| In this paper,we focused on the shales from the Lower Cambrian Shuijingtuo Formation in the south of Huangling anticline in Western Hubei Province.Based on the geochemical and mineralogical characteristics,the excess silica and the existing form of the quartz in the target Shuijingtuo Shales were analyzed and discussed by using the techniques of major elements,scanning electron microscopy-cathodoluminescence(SEM-CL)and scanning electron microscopy-energy dispersive spectroscopy(SEM-EDS).The types of organic pores were analyzed by argon ion polishing SEM and SEM petrology characteristics.The formation,evolution and preservation of organic pores were discussed.The nano pore structure of shale was quantitatively characterized by low pressure gas adsorption experiment and the influence of organic matter and mineral composition on the pore structures were analyzed.The occurrence mechanism of shale gas was discussed by high pressure methane adsorption experiment.Finally,based on the adsorption potential theory and SDR adsorption isotherm model,geological model of shale gas content was established,and the content of adsorbed gas and free gas and their mutual conversion relationship in the geological history of Shuijingtuo formation shale in the study area were quantitatively analyzed.The research results and understanding of this paper are as follows:1.Due to the high quartz content in the Shuijingtuo Shales,a combination of XRD mineral analysis and major element analysis were conducted to analysis the silica origin and quantitative calculated the excess silica and to discuss the effect of sedimentary environment on the excess silicon content vertical and vertical distribution.(1)XRD mineral analysis shows that the brittle minerals of the Shuijingtuo Shales in the EYY 1?ZD1 and ZD2 wells are mainly quartz and clay minerals.The clay minerals are dominated by the illite and mixed-layer illite/smectite.The Shuijingtuo Shales can be divided into upper,middle and lower members with the results of the lithofacies classified by the ternary diagram of siliceous minerals-carbonates minerals-clay minerals.The lithofacies of the lower and upper members mainly are silica-rich argillaceous shale lithofacies(CM-1),calcareous/siliceous mixed shale lithofacies(M-1),argillaceous/siliceous mixed shale lithofacies(M-2),argillaceous/calcareous mixed shale lithofacies(M-3),and siliceous calcareous shale lithofacies(C-1).The lithofacies of the middle member are dominated by siliceous shale lithofacies(S),mixed siliceous shale lithofacies(S-2),and clay-rich siliceous shale lithofacies(S-3).The quartz content has a positive relationship with the TOC content,the TOC content of the shales with siliceous shale lithofacies(S)is higher than 5.0%,while shales with mixed shale lithofacies(M)and clay shale lithofacies has a lower TOC content(<2.0%).(2)The positive relationship between quartz and TOC content indicates that the quartz has biogenic silicon source.The observed sponge bone needle further confirmed the biogenic silica source.The dominated illite and mixed-layer illite/smectite in the clay minerals indicates that the silicon during the transformation of montmorillonite to illite provides part of the siliceous source.The cross plot of SiO2 and Al2O3,SiO2 and Zr shows that some shale samples fall on the normal sedimentary line,and some samples fall above the normal sedimentary line.The shale samples on the normal sedimentary line indicate the source of the detrital silicon,and the shales above the normal sedimentary line has excess silicon containing the biogenic silicon and silicon released from clay minerals transformation.(3)The results of calculation of excess silicon content show that the content of excess silicon and the ratio of excess silicon to total silicon in EYY 1 well,ZD1 well and ZD2 well have similar vertical variation trend,showing a trend of increasing first and then decreasing from the lower member to the upper member,which is consistent with the rapid rise to slow decline of sea level.The EYY1 well located in the deep shelf facies has the highest excess silicon content and the ratio of excess silicon to total silicon,while the YD2 well has the lowest excess silicon content and the ratio of excess silicon to total silicon.This is due to EYY 1 well is far away from terrigenous clastic input and has less detrital silicon content.Meanwhile,deeper water is conducive to the development of siliceous organisms and provides more biogenic silicon.Above all,the vertical and vertical distribution of excess silicon content in the study area is controlled by sedimentary facies,paleo water depth and source distance of detrital materials.(3)The SEM-CL and SEM-EDS images show two types of quartz in the Shuijingtuo Shales:detrital quartz and aggregates of microcrystalline quartz.The detrital quartz has a sub-rounded shape and has a uniform and bright luminescence.The microcrystalline quartz has a euhedral shape.The microcrystalline quartz belongs to the authigenic quartz and is characterized by a dark to low luminescence.2.The pore types of the Shuijingtuo Shales reservoir are described by FE-SEM.The organic pores as the dominated pore types are identified by FE-SEM petrography,and the formation and evolution of organic pores are discussed.(1)FE-SEM images show that organic pores,interparticle pores,intraparticle pores and microfractures are formed in the Shuijingtuo Shale.Organic pore is the main pore type of shale reservoir,which is mainly circular and sub circular,with sponge like characteristics.The pore diameter is small with 4-84 nm.The pores in the early-formed framboidal pyrite,pyrite dissolution pore and quartz dissolution pore are the places where the porous organic matter develops.The porous organic matter and microcrystalline quartz aggregate mostly co-exist in the interparticle pores.(2)The pores developed in the migrated organic matter are secondary organic pores.The key to judge whether the organic matter was migrated is to determine whether the pore space is unfilled before oil and bitumen migration.The FE-SEM organic matter petrography show that the organic matter filled with microcrystalline quartz aggregate and surrounded by autogenous minerals,the organic matter filled with the pores in the framboidal pyrite,and the organic matter filled with dissolution pores of quartz particles is migration organic matter.According to the high thermal maturity of the Shuijingtuo Shale in the study area,it can be inferred that the main type of organic matter in the Shuijingtuo Shale is porous pyrobitumen,and the pyrobitumen-hosted pore is the main pore type in the Shuijingtuo Shale.(3)The development location,formation time and reservoir formation pressure can affect the preservation of organic pores.The pores in early-formed framboidal pyrite,dissolution pores in pyrite and quartz grains and intergranular pores filled with microcrystalline quartz aggregate can protect the organic pores from mechanical compaction;the smaller pore diameter of the Shuijingtuo Shale also has a positive impact on its preservation;the abnormally high pore pressure can reduce the effective stress,which is conducive to the preservation of organic pores;later compaction can lead to the deformation and collapse of the early-formed organic pores.3.In view of the complex nano-scale pore network of the Shuijingtuo Shales,the pore structure was identified by means of quantitative(helium porosity,low pressure gas adsorption)test,and the effect of organic matter and mineral composition on pore structure were discussed.(1)The adsorption isotherm of low pressure carbon dioxide belongs to type I adsorption isotherm,which indicates that there are a large number of micropores in shale reservoir;the pore size distribution curve obtained by DFT model has multi peak type,mainly including four main peaks of 0.35 nm,0.48 nm,0.57 nm and 0.82 nm.The low pressure nitrogen adsorption isotherms are belong to the ? adsorption isotherm.Most of the shale samples have a hysteresis loop with H3 type,and part of the shale samples have a hysteresis loop with H2 type,indicating that the pore types of the Shuijingtuo Shales in the study area are mainly slit or parallel plate pores,with a small amount of ink bottle pores;The BJH pore size distribution curve shows that the pore volume of Shuijingtuo Shales is mainly provided by pores with diameter less than 10 nm and macropores,accounting for 16.95-77.52%,15.04-57.74%with average of 52.69%and 32.64%,respectively.The BET specific surface area of shale is mainly provided by pores with pore diameter less than 10 nm,and the proportion is 65.81-98.80%with an average of 94.91%.(2)The main pore structure parameters of the Shuijingtuo Shales,such as helium porosity,BET specific surface area,BJH total pore volume,and DR micropore volume are positively correlated with TOC content,which indicate that organic matter is the main controlling factor of the shale pore structure;with the increase of clay mineral content in Shuijingtuo Shales,the pore structure parameters have a decreasing trend.This is mainly because the organic matter content conceals the contribution of clay minerals to the pore volume and specific surface area,and the TOC normalized pore structure parameters have a positive correlation with clay minerals.4.The adsorption capacity of the Shuijingtuo Shales was determined by high pressure methane adsorption experiment,and the methane adsorption process was analyzed.(1)The excess methane adsorption isotherm of shale has parabola shape,and the excess adsorption capacity first increases rapidly with the increase of pressure,reaches the maximum value,and then decreases gradually with the further increase of pressure;For a same shale sample,the excess adsorption isotherms at different temperatures have similar shape;under the same pressure,the excess adsorption capacity at lower temperature is greater than that at higher temperature.(2)The SDR adsorption model based on the adsorption potential theory can well fit the excess adsorption isotherm.The range of adsorption capacity of the Shuijingtuo Shale at 30? is 1.68-5.47 m3/t.The methane adsorption capacity at different temperatures decreases with the increase of temperature.(3)The parabola-like shape of excess adsorption isotherm is the result of the difference of density increasing rate between adsorption phase and free phase.At the critical pressure,the change rate of free gas density(pfree)with pressure(P)((?)?free/(?)P)is equal to the change rate of adsorption gas density(pads)with pressure(P)((?)?free/(?)P),that is,(?)?free/(?)P=(?)?free/(?)P.Under the critical pressure,(?)?free/(?)P<(?)?free/(?)P,resulting in the increase of excess adsorption capacity with the increase of pressure.Above the critical pressure,(?)?free/(?)P>(?)?free/(?)P,resulting in the decrease of excess adsorption capacity with the increase of pressure.The volume of adsorbed phase(Vads)is about equal to DR microporous volume(VmicroP),but far lower than BJH total pore volume(VtotalP),that is,VmicroP?Vads<VtotalP.This volume relationship shows that the volume of adsorbed phase only accounts for a small part of the total pore volume,and most of the empty space in the large pore is still filled by free gas.(4)Internal factors(temperature and pressure)and external factors(TOC content,mineral composition and pore structure)control the methane adsorption capacity.Shale adsorption is a physical adsorption with an exothermic process.Therefore,the increase of temperature will lead to the decrease of adsorption.Pressure has a positive effect on the shale methane adsorption.There is a strong positive correlation between TOC content and adsorption capacity.This positive correlation shows that TOC content is the main factor to control methane adsorption capacity of the Shuijingtuo Shales.The essential effect of TOC content on methane adsorption capacity is due to the abundant nanoscale organic pores in organic matter.The adsorption sites and volumes of methane molecules were significantly increased because of the existence of organic pores.In order to eliminate the influence of organic matter content on the adsorption capacity of clay minerals,the relationship between clay mineral content and methane adsorption capacity can be obtained by standardizing the TOC content of adsorption capacity.5.The model of shale gas storage capacity in two basic geological processes(burial stage with hydrocarbon generation and increasing total gas content and uplift stage with constant total gas content)is established to explore the conversion relationship between adsorbed gas and free gas in the Shuijingtuo Shale in the study area.(1)Based on the adsorption potential theory and SDR adsorption isotherm model,considering the influence of adsorption phase volume,gas saturation and water content,combined with the high pressure methane adsorption experiment under different temperature conditions,the storage capacity formula of free gas,adsorbed gas and total gas content under geological conditions is deduced.(2)During the burial process,the adsorbed gas storage capacity initial rapidly increase in the shallow depth and continuous decrease after reaching the maximum value,indicating that the adsorbed gas is primarily controlled by pore pressure at shallow burial depths,but at deep burial depths,temperature is the primary controlling factor.The storage capacity of free gas and total gas has the same trend,both of which increase continuously with the increase of burial depth.(3)The effects of pore pressure on gas storage capacities are investigated.The storage capacity of free gas and total gas significantly increase with the increasing pore pressure.However,the influence of pore pressure on the storage capacity of adsorbed gas is limited,particularly at deep burial depths.(4)In the uplift process with constant total gas content,shale gas needs to be transformed from free phase to adsorbed phase to keep the total gas constant.The simulation results show that the adsorbed gas content increases linearly,while the free gas content decreases linearly.(5)The calculated results agree well with the canister desorption data,indicating that the established equations for the gas storage capacities and the model parameter values are reasonable.The total gas content of the Shuijingtuo Shale in the maximum burial depth range from 1.33 m3/t to 3.90 m3/t for EYY 1 well,ZD 1 well and ZD 2 well.The total gas content in the maximum burial depth is higher than the present-day burial depth in the both three shale gas wells.The difference of gas content before and after uplift indicates that part of shale gas is lost during uplift.The gas loss content and gas loss rate are 0.05-1.92 m3/t and 23-28%for the EYY 1 well,0.73-1.96 m3/t and 49-57%for the ZD 1 well,0.55-1.48 m3/t and 36-43%for the ZD 2 well.The three shale gas wells of EYY 1,ZD 1 and ZD 2 have similar uplift amplitude,but there are great differences in gas loss rate.The EYY 1 well with the largest present-day burla depth(2601-3069 m)has the lowest gas loss rate,while ZD 1 well with the shallowest present-day burial depth(66-386 m)has the has the highest gas loss rate,which indicates that deep shale is more favorable for shale gas exploration than shallow shale in the same area. |
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