Characteristics And Petroleum Geological Significance Of Lacustrine Carbonate Microscopic Storage Spaces Of The Jurassic Da'Anzhai Member In The Central Sichuan Basin,China

Refinement and quantification are the basic trends in reservoir research.The study of reservoir has expanded to the unconventional scale(?m-nm)from the conventional scale(mm-?m).Although numerous progresses have been achieved in characterizing shales,tight sandstone and coal reservoirs,studies that are associated with lacustrine tight carbonate are limited.The Da'anzhai Member in the central Sichuan Basin is an important lacustrine carbonate reservoir in China,and the study of its microscopic reservoir characteristics is lacking.In this dissertation,the Da'anzhai Member was systematically studied,and some new technical means were used to finely characterize its microscopic reservoir.Characteristics,petroleum geological significance,and influencing factors including sedimentation and diagenetic evolutions of the lacustrine tight carbonate microscopic reservoirs were discussed.In addition,based on the microscopic reservoir characteristics,the full-scale reservoir evaluation criteria have been established,and it was used to predict the favorable reservoir and areas.The main understandings are as follows.The microscopic storage spaces(<1?m)and the pore-fracture double media system are both well-developed in the Da'anzhai lacustrine carbonate reservoir.However,it has more wedge-shaped pores,dead end pores,and more complex microstructures than found in other unconventional reservoirs shown in the dissertation.As for the storage spaces,the nano-scale reservoir space(<1?m)occupies a large proportion(92.27%)mainly including the well-developed intraparticle pores,interparticle pores,and organic matter pores,and the V_BJH for pore volume and S_BET for the specific surface area can reach 2.50 cm³/100g and 6.02 m²/g,respectively.They all indicate that the microscopic storage spaces(<1?m)are well-developed.As for the pore structure,the isotherms and hysteresis loops are of Type?and Type H₃,respectively,which indicates wedge-shaped pores.However,the hysteresis loops indicate that the lacustrine carbonate has more dead-end pores and these pores with more complex microstructures than other reservoirs in this dissertation.The high values of the fractal dimensions based on the FHH model indicate the strong complexity and heterogeneity of the lacustrine tight carbonate reservoir surface.Different sedimentary facies and paleoenvironments effectively lead to different original sediments,and they can show different characteristics of microscopic reservoir after different diagenetic evolution.The terrestrial minerals,primarily including quartz,and clay,have well-developed nanopores and are positively correlated with the nanopore storage space,whereas the authigenic minerals,primarily including calcite,have strong cementation and filling effects and are negatively correlated with nanopore storage space.The sedimentary facies of the shell beach edge and lake slope in front of(or near)the shell beach and the palaeoenvironments with strong paleo-weathering,low salinity,humid(rainy)paleo-climate and low hydrodynamic conditions are favorable for improving the storage spaces and the terrestrial mineral contents but are harmful to the formation of authigenic minerals.The characteristics of the size,shape,heterogeneity,and the pore-fracture double media system for lacustrine tight carbonate microscopic reservoirs can effectively control the distribution and development characteristics of oil and gas in the Jurassic Da'anzhai Member in the central Sichuan Basin.The stable production is guaranteed by the well-developed microscopic storage spaces(<1?m)and the pore-fracture double media system.The microscopic pore system has lots of wedge-shaped pores,dead-end pores,and high fractal dimension,and they are not conducive to the charging and migration of oil and gas,which can lead to poor oiliness and low production.Under the evaluation criteria of the microscopic reservoir,the shell beach edge sedimentary facies with medium-low energy water have advantages in the microscopic reservoir spaces,pore structure,and full-scale migration transportation system compared to the high energy shell beach sedimentary facies.So,the direction of exploration in the study area should shift from crystalline shell limestones in the high energy shell beach sedimentary facies to the argillaceous shell limestones in the shell beach edge sedimentary facies with medium-low energy.In addition,the thick shales in deep lake or semideep lake are also worth studying.