Study on the long-term performance of shield tunnel passing through the gas-bearing strata

When the shield tunnel passes through the gas-bearing strata,gas and water leakage may occur depending on the sealing performance of the segment joints.This process involves the complex multiphase seepage flow phenomenon in unsaturated soil.In this study,a fully coupled solid-liquid-gas model of the GIL Utility Tunnel was established to investigate the influence of the high-pressure gas on the mechanical properties of the tunnel segments and joints.The constitutive model of the Extended Barcelona Basic Model was imple-mented to simulate the effect of the seepage process on soil deformation.The results show that significant upward displacement occurred in the gas reservoir and its overlying strata,and the maximum displacement reached 30 mm.In addition,during the leakage of the gas and the water,an increase in the average soil effective stress was observed.It would induce a reduction in the suction and expansion of the yield surface.The tunnel tended to be stable from 20 years onwards,thus the soil deformation due to the water leakage only occurred at the early stage.In addition,the joint opening under the most unfavorable internal force combination was 0.69 mm,and the correspond-ing bolt stress was 119.5 MPa,which is below the yield limit.The results of this study help to understand the influence of high-pressure gas on tunnel safety and the sealing performance of the joints.

Pore Connectivity of Deep Lacustrine Shale and its Effect on Gas-bearing Characteristics in the Songliao Basin:Implications from Continental Scientific Drilling

The lacustrine shale of deep Shahezi Formation in the Songliao basin has great gas potential,but its pore evolution,heterogeneity,and connectivity characteristics remain unclear.In this work,total organic carbon analysis,rock pyrolysis,X-ray diffraction field emission scanning electron microscopy,the particle and crack analysis system software,low-temperature nitrogen adsorption experiment,fractal theory,high-pressure mercury injection experiment and nuclear magnetic resonance experiment were used to study the Shahezi shale from Well SK-2.The result indicated that the organic pores in Shahezi shale are not developed,and the intergranular and intragranular pores are mainly formed by illitedominated clay.As the burial depth increases,the pore size and slit-shaped pores formed by clay decrease,and dissolved pores in the feldspar and carbonate minerals and dissolved fractures in the quartz increase.The pore evolution is affected by clay,compaction,and high-temperature corrosion.Based on the pore structure characteristics reflected by the pore size distribution and pore structure parameters obtained by multiple experimental methods,the pore development and evolution are divided into three stages.During stageⅠandⅡ,the pore heterogeneity of the shale reservoirs increases with the depth,the physical properties and pore connectivity deteriorate,but the gas-bearing property is good.In stageⅢ,the pore heterogeneity is the highest,its gas generation and storage capacity are low,but the increase of micro-fractures makes pore connectivity and gas-bearing better.

LA-ICP-MS zircon U-Pb dating of tuffites in the Sachakou Pb-Zn mining area, Karakorum, Xinjiang and its establishment of Early Triassic strata

1.Objective In the past decade,a group of medium to giant lead-zinc deposits,represented by Huoshaoyun,Sachakou,and Yuanbaoling,have been discovered in the Aksai Chin region of Karakoram,Xinjiang.They are all located in the Mesozoic carbonate and clastic rock formations.The Sachakou leadzinc mining area is adjacent to the northwest of the Huoshaoyun lead-zinc mining area and is in the same stratigraphic layer as Huoshaoyun.Although many scholars have been arguing about the type and age of Huoshaoyun lead-zinc mineralization,few scholars have paid attention to the classification of the ore-bearing strata in the area.The stratigraphy of the Lower Permian Shenxianwan Group to the Upper Cretaceous Tielongtan Group is exposed in the Sachakou area of Karakorum,Xinjiang,however,the Late Permian-Early Triassic stratigraphy is missing(Fig.1a).Due to the harsh natural conditions in the area and the low level of work,the stratigraphic delineation is not exhaustive,and the regional lithology is dominated by carbonates and clastic rocks,which makes it difficult to identify the age of the regional lithology and causes problems for the exploration and research of lead-zinc in the area.

