Maceral evolution of lacustrine shale and its effects on the development of organic pores during low mature to high mature stage:A case study from the Qingshankou Formation in northern Songliao Basin,northeast China

Organic matter(OM)hosted pores are crucial for the storage and migration of petroleum in shale reservoirs.Thermal maturity and macerals type are important factors controlling the development of pores therein.In this study,six lacustrine shale samples with different thermal maturities from the first member of the Qingshankou Formation in the Songliao Basin,of which vitrinite reflectance(R_(o))ranging from 0.58% to 1.43%,were selected for a comparative analysis.Scanning electron microscopy(SEM)and reflected light microscopy were combined to investigate the development of organic pores in different macerals during thermal maturation.The results show that alginite and liptodetrinite are the dominant primary macerals,followed by bituminite.Only a few primary organic pores developed in the alginite at the lowest maturity(R_(o)=0.58%).As a result of petroleum generation,oil-prone macerals began to transform to initial-oil solid bitumen at the early oil window(R_(o)=0.73%)and shrinkage cracks were observed.Initial-oil solid bitumen cracked to oil,gas and post-oil bitumen by primary cracking(R_(o)=0.98%).Moreover,solid bitumen(SB)was found to be the dominant OM when R_(o)>0.98%,which indicates that SB is the product of oil-prone macerals transformation.Many secondary bubble pores were observed on SB,which formed by gas release,while devolatilization cracks developed on migrated SB.Additionally,at the late oil window(Ro?1.16%),migrated SB filled the interparticle pore spaces.With further increase in temperature,the liquid oil underwent secondary cracking into pyrobitumen and gas,and spongy pores developed on the pyrobitumen at higher levels of maturity(Ro=1.43%),which formed when pyrobitumen cracked into gas.Vitrinite and inertinite are stable without any visible pores over the range of maturities,verifying their low petroleum generation potential.In addition,it was concluded that clay minerals could have a catalytic effect on the petroleum generation,which may explain why organicclay mixtures had more abundant pores than single OM particles.However,after R_(o)>0.98%,authigenic minerals occupied the organic pore spaces on the organic-clay mixtures,resulting in fewer pores compared to those observed in samples at the early to peak oil window.

Thermal simulation experiment of organic matter-rich shale and implication for organic pore formation and evolution

A thermal simulation experiment of diagenesis,hydrocarbon generation and evolution of the organic matter-rich shale was carried out to investigate formation and evolution of organic pores under the constraint from immature,low mature,mature,high mature to overmature geological conditions.The argon ion polishingefield emission scanning electron microscope was used to analyze microscopic features of original samples and simulated samples of various evolution stages.Results showed organic pores could be formed during hydrocarbon generation from biochemical and hypothermal processes in the immature and low mature stages,and the shale shallow-buried depth might be favorable for preservation of organic pores;the generation and evolution of organic pores were of heterogeneity,and the maturity was not a decisive factor which controlled formation and development of organic pores,while the difference in physiochemical structure of organic matter played an important role in formation and evolution of organic pores;the organic pore development was obviously related with the retained oil,and the organic pores formed in the oil generation stage were easily filled by pyrolysis asphalt;organic contraction fractures/organic marginal pores might be important storage spaces for shale gas occurrence,and their development was mainly controlled by the physiochemical structure and evolution degree of organic matters when the chemical adsorbed organic matter was converted into the physical adsorbed organic matter and the free organic matter.

Controlling Factors of Organic Nanopore Development: A Case Study on Marine Shale in the Middle and Upper Yangtze Region, South China

The Upper Ordovician Wufeng-Lower Silurian Longmaxi and the Lower Cambrian Qiongzhusi shales are the major targets for shale gas exploration and development in China.Although the two organic-rich shales share similar distribution ranges and thicknesses,they exhibit substantially different exploration and development results.This work analyzed the nanopore structures of the shale reservoirs in this region.Pore development of 51 shale samples collected from various formations and locations was compared using the petromineralogical,geochemical,structural geological and reservoir geological methods.The results indicate that the reservoir space in these shales is dominated by organic pores and the total pore volume of micropores,mesopores,macropores in different tectonic areas and formations show different trends with the increase of TOC.It is suggested that organic pores of shale can be well preserved in areas with simple structure and suitable preservation conditions,and the shale with smaller maximum ancient burial depth and later hydrocarbongeneration-end-time is also more conducive to pore preservation.Organic pore evolution models are established,and they are as follows:①Organic matter pore development stage,②Early stage of organic matter pore destruction,and③late stage of organic matter pore destruction.The areas conducive to pore development are favorable for shale gas development.Research results can effectively guide the optimization and evaluation of favorable areas of shale gas.

