Influence of structural damage on evaluation of microscopic pore structure in marine continental transitional shale of the Southern North China Basin:A method based on the low-temperature N2 adsorption experiment

Structural damage from sample preparation processes such as cutting and polishing may change the pore structure of rocks.However,changes in pore structure caused by this structural damage from crushing and its effect on marine continental transitional shale have not been well documented.The changes of microscopic pore structure in marine continental transitional shale during the sample preparation have important research value for subsequent exploration and development of shale gas.In this study,the pore structures of transitional shale samples from the Shanxi-Taiyuan Formation of the Southern North China Basin under different degrees of damage were analyzed through low-temperature N;adsorption experiments,combined with X-ray diffraction,total organic carbon,vitrinite reflectance analysis,and scanning electron microscopy.The results showed that(1)With increasing structural damage,the specific surface area(SSA)changed within relatively tight bounds,while the pore volume(PV)varied significantly,and the growth rate(maximum)exhibited a certain critical value with the crushing mesh number increasing from 20 to 200.(2)The ratio of SSA to PV can be used as a potential proxy for evaluating the influence of changes in the pore structure.(3)Correlation analysis revealed that the microscopic pore structure of marine continental transitional shale from the Shanxi-Taiyuan Formations is mainly controlled by organic matter and clay minerals.Clay minerals play a leading role in the development of microscopic pores and changes in pore structure.

Full-Sized Pore Structure and Fractal Characteristics of Marine-Continental Transitional Shale: A Case Study in Qinshui Basin, North China

Based on 10 shale samples collected from 4 wells in Qinshui Basin,we investigate the full-sized pore structure and fractal characteristics of Marine-Continental transitional shale by performing organic geochemistry,mineralogical composition,Nitrogen gas adsorption(N2 adsorption)and Nuclear Magnetic Resonance(NMR)measurements and fractal analysis.Results show that the TOC content of the shale samples is relatively high,with an average value of 2.44wt%,and the thermal evolution is during the mature-over mature stage.The NMR T2 spectrum can be used to characterize the fullsized pore structure characteristics of shale.By combining N2 adsorption pore structure parameters and NMR T2 spectrums,the surface relaxivity of samples are calculated to be between 1.7877 um/s and 5.2272 um/s.On this basis,the T2 spectrums are converted to full-sized pore volume and surface area distribution curves.The statistics show that the pore volume is mainly provided by mesopore,followed by micropore,and the average percentages are 65.04%and 30.83%respectively;the surface area is mainly provided by micropore,followed by mesopore,and the average percentages are 60.8004%and 39.137%respectively;macropore contributes little to pore volume and surface area.The pore structure characteristics of shale have no relationship with TOC,but strong relationships with clay minerals content.NMR fractal dimensions Dmicro and Dmeso have strong positive relationships with the N2 adsorption fractal dimensions D1 and D2 respectively,indicating that Dmicro can be used to characterize the fractal characteristics of pore surface,and Dmeso can be used to characterize the fractal characteristics of pore structure.The shale surface relaxivity is controlled by multiple factors.The increasing of clay mineral content,pore surface area,pore surface fractal dimension and the decreasing of average pore size,will all lead to the decreasing of shale surface relaxivity.

Pore-scale mechanisms and characterization of light oil storage in shale nanopores:New method and insights

A new method is proposed to analyze the pore-scale mechanisms and characterization of light oil storage in shale nanopores,which is based on the Hydrocarbon Vapor Adsorption(HVA)and Pore Calculation Model(PCM).First,the basic principle of the HVA-PCM method is introduced,and the experimental/mathematical analysis processes are given.Then,the HVA-PCM method is applied to shale samples to analyze the mechanisms and characterization of light oil storage in shale nanopores.The results provide insights into the pore-scale oil storage mechanisms,oil storage structure,oil film thickness,oil distribution within different sized pores,and the oil storage state.Finally,the advantages and limitations of the HVA-PCM method are discussed,and suggestions for further improvement are proposed.Overall,the HVA-PCM method is a powerful tool for extracting quantitative information on the light oil storage in shale nanopores.

Microbial Geochemical Characteristics of the Coalbed Methane in the Shizhuangnan Block of Qinshui Basin, North China and their Geological Implications

Methanogens and sulfate reducing bacteria were detected by the 16SrRNA sequencing of coalbed methane(CBM)co-produced water in the south of the Qinshui Basin,which is indicative of the presence of secondary biological gas in the south of this basin,in contradiction to the previous understanding of thermogenic gas.This work systematically collected water samples from the CBM wells in the Shizhuangnan Block and analyzed the microbial geochemical characteristics from the aspects of water ions,hydrogen and oxygen isotopes,dissolved inorganic carbon and microbial diversity.It is shown that the Shizhuangnan Block has a nearly SN-trending monoclinic structure,and the elevation of coal seam decreases gradually from the east to west.Because of the water blocking effect of Sitou fault in the west,the precipitation flowed from the east to west,and gradually transited to stagnant flow area.The concentration variation of some ions such as Na^+,K^+,Ca2^+,Mg2^+,Cl^-,HCO3^-and total dissolved solids(TDS)suggest the variation of redox condition in the coal reservoir water.The 16SrDNA sequencing analysis of the collected water samples detected the presence of methanogens and sulfate reduction bacteria.The presence of methane production zone and sulfate methane transition zone(SMTZ)was identified.The effect of methanogens in the methane production zone leads to an increase in the methane concentration,resulting in a high gas content in the study area.In the SMTZ,most methane is consumed by anaerobic oxidation due to high sulfate concentrations.

Study on the relationship between hydrocarbon generation and pore evolution in continental shale from the Ordos Basin,China

The relationship between hydrocarbon generation and the evolution of shale pore structure and its heterogeneity of continental shale from the Ordos Basin,China was quantitatively studied based on thermal simulation experiment,mercury injection capillary pressure(MICP),gas adsorption,vitrinite reflectance(R_(o))analysis,and hydrocarbon generation test combined with Frenkel-Halsey-Hill(FHH)fractal model.The result shows that the pore volume(PV)and specific surface area(SSA)of pores with different pore sizes show a trend of decreasing first and then increasing as the maturity increases in general,R_(o)>1.59%is initially defined as a favorable stage for pore development in continental shale.Hydrocarbon generation has different effects on pore heterogeneity of different scales.For the N_(2)adsorption,the roughness of small pore surface(D_(1))decreases in the oil window;the complexity of large pore structure(D_(2))increases in the oil window but decreases in the gas window.For the MICP,the heterogeneity of small pore(D_(1))increases in the oil window and increases first and then decreases in the gas window;D_(2)remains basically constant during the whole stage and is close to 3,indicating that the heterogeneity of large pores is extremely strong and is not affected by hydrocarbon generation.

Geochemical Characteristics and Development Significances of Constant and Trace Elements from Coalbed Methane Co-Produced Water:A Case Study of the Shizhuangnan Block,the Southern Qinshui Basin

As an unconventional natural gas resource,coalbed methane(CBM)development releases a large amount of CBM wells co-produced water.Geochemical characteristics of the co-produced water provide an essential foundation for the production dynamics of CBM reservoirs if the impacts of fracturing fluids and other aquifers can be ignored.In the Shizhuangnan Block of the southern Qinshui Basin,constant and trace elements in CBM co-produced water from the wellheads were collected and determined,which is applied to assess water source,fracturing fluid effect,and CBM production.Based on principle component analysis and hierarchical clustering analysis,the water samples are divided into four categories.It suggests that different characteristics affected by water-rock interaction,reservoir environment,aquifer recharge,and hydraulic fracturing result in the various ratios of Na^(+)/Cl^(-),alkalinity(HCO_(3)^(-)+CO_(3)^(2-))/Cl^(-)and other specific rules.Moreover,Cl^(-)is selected as a dividing line for complete fracturing fluid flow back,associated with organic-bound chlorine complexes in the original coal seam water.Compared to constant elements,there is a significant correlation between Li and Sr concentrations and CBM productivity,so templates regarding trace elements can be used to distinguish various sources of the co-produced water.

Research status of lacustrine mudrock deposition constrained from astronomical forcing

The sources,transportation and depositional processes of lacustrine mudrock are still poorly understood.Existing studies have demonstrated the controlling effect of astronomical forcing on lacustrine mudrock deposition,but its depositional mechanism and evolution are still not systematically investigated.Most research related to astronomical forcing exclusively highlights the sedimentation of carbonate rocks in deep-water lacustrine setting,with insufficient attention paid to the thick organic-rich,deep-lake mudrock.With the increasing interest in exploration and development of shale oil and gas accumulations,it is urgent to deeply understand depositional rules of lacustrine mudrock.This study reviews sediment sources,depositional mechanism and evolution process of mudrock through expounding the correlations between the periodic changes of astronomical forces,the parameters of Earth orbital and mudrock compositions.By investigating the existing literature and using some actual data of Jiyang Depression,Bohai Bay Basin in East China,this study expounds on the influence of astronomical cycles on the deposition of lacustrine mudrock.Moreover,efforts are made to analyze the effects of various orbital parameters(e.g.,precession,obliquity,and eccentricity with the periods ranging from tens of thousands years to million years)on the deposition of mudrock from small-scale(decimeters to meters)to large-scale(10s to 100s meters).Further,it is feasible to apply the high-precision isochronous stratigraphic correlation into clarifying the distribution of favorable shale oil and gas reservoirs.To conclude,this study enunciates the sedimentation of mudrock from a new perspective(astronomical forcing)and provides a direction for the research on sedimentation of fine-grained sedimentary rocks.

