Evaluation Of Pore Structure Of Tight Mixed Rocks And Its Effect On Reservoir Validity

The oil and gas resources,contained in lacustrine mixed rocks of the upper member of the Lower Ganchaigou Formation（E₃²）in the Yingxi area of the Qaidam Basin,are very rich.However,due to the characteristics of tight reservoir,complex mineral compositions,diverse types of reservoir spaces,frequent thin interbedding and strong heterogeneity,the exploration and exploitation of oil and gas in this area are seriously restricted.Firstly,via clarifying the background of structural,sedimentary and stratigraphic characteristics of Yingxi area,and based on the characteristics of minerals,lithologies,physical properties,reservoir spaces and lithofacies,this paper establishes the pore structure classification and evaluation criteria of E₃² mixed rock reservoirs in Yingxi area by deeply analyzing the characteristics of pore-throat size,pore space distribution and pore-throat connectivity of the E₃² mixed rock reservoirs.Secondly,the main controlling factors of the pore structure of the E₃² mixed rock reservoir are identified from the four perspectives of diagenesis,macro-fractures,mineral composition and stratigraphic heterogeneity.Finally,the influences of the different pore structure types of E₃² mixed rock on the reservoir capacity and seepage capacity are studied,and the constraint effects of the combination of pore structure types on the economically recoverable reserves of the reservoirs are discussed.The porosity of E₃² mixed rock in Yingxi area is mainly between 4～12%,and the permeability is generally less than 1m D.The lithologies mainly consist of mixed rocks,limy dolomites,marl-bearing dolomites,argillaceous limy dolomites and sandy limy dolomites,based on four types of laminar structure patterns,which are strong layered,weak layered,porphyritic and massive,the E₃² mixed rock facies can be classified into12 types.The reservoir spaces are dominated by nano-micron and sub-micron pores,supplemented by small pores.The pore radius is mainly between 0.01 and 0.3μm.The throat radius is generally less than 0.1μm,but the advantage of number of intercrystalline pores can make up for the insufficience of small pore sizes,and can provide considerable storage spaces.The total pore space of E₃² mixed rock has a wider pore size distribution range than that of the movable fluid pore space,with the higher content of small pores and the stronger heterogeneity.When the pore radius increases,the radius of the throat directly connected to it has a tendency to increase accordingly.However,compared with the pore space in the middle section of the pore size distribution,the relationship of the pore throat configuration of large pore space and small pore space becomes more complicated.For the E₃² mixed rock,the movable fluid porosity can characterize the effective storage capacity,the movable fluid pore space fractal dimension can reflect the connectivity of the pore throat,and the movable pore content has a significant control effect on both.In addition,based on the movable fluid porosity and the movable fluid pore space fractal dimension,the pore structures of the E₃² mixed rock reservoirs in Yingxi area are divided into three types,which are type 1 contains large pores and thin throats,type 2 contains small pores and micro throats,and type 3 contains micropores and micro throats.Principal component analysis method is used to establish the logging quantitative identification chart of E₃² mixed rock pore structures in Yingxi area,which realizes the macroscopic and vertical continuous classification and evaluation of pore structures.The development of dolomites in the E₃² mixed rock reservoirs in Yingxi area are conducive to the increase of porosity.Sand-bearing or sandy limy dolomites are conducive to the formation of micro-fractures.The dissolution,associated with micro-fractures,mostly occurs in carbonate minerals to form the dissolved fractures,which can effectively improve the petrophysical properties of E₃² mixed rocks.The high content of clay minerals,strong cementation,and widely developed silt-level grains are the main reasons for the deterioration of the pore structures.The development of macro fractures is a sufficient condition for the formation of good pore structures,but the layers with good pore structures may not necessarily have the development of macro fractures.The influence of multi-scale heterogeneity of strata on pore structures is obviously different.The sizes and distribution of the pore throats are mainly controlled by the heterogeneity of the component,and the heterogeneity of the laminar layers has the strongest influence on the connectivity of the pores and throats.At the same time,the peracid dissolution experiment shows that the mineral composition and pore size distribution characteristics of E₃² mixed rock have a significant control effect on the storage capacity of the reservoir,and the fractal modeling of permeability reveals that the non-uniform adsorption of water film on pore surface and the pore space of movable fluid have significant influence on the seepage capacity of reservoir.In addition,the economic recoverable reserves of a single well in the Yingxi area are obviously restricted by the pore structures.Among them,the economic recoverable reserves of the reservoirs with large pores and thin throats are the highest,the reservoirs with large pores and thin throats-small pores and micro throats have certain economic benefits,while the reservoirs with small pores and thin throats have the lowest economic recoverable reserves,which means that it’s difficult to make profits.