Dynamic evolution characteristics of the “source-reservoir” integration of gray marl and its geological significance to unconventional gas: Insights from pyrolysis experiments

The marl–limestone rhythmic strata of the Permian Maokou Formation have been identified as hosts of unconventional gas reservoirs with “source–reservoir” integration. The lack of research on the pore structure evolution of organic-rich carbonate rock restricts gas exploration of these strata. Here, pyrolysis experiments were performed on the Mao-1 carbonate to simulate hydrocarbon generation, expulsion and diagenesis in gray marl from low maturity to overmaturity. The pore structure of this marl is dominated by mesopores and macropores, and the proportion of macropores increases gradually with temperature. The macropores are mainly pores in the organic matter and shrinkage microcracks. Additionally, micropores and mesopores, dominated by clay mineral interlayer pores and pyrite intergranular pores, are developed in the high mature stage and subsequently compressed in the overmature stage. The main contributors to the specific surface area are micropores and mesopores, which are conducive to natural gas adsorption. After the same pyrolysis treatment, the available porosity of grey marl is higher than that of marine/lacustrine shales, and exhibits an obvious decrease in the low mature–mature stage. These suggest that the abundant residual oil generated blocked the organic and inorganic pores in the gray marl, providing a pivotal material foundation for the gas generation. Micropores and mesopores developed during the high mature stage ensure the gas accumulation and preservation. The above indicate the organic-rich carbonate at the high mature–overmature stage (Ro = 1.7%–2.5%) in the Sichuan Basin may be a favorable exploration horizon for unconventional oil and gas.

Pore characteristics and formation mechanism of high-maturity organic-rich shale in Lower Cambrian Jiumenchong Formation,southern Guizhou

In order to investigate pore characteristics and formation mechanism in the high-maturity organic-rich shale of Lower Cambrian Jiumenchong Formation in southern Guizhou,the pore structure,pore type and storage properties are well studied through the rock thin section,total rock X-ray diffraction,lowtemperature nitrogen adsorption,high pressure mercury injection-adsorption test,helium porosity test,argon ion polishing-scanning electron microscope,thermal evolution and pore evolution history reconstruction,and based on the diagenesis and compaction as well as thermal evolution process,a microscopic pore formation and evolution model of high-maturity organic-rich shale are established.The result shows that the high-maturity organic-rich shale of Jiumenchong Formation has the average total specific surface area of 12.66m^(2)/g and the total pore volume of 11.54×10^(-3)cm^(3)/g,and the total specific surface area have a positive correlation with total pore volume;the total specific surface area and the total pore volume are slightly lower compared with the Lower Silurian shale.The pores are dominated by micropores and mesopores,while macropores are very rare.The pores of the organic-rich shale mainly are organic pores with small diameter usually less than 30 nm,and the pore boundary form is irregular,the inorganic mineral pores are not developed.Compared with the Silurian shale,the shale has poorer reservoir property,the average porosity is only 2.80%;the horizontal permeability is 1e3 times of the vertical permeability,indicating the horizontal lamellations are not developed.The formation and evolution of pores in high-maturity organic-rich shale is jointly influenced by the evolutionary process of intergranular pores of inorganic mineral under the control of the diagenesis and compaction,the organic pore formation process in the hydrocarbon generation-oil formation-oil and gas transformation sequence under the control of thermal evolution,and the natural gas loss-supply equilibration process under the condition of later pore preservation.

Pore evolution characteristic of shale in the Longmaxi Formation,Sