Micropores Characterization Method And Gas Content Analysis For Marine Shale Reservoirs In Southern China

In this paper,Qiongzhusi and Wufeng-Longmaxi marine shales in southern China were selected as research objects.In order to explore the micropores structure and the internal fluid distribution of marine shale reservoir,production data,outcrops and laboratory experiments were combined for comparative research for these two shales,some conclusions were obtained as follows:From the sedimentary,tectonic setting and outcrops research of the Qiongzhusi shale and Longmaxi shale,we can conclude that:these two sets of shales are widely distributed in the Sichuan basin and the surrounding area,and the thickness is great,and the high abundance of organic matter in these two shales proved that they have great potential in the future.There are two main characteristics of these two marine shales:high degree of evolution?diagenetic evolution,thermal evolution?,and strong structural transformation in the late stage?Yanshan-Xishan period?.The results of geochemical parameters and mineral characteristics show that,the organic matter types of Longmaxi shale and Qiongzhusi shale are mainly type I,and partly II₁.The organic matter abundance of the shale in the Qiongzhusi Formation is generally high,and the TOC content is generally between 0.04%and 15.31%,mainly concentrated in the low value of 0%²%.The samples with high TOC value mainly came from Changning groove.The TOC values of the Longmaxi ranged from 1.0%to6.0%,and majority of black shale samples have high TOC?>2.0%?.The high TOC samples are mainly from black shale in lower part of the Longmaxi Formation,which is regarded to form in the deep-water sediment.Silty shale and argillaceous siltstone in the upper part of the Longmaxi Formation have lower TOC?less than 1%?.The high TOC samples are found from two sedimentary centers in the south and northeastern of Sichuan basin.Ro values of the two sets of shales range from 1.55%to 5.10%,indicating they are in the over-mature stage,and they are in the gas window.Mineral compositions of the Longmaxi and the Qiongzhusi are similar:the content of quartz is highest,ranging from 20%to 85%;clay mineral is between 5%and 70%,and the content of carbonate mineral ranges from 5%to 60%.The content of feldspar is low?only 15%?.A new lithofacies ternary diagrams?with TOC content?was proposed.We combied SEM and JMicroVision software to get a quantitative method for different kinds of pores.The parameters of different types of pores?organic pores,intergranular pores,intragranular pores?were obtained.The pore parameters such as porosity,permeability and displacement pressure,median pressure,sorting coefficient,maximum pore throat radius,residual mercury saturation and exit efficiency were obtained by high pressure mercury intrusion.The Longmaxi shale can be divided into three categories based on the twist and connectivity using mercury intrusion results.The specific surface area,pore volume and pore size distribution of shale can be obtained through low temperature liquid nitrogen adsorption and carbon dioxide adsorption.The results show that the shale pores of the Longmaxi are mainly distributed in the micropores and mesopores,while the pores quantity of Lower Longmaxi shales are higher than that in the Upper Longmaxi.The pores of Lower Longmaxi shales is mainly composed of nanopores,and the pores are smaller.The peak of PSD is mainly concentrated at 3nm and 4nm.The lower part of Longmaxi have more micropores than upper part.A combination of high pressure mercury intrusion,low-temperature nitrogen adsorption and carbon dioxide adsorption is used for all-aperture pore characterization techniques.Different techniques can detect different pore size ranges:CO₂ adsorption can cover pore size from 0.4nm to 1.5nm;N₂ adsorption can cover the size ranging from 2nm to 180nm;and mercury intrusion can characterize a wide pore size ranging from 6 nm to 8000 nm.The inconsistence occurs in the overlap region for some samples with different techniques,and there may be four main possibilities for inconsistence.Magnetic and fluorescent nanoparticle material imaging technique,fluid tracer spontaneous imbibition experiment and fluid diffusion experiment were used for this study.These three new tracer technologies and methane isotherm adsorption were used to characterize the occurrence and flow characteristics of fluids in shale.According to the concentration of the tracer,we can detect the fluids migration path and final fluid distribution.It is a very intuitive characterization method.According to the experimental results and theoretical research,the fluid migration and distribution pattern of the shale was established.Geological evidences such as pressure coefficient,organic pores,isotope geochemical parameters and solid bitumens development and distribution in adjacent strata were used to prove that Tongwan unconformity is the main reason causing gas content differences between Wufeng-Longmaxi and Qiongzhusi shales.Qiongzhusi shale is considered as a relatively open system according to Tongwan unconformity.A large number of hydrocarbons migrated out to other formations,which resulting that only a small amount of residual natural gas left in the shale.So it is difficult to keep a high pressure coefficient.Meanwhile,the amount of organic pores is small.Based on all these experiments,the hydrocarbon expulsion mode of the Qiongzhusi shale system and the Wufeng-Longmaxi shale system in the Sichuan basin was established.