Quartz types,genesis and their geological significance within the Wufeng-Longmaxi Formation in north-western Hunan,China

Quartz is an important mineral component in the Late Ordovician–Early Silurian Wufeng-Longmaxi Formation with various forms and sources and has a significant impact on the properties of shale gas reservoirs.In this study,geochemical analysis,scanning electron microscopy(SEM)observation,and rock mechanics testing were performed on shale samples from the Wufeng-Longmaxi Formation in north-western Hunan,South China.Quartz is classified into four types based on morphological features and cathodoluminescence(CL)images under SEM–terrigenous detrital quartz,quartz overgrowths,biogenic skeletal quartz and microquartz.The quartz in Upper Longmaxi Formation is predominantly of terrigenous origin and contains a small amount of quartz formed by clay transformation.The quartz in the Wufeng-Lower Longmaxi Formation is predominantly biogenic.The biogenic quartz has a direct effect on organic matter(OM)abundance,pore structure and brittleness.It is indicated by the positive correlation with TOC content and biogenic Ba content that biogenic quartz-rich strata have high paleoproductivity.The rigid frameworks formed by biogenic quartz during the early diagenesis stage facilitated the preservation of the primary pores.The interparticle pores of biogenic quartz are the space for OM preservation and migration,creating conditions for the development of OM pores.Additionally,the calculated brittleness index(BI)shows a positive correlation with biogenic quartz content,indicating that layers rich in biogenic quartz are more conducive to fracture.Therefore,the Wufeng-Lower Longmaxi Formation has higher OM content,porosity and represents a more favorable exploration and development target.

Diagenetic Sequence and Genetic Mechanism of Silurian Tight Sandstone Reservoirs in the Eastern Tarim Basin, Northwest China

The Silurian stratigraphic sequence has recently become one of the most important exploration targets in the Tarim Basin, with a considerable amount of profitable hydrocarbon pools discovered in the central Tarim Basin. Previous exploration activities indicate that the Silurian stratigraphic sequence in the eastern Tarim Basin has great hydrocarbon exploration potential. The Silurian reservoirs comprise a set of tight marine sandstones, whose diagenetic sequence and genetic mechanism are still poorly understood. The complex relationship of hydrocarbon generation, the timing of the peak expulsion of the source rocks and the evolution of the reservoirs remains unclear. An integrated description and analysis have been carried out on core samples from eleven wells selected from the eastern Tarim Basin. A range of petrographic and geochemical analyses were conducted. By using an integrated approach with thin-section petrography, scanning electron microscopy（SEM）, cathodoluminescence（CL）, carbon and oxygen isotope geochemistry, formation water analysis, X-ray diffractometry（XRD）, electron probe microanalysis and fluid inclusion microthermometry, the genesis and occurrence of individual diagenetic events were documented to reconstruct the diagenetic sequence and diagenetic model for the Silurian sandstone. Additionally, the tight nature of the Silurian reservoirs can mainly be attributed to the compaction processes and cementation. In particular, the destructiveness of the compactional processes to the original porosity is far greater than that from the cementation. Furthermore, fluid inclusion analyses also indicate that the Silurian sandstone has experienced three phases of hydrocarbon charge. The first two phases occurred during the eodiagenesis stage（from the Late Silurian to the Early Devonian and from the Late Carboniferous to the end of the Late Permian）, when the Silurian sandstone was not tight and had a porosity of greater than 20%. The third phase occurred during the stage B of mesodiagenesis（since the Late Cretaceous）, when the Silurian sandstone was fully tight.

A review on microbial metabolism to increase coalbed methane generation and coal pretreatment to improve its bioavailability

Abundant unminable coal in deep strata and abandoned mines are also precious sources of clean gas energy,under which biotransformation is a potential path.In recent years,substantial progress has been made in laboratory research on coal degradation to produce methane by microbial metabolism.This paper systematically reviews the research progress of microbial enhancement and microbial stimulation of coal,physicochemical pretreatments of coal,and environmental factors affecting coal biotransformation.The research idea of coal biotransformation should aim at field production increase and gradually clarify the microbial mechanism of coal degradation and the regional distribution and functional composition of microbial communities on the block scale.The research on coal biotransformation helps improve the development level of coalbed methane and the sustainable development of unconventional natural gas resources.