Influence of confined water on the limit support pressure of tunnel face in weakly water-rich strata

In the process of shield tunneling through soft soil layers,the presence of confined water ahead poses a significant threat to the stability of the tunnel face.Therefore,it is crucial to consider the impact of confined water on the limit support pressure of the tunnel face.This study employed the finite element method(FEM)to analyze the limit support pressure of shield tunnel face instability within a pressurized water-containing layer.Subsequently,a multiple linear regression approach was applied to derive a concise solution formula for the limit support pressure,incorporating various influencing factors.The analysis yields the following conclusions:1)The influence of confined water on the instability mode of the tunnel face in soft soil layers makes the displacement response of the strata not significant when the face is unstable;2)The limit support pressure increases approximately linearly with the pressure head,shield tunnel diameter,and tunnel burial depth.And inversely proportional to the thickness of the impermeable layer,soil cohesion and internal friction angle;3)Through an engineering case study analysis,the results align well with those obtained from traditional theoretical methods,thereby validating the rationality of the equations proposed in this paper.Furthermore,the proposed equations overcome the limitation of traditional theoretical approaches considering the influence of changes in impermeable layer thickness.It can accurately depict the dynamic variation in the required limit support pressure to maintain the stability of the tunnel face during shield tunneling,thus better reflecting engineering reality.

Failure mechanism and safety control technology of a composite strata roadway in deep and soft rock masses:a case study

The construction of coal mines often encounters deep composite soft rock roadways,which is characterized by significant deformation and poor stability.To deeply study the failure mechanism and large deformation challenges of a composite strata roadway in deep and soft rock masses,a numerical model of 3DEC tetrahedral blocks was established based on the method of rock quality designation(RQD).The results showed that original support cannot prevent asymmetric failure and large deformation due to the adverse geological environment and unsuitable support design.According to the failure characteristics,a coupling support of“NPR bolt/cable+mesh+shotcrete+steel pipe”was proposed to control the stability of the surrounding rock.The excellent mechanical properties of large deformation(approximately 400 mm)and high constant resistance force(bolt with 180 k N;cable with 350 k N)were evaluated by the tensile tests.The numerical results showed that the maximum deformation was minimized to 243 mm,and the bearing capacity of the surrounding rock of the roadway was enhanced.The field test results showed that the maximum deformation of the surrounding rock was 210 mm,and the forces of the NPR bolt and cable were stable at approximately 180 k N and 350 k N,respectively.This demonstrated the effectiveness of the coupling support with the NPR bolt and cable,which could be a guiding significance for the safety control of large deformation and failure in deep composite soft rock roadways.

Types,composition and diagenetic evolution of authigenic clay minerals in argillaceous limestone of sepiolite-bearing strata:A case study of Mao-1 Member of Middle Permian Maokou Formation,eastern Sichuan Basin,SW China

