Study On The Difference In Characteristics Of The Tight Sandstone Reservoir And Its Restrictive Mechanism To The Fluid Availability Of Chang 6 Member In The Ordos Basin

Tight sandstone oil is one of the most realistic unconventional oil and gas resources in China.The delta sandstone reservoir and gravity flow sandstone reservoir are the two most important tight oil reservoirs.At present,the Yanchang Formation of Ordos Basin has been developed on a large scale.The foundation of predicting the tight oil sweet areas area is the deeply analyzing reservoir characteristics differences between these two sandstone reservoirs types and its restrictive mechanism on the fluid availability.Using a series of experiments,based on the analysis of sedimentary characteristics,petrological characteristics,the diagenetic evolution,physical properties,pore structure characteristics,fluid flow characteristics and the characteristics of fractal dimension,the differences of two types reservoir characteristics have been expounded,and the controlling factors on the ability of the reservoir fluid flow have been determined,and the effect of different pore-throat combination type on development performance have also been discussed.The significant differences exist in the micro-structure of pore throat between the delta front sand body and turbidite fan sand body in the study area.The feldspar content in the sand body area of the delta front is higher,while the quartz,lithic fragments and interstitial materials contents in the turbidite fan sand body are higher,among which the rigid lithic fragments,illite and illite/smectite contents have obvious advantage.As a result,feldspar in the delta front has relatively more solution pore,while the turbidite fan has relatively more intergranular pore and micro-pore.Compared with the delta front,the turbidite fan sand body area has wider granularity distribution and poorer sorting,which cause wider porosity distribution and lower permeability.The porosity reduction rate was higher in the delta front sand body during the early cementation stage,while that of turbid fan was higher during late cementation stage.Due to the differences of material composition and diagenetic evolution,the pore throat structure,throat structure and fluid flow characteristics in the delta front area are better than those in the turbidite fan area,while the phase permeability characteristics are relatively better than those in the turbidite fan area.The fluid availability is obvious controlled by reservoir characteristic parameters.Increasing lithic fragments,feldspar or interstitial materials,especially the illite and illite/smectite of clay mineral,less quartz contents,combining with decreasing the mineral particle size reinforce the roughness of the pore throat surface,reduce the connectivity of the pore throat,and enhance microscopic heterogeneity.The strong heterogeneity of pore throat reduces the effective porosity,effective oil-displacing porosity and the maximum effective oil-phase permeability,namely,the fluid flow ability decreases.The results indicate that the fluid availability in the delta front sandy body is relatively better than that in the turbidite fan sand body.Moreover,fractal dimension is mainly based on pore throat connectivity and heterogeneity,while gas permeability is mainly related to pore size.Large pore throat surface is rough and has more contribution to the overall roughness of the pore throat surface,whereas based on its limited amount,large pore diameter,and limited surface roughness changes,the connectivity and heterogeneity of the pore throat of the sample are mainly controlled by the small throat and pores.The pore-throat structure is mostly related to the self-similarity of pore-throat surface,and the roughness of the pore throat surface has great influence on the fluid ability.In addition,the lower limit of reservoir production in two types of sand body was calculated based on the physical property cross plot,J function,method of inflexion and effective porosity.Reservoirs with pore-throat type ? show a high oil production rate in the early-to-mid period and early water breakthrough time,thus the rate of water-injection needs to be controlled during oil production.Reservoirs with pore-throat type ? show the best comprehensive production effect.