly densified with an average porosity of 7.82%and an average permeability of 0.255×10-~3m~2.Most of the clastic particles are sub-angular and can be classified well.They are several differrnt types of particle contactions containing linear contaction,concessional contaction and a few point contaction.The particles are mainly supported by contact-pore cementation with low component maturity and medium structure maturity.The types of pores can be divided into primary pores and secondary pores,of which the primary pores are mainly residual intergranular pores and the secondary pores are pramarily dissolution secondary pores including dissolution intergranular pores and the dissolution granular internal pores.Besides,the high angled structural cracks and microcracks are locally developed in the dissolution secondary pores.(3)Through the field data sets including the cores,loggings and geological prior knowledge,the reservoir heterogeneity can be described from three different aspects:vertical heterogeneity,plane heterogeneity and microscopic heterogeneity.The composite rhythm patterns composed by the superposition of positive rhythm dominiates the reservoir heterogeneity.The positive and negative rhythm occur periodically in both high and low permeability.Interlaminar heterogeneity can be obviously observed and chang 6~3 has the strongest heterogeneity.On the plane,the areas with high porosity and permeability are mainly distributed in block or short strip.The microscopic heterogeneity analysis showes that the relationship between porosity and permeability of the chang 6 was poor because the distribution of pore throat was irregular and the structures of pore was not connected.Overrally speaking,the heterogeneity of chang 6 tight oil reservoirs is medium to strong.(4)We firstly utilized multiple analytical techniques,including mercury injection test,nuclear magnetic resonance(NMR)test,CT scan and physical property analysis of experimental data,to evaluate reservoir classification,optimizing physical properties,pore structure,sedimentary microfacies and sand body thickness,which are used as reservoir classification parameters.Then,based on the stasticiscal methods(e.g.,determining the evaluation indexes),we can classify the tight sandstone reservoris of study area into 4 types:Class I reservoirs are special types of low permeability reservoir,the main sedimentary facies for muddy water deposition and sandy debris flow tongue shape body sedimentation,lithology mainly fine sandstone,belongs to the study area the best reservoir;Class II reservoirs belong to dense type reservoir,Class III and IV reservoirs have poor reservoir performance.(5)The characteristics of chang 6 small reservoirs are identified by evaluation.6~1 small layer?class of sandy clastic reservoir is main developd in flow tongue shape body,with short stripe or dotted patterns.Specifically,it is pramarily distributed in S205,S106,S27wellblock.6~2 small layer?class reservoir distributes in S1307 wellblock and S208wellblock,with dotted patterns as well,where sand body thickness is normally greater than12 m.6~3 small layer?class reservoir mainly develops in muddy water and is distributed as a short article zonal in the H146,SZ1314 wellblock.6~4 small layer?class reservoir develops in muddy water and the tongue body in SZ1313 wellblock.The distribution of sedimentary microfacies controls the distribution of high quality reservoirs of tight sandstone,and diagenesis is the key to control the seepage ability of reservoirs.