The types,occurrence and composition of authigenic clay minerals in argillaceous limestone of sepiolite-bearing strata of the first member of the Middle Permian Maokou Formation(Mao-1 Member)in eastern Sichuan Basin were investigated through outcrop section measurement,core observation,thin section identification,argon ion polishing,X-ray diffraction,scanning electron microscope,energy spectrum analysis and laser ablation-inductively coupled plasma-mass spectrometry.The diagenetic evolution sequence of clay minerals was clarified,and the sedimentary-diagenetic evolution model of clay minerals was established.The results show that authigenic sepiolite minerals were precipitated in the Si4+and Mg2+-rich cool aragonite sea and sepiolite-bearing strata were formed in the Mao-1 Member.During burial diagenesis,authigenic clay minerals undergo two possible evolution sequences.First,from the early diagenetic stage A to the middle diagenetic stage A1,the sepiolite kept stable in the shallow-buried environment lack of Al3+.It began to transform into stevensite in the middle diagenetic stage A2,and then evolved into disordered talc in the middle diagenetic stage B1and finally into talc in the period from the middle diagenetic stage B2to the late diagenetic stage.Thus,the primary diagenetic evolution sequence of authigenic clay minerals,i.e.sepiolite-stevensite-disordered talc-talc,was formed in the Mao-1 Member.Second,in the early diagenetic stage A,as Al3+carried by the storm and upwelling currents was involved in the diagenetic process,trace of sepiolite started to evolve into smectite,and a part of smectite turned into chlorite.From the early diagenetic stage B to the middle diagenesis stage A1,a part of smectite evolved to illite/smectite mixed layer(I/S).The I/S evolved initially into illite from the middle diagenesis stage A2to the middle diagenesis stage B2,and then totally into illite in the late diagenesis stage.Thus,the secondary diagenetic evolution sequence of authigenic clay minerals,i.e.sepiolite-smectite-chlorite/illite,was formed in the Mao-1 Member.The types and evolution of authigenic clay minerals in argillaceous limestone of sepiolite-bearing strata are significant for petroleum geology in two aspects.First,sepiolite can adsorb and accumulate a large amount of organic matters,thereby effectively improving the quality and hydrocarbon generation potential of the source rocks of the Mao-1 Member.Second,the evolution from sepiolite to talc is accompanied by the formation of numerous organic matter pores and clay shrinkage pores/fractures,as well as the releasing of the Mg2+-rich diagenetic fluid,which allows for the dolomitization of limestone within or around the sag.As a result,the new assemblages of self-generation and self-accumulation,and lower/side source and upper/lateral reservoir,are created in the Middle Permian,enhancing the hydrocarbon accumulation efficiency.

Shield Excavation Analysis: Ground Settlement & Mechanical Responses in Complex Strata

This study delves into the effects of shield tunneling in complex coastal strata, focusing on how this constructionmethod impacts surface settlement, the mechanical properties of adjacent rock, and the deformation of tunnelsegments. It investigates the impact of shield construction on surface settlement, mechanical characteristics ofnearby rock, and segment deformation in complex coastal strata susceptible to construction disturbances. Utilizingthe Fuzhou Binhai express line as a case study, we developed a comprehensive numerical model using theABAQUS finite element software. The model incorporates factors such as face force, grouting pressure, jack force,and cutterhead torque. Its accuracy is validated against field monitoring data from engineering projects. Simulationswere conducted to analyze ground settlement and mechanical changes in adjacent rock and segments acrossfive soil layers. The results indicate that disturbances are most significant near the excavation zone of the shieldmachine, with a prominent settlement trough forming and stabilizing around 2.0–3.0 D from the excavation. Theexcavation face compresses the soil, inducing lateral expansion. As grouting pressure decreases, the segmentexperiences upward buoyancy. In mixed strata, softer layers witness increased cutting, intensifying disturbancesbut reducing segment floatation. These findings offer valuable insights for predicting settlements, ensuring segmentand rock safety, and optimizing tunneling parameters.

An intelligent automatic correlation method of oilbearing strata based on pattern constraints:An example of accretionary stratigraphy of Shishen 100 block in Shinan Oilfield of Bohai Bay Basin,East China

Aiming at the problem that the data-driven automatic correlation methods which are difficult to adapt to the automatic correlation of oil-bearing strata with large changes in lateral sedimentary facies and strata thickness,an intelligent automatic correlation method of oil-bearing strata based on pattern constraints is formed.We propose to introduce knowledge-driven in automatic correlation of oil-bearing strata,constraining the correlation process by stratigraphic sedimentary patterns and improving the similarity measuring machine and conditional constraint dynamic time warping algorithm to automate the correlation of marker layers and the interfaces of each stratum.The application in Shishen 100 block in the Shinan Oilfield of the Bohai Bay Basin shows that the coincidence rate of the marker layers identified by this method is over 95.00%,and the average coincidence rate of identified oil-bearing strata reaches 90.02% compared to artificial correlation results,which is about 17 percentage points higher than that of the existing automatic correlation methods.The accuracy of the automatic correlation of oil-bearing strata has been effectively improved.

冲击载荷下叠合岩层巷道围岩的应力演化数值模拟研究——基于岩层运动并行计算系统StrataKing

煤系地层属于典型的层状沉积岩层,这在传统连续介质模型中不能被较好反映。为了探究冲击载荷下叠合岩层巷道围岩的应力演化规律,采用以拉格朗日元与离散元耦合连续-非连续方法为基础发展的岩层运动并行计算系统StrataKing,基于理想化模型分析了巷道上表面附近应力波的叠加。阐明了应力波叠加导致顶板开裂机理,并且探讨了冲击载荷幅值的影响规律。研究发现:在原始压应力波传至巷道上表面附近测点后,该测点的最大主应力刚开始呈下降-上升-下降的变化趋势,这是由原始压、拉应力波不同时刻的叠加不同导致的;由于次级应力波的波长较原始应力波的小,次级压、拉应力波的单独作用更明显,它们的叠加使巷道上表面附近测点能产生更低和更高的最大主应力,甚至导致顶板开裂;冲击载荷幅值越大,近似阶梯增长阶段中拉裂纹平均发展速度越快,巷道围岩最终开裂范围越大,巷道围岩平衡越困难。

岩层运动并行计算系统StrataKing及云平台StrataKing.UniXDE参数确定原则及应用

StrataKing(SK)及StrataKing.UniXDE(SK.U)是针对采动诱发岩层运动的首款GPU并行计算系统和云平台。介绍了主要参数的确定原则。单元尺寸的选取应综合考虑GPU的性能和计算精度。准静力计算时,局部自适应阻尼系数尽量接近上限取。通过浅埋厚煤层岩层运动模拟,展示了SK的大规模计算能力、丰富功能和鲜明矿业特色。对本文结果与相似材料模拟实验结果的异同进行了分析。对已经处于试用阶段的SK.U的使用进行了介绍。

Linear numerical calculation method for obtaining critical point,pore fluid,and framework parameters of gas-bearing media

Up to now, the primary method for studying critical porosity and porous media are experimental measurements and data analysis. There are few references on how to numerically calculate porosity at the critical point, pore fluid-related parameters, or framework-related parameters. So in this article, we provide a method for calculating these elastic parameters and use this method to analyze gas-bearing samples. We first derive three linear equations for numerical calculations. They are the equation of density p versus porosity Ф, density times the square of compressional wave velocity p Vp^2 versus porosity, and density times the square of shear wave velocity pVs^2 versus porosity. Here porosity is viewed as an independent variable and the other parameters are dependent variables. We elaborate on the calculation steps and provide some notes. Then we use our method to analyze gas-bearing sandstone samples. In the calculations, density and P- and S-velocities are input data and we calculate eleven relative parameters for porous fluid, framework, and critical point. In the end, by comparing our results with experiment measurements, we prove the viability of the method.

Types,Characteristics and Significances of Migrating Pathways of Gas-bearing Fluids in the Shenhu Area,Northern Continental Slope of the South China Sea

The first marine gas hydrate expedition in China has been conducted by Guangzhou Marine Geological Survey in the Shenhu Area, northern continental slope of the South China Sea. Previous study has analyzed the P-T conditions, geophysical anomalies and saturation calculations of these gas hydrates, but has not documented in detail the migration of gas-bearing fluids in the study area. Based on the interpretations of 2D/3D seismic data, this work identified two types of migration pathways for gas-bearing fluids in the Shenhu area, i.e., vertical and lateral pathways. The vertical pathways（largescale faults, gas chimneys and mud diapirs） presented as steep seismic reflection anomalies, which could be traced downward to the Eocene source rocks and may penetrate into the Late Miocene strata. The deeper gases/fluids might be allowed migrating into the shallower strata through these vertical conduits. However, the distributions showed distinct differences between these pathways. Large-scale faults developed only in the north and northeast of the Shenhu area, while in the drilling area gas chimneys were the sole vertical migration pathways. Since the Pliocene, normal faults, detachment faults and favorable sediments have constituted the lateral pathways in the Shenhu gas hydrate drilling area. Although these lateral pathways were connected with gas chimneys, they exerted different effects on hydrate formation and accumulation. Gas-bearing fluids migrated upward along gas chimneys might further migrate laterally because of the normal faults, thereby enlarging the range of the chimneys. Linking gas chimneys with the seafloor, the detachment faults might act as conduits for escaping gases/fluids. Re-deposited sediments developed at the early stage of the Quaternary were located within the gas hydrate stability zone, so hydrates would be enriched in these favorable sediments. Compared with the migration pathways（large-scale faults and mud diapirs） in the LW3-1 deep-sea oil/gas field, the migration efficiency of the vertical pathways（composed of gas chimneys） in the gas hydrate drilling area might be relatively low. Description and qualitative discrimination of migration pathways in the Shenhu gas hydrate drilling area are helpful to further understand the relationship between good-quality deep source rocks and shallow, mainly biogenicallyproduced, hydrates. As the main source rocks of the Baiyun sag, lacustrine mudstones in the Wenchang and Enping Formations may provide thermogenic methane. Gas chimneys with relatively low migration efficiency created the vertical pathways. Caused by the Dongsha tectonic movement, the release of overpressured fluids might reduce the vertical migration rates of the thermogenic methane. The thick bathyal/abyssal fine-grained sediments since the Late Miocene provided migration media with low permeability. These preconditions may cause carbon isotopic fractionation ofthermogenic methane during long-distance vertical migrations. Therefore, although geochemical analyses indicate that the methane forming gas hydrate in the Shenhu area was mainly produced biogenically, or was mixed methane primarily of microbial origin, thermogenic methane still contribute significantly.

Study of A Geo-Acoustic Model of Gas-Bearing Sediment and Its Application in Sediment with Low Acoustic Veloctiy

A new geo-acoustic model for gas-bearing sediment is proposed based on the work of Dvorkin and Prasad, and Biot theory. Only five geophysical parameters： sediment mineral composition, free gas saturation, tortuosity （also known as the structure factor）, permeability, and porosity, are considered in the model. A benefit of this model is that we need only five parameters instead of ten parameters in the Blot＇ s formulas for acoustic velocity and attenuation calculation. Here the model is demonstrated with the in-situ experimental data collected from the Hangzhou Bay, China. The results of this study suggest that free gas content in sediment is the most critical condition resulting in a low acoustic velocity （compressional wave）. The respective contributions of the other four parameters in the model are also discussed.

Sinian gas sources and effectiveness of primary gas-bearing system in Sichuan Basin, SW China

Based on correlation between geochemical characteristics of Sinian and Cambrian source rocks and discovered gas reservoirs,paleoand the analysis on geological conditions of reservoir formation,the sources of natural gas in the Sinian of Sichuan Basin have been discussed to sort out the contribution of Sinian source rocks to the gas reservoirs and effectiveness of Sinian primary gas-bearing system.Through the analysis of natural gas composition,carbon and hydrogen isotopes and effectiveness of Sinian accumulation assemblages,it is concluded that:(1)The natural gas derived from the Sinian source rock is characterized by low ethane content,heavy ethane carbon isotope and light methane hydrogen isotope,and obviously different from the gas generated by the Cambrian source rock.(2)The gas reservoirs discovered in Sinian Dengying Formation are sourced by Sinian and Cambrian source rocks,and the Sinian source rock contributes different proportions to the gas in the 4th member and the 2nd member of the Dengying Formation,specifically,39%and 55%to the 4th member in marginal zone and intra-platform,54%and 68%to the 2th member in the marginal zone and intra-platform respectively.(3)The effectiveness of the Sinian primary gas-bearing system depends on the gas generating effectiveness of the source kitchen,reservoir and combination of gas accumulation elements.For high-over mature marine source rocks at the Ro of less than 3.5%,besides gas generated from the thermal cracking of liquid hydrocarbon,the kerogen still has some gas generation potential by thermal degradation.In addition,the Sinian microbial dolomite still preserves relatively good-quality reservoirs despite large burial depths,which match well with other basic conditions for gas accumulation in central Sichuan paleo-uplift,increasing the possibility of Sinian primary gas-bearing system.The research results confirm that the Sinian primary gas-bearing system is likely to form large-scale accumulation.

The influence factors of gas-bearing and geological characteristics of Niutitang Formation shale in the southern margin of Xuefeng Mountain ancient uplift: A case of Well Huangdi 1

In order to evaluate the geological characteristics and gas-bearing factors of Niutitang Formation within the Lower Cambrian of northern Guizhou,the Huangping area located at the southern edge of the ancient uplift belt of Xuefeng Mountain was selected as the target area,and Well Huangdi 1 was drilled for the geological survey of shale gas.Through geological background analysis and well logging and laboratory analysis such as organic geochemical test,gas content analysis,isothermal adsorption,and specific surface area experiments on Well Huangdi 1,the results show that the Niutitang Formation is a deep-water shelf,trough-like folds and thrust fault.The thickness of black shale is 119.95 m,of which carbonaceous shale is 89.6 m.The average value of organic carbon content is 3.55%,kerogen vitrinite reflectance value is 2.37% and kerogen type is sapropel-type.The brittle mineral content is 51%(quartz 38%),clay mineral content is 38.3%.The value of porosity and permeability are 0.5%and 0.0014 mD,which the reservoir of the Niutitang Formation belongs to low permeability with characteristics of ultra-low porosity.The gas content is 0.09‒1.31 m^3/t with a high-value area and a second high-value area.By comparing with the geological parameters of adjacent wells in the adjacent area,the accumulation model of“sediment control zone,Ro control zone,structure controlling reservoir”in the study area is proposed.Therefore,deep-water shelf-slope facies,Ro is between high maturity-early stage of overmaturity and well-preserved zones in the Niutitang Formation in this area are favorable direction for the next step of shale gas exploration.

An alternative approach to match field production data from unconventional gas-bearing systems

Nowadays,the unconventional gas-bearing system plays an increasingly important role in energy market.The performances of the current history-matching techniques are not satisfied when applied to such systems.To overcome this shortfall,an alternative approach was developed and applied to investigate production data from an unconventional gas-bearing system.In this approach,the fluid flow curve obtained from the field is the superposition of a series of Gaussian functions.An automatic computing program was developed in the MATLAB,and both gas and water field data collected from a vertical well in the Linxing Block,Ordos Basin,were used to present the data processing technique.In the reservoir study,the automatic computing program was applied to match the production data from a single coal seam,multiple coal seams and multiple vertically stacked reservoirs with favourable fitting results.Compared with previous approaches,the proposed approach yields better results for both gas and water production data and can calculate the contributions from different reservoirs.The start time of the extraction for each gas-containing unit can also be determined.The new approach can be applied to the field data prediction and designation for the well locations and patterns at the reservoir scale.

Evaluation and prediction of earth pressure balance shield performance in complex rock strata:A case study in Dalian,China

This research explores the potential for the evaluation and prediction of earth pressure balance shield performance based on a gray system model.The research focuses on a shield tunnel excavated for Metro Line 2 in Dalian,China.Due to the large error between the initial geological exploration data and real strata,the project construction is extremely difficult.In view of the current situation regarding the project,a quantitative method for evaluating the tunneling efficiency was proposed using cutterhead rotation(R),advance speed(S),total thrust(F)and torque(T).A total of 80 datasets with three input parameters and one output variable(F or T)were collected from this project,and a prediction framework based gray system model was established.Based on the prediction model,five prediction schemes were set up.Through error analysis,the optimal prediction scheme was obtained from the five schemes.The parametric investigation performed indicates that the relationships between F and the three input variables in the gray system model harmonize with the theoretical explanation.The case shows that the shield tunneling performance and efficiency are improved by the tunneling parameter prediction model based on the gray system model.

Numerical modeling of destress blasting for strata separation

Destress blasting(DB)implemented along the perimeter of safety pillars is a special application of destressing in coal longwall mining.The goal is to separate relatively more deformed mined areas from safety pillars,such as shaft pillars or cross-cut pillars,to reduce the transfer of high stresses to the protective pillar.This case study aims to numerically simulate selected destress blasts in the Czech part of the Upper Silesian Coal Basin and examine its impact on stress transfer to the safety pillar area.To separate the area between the protective pillar and the longwall(LW),two fans of five 93-mm blast holes(length of 93e100 m)were drilled from the gate roads into the overburden strata.Each set of blast holes was fired separately in two stages without time delay.The explosive charge(gelatin-type of explosive)of each stage is 3450 kg.The two DB stages were fired when the longwall face was approximately 158 m and 152 m away from the blast.A 3D mine-wide model is built and validated with in situ stress measured with hydrofracturing.Mining and destressing in three 5-m thick coal seams are simulated in the region.Numerical modeling of DB is successfully conducted using a rock fragmentation factor a of 0.05 and a stress reduction/dissipation factor β of 0.95.Buffering of transfer of additional stress from the mining area into the safety pillar is evaluated by comparison of yielding volume before and after DB.It is shown that yielding volume drops after DB by nearly 80%in the area of the destressing panel and near the safety shaft pillar.

Methods of identifying gas-bearing reservoirs by logs and applications of volcanic formations

The authors generalized the methods how to identify and evaluate gas zones using logs,and put forward the methods of crossplots and overlays of porosity logs on the identification of volcanic gas-bearing reservoirs in the northern Songliao Basin with good results.This study provides technical clues in deep formations and offers references for other areas to identify oil and gas layers.

Reservoir characteristics and genetic mechanisms of gas-bearing shales with different laminae and laminae combinations: A case study of Member 1 of the Lower Silurian Longmaxi shale in Sichuan Basin, SW China

Based on thin-section,argon-ion polished large-area imaging and nano-CT scanning data,the reservoir characteristics and genetic mechanisms of the Lower Silurian Longmaxi shale layers with different laminae and laminae combinations in the Sichuan Basin were examined.It is found that the shale has two kinds of laminae,clayey lamina and silty lamina,which are different in single lamina thickness,composition,pore type and structure,plane porosity and pore size distribution.The clayey laminae are about 100μm thick each,over 15%in organic matter content,over 70%in quartz content,and higher in organic pore ratio and plane porosity.They have abundant bedding fractures and organic matter and organic pores connecting with each other to form a network.In contrast,the silty laminae are about 50μm thick each,5%to 15%in organic matter content,over 50%in carbonate content,higher in inorganic pore ratio,undeveloped in bedding fracture,and have organic matter and organic pores disconnected from each other.The formation of mud lamina and silt lamina may be related to the flourish of silicon-rich organisms.The mud lamina is formed during the intermittent period,and silt lamina is formed during the bloom period of silicon-rich organisms.The mud laminae and silt laminae can combine into three types of assemblages:strip-shaped silt,gradating sand-mud and sand-mud thin interlayers.The strip-shaped silt assemblage has the highest porosity and horizontal/vertical permeability ratio,followed by the gradating sand-mud assemblage and sand-mud thin interlayer assemblage.The difference in the content ratio of the mud laminae to silt laminae results in the difference in the horizontal/vertical permeability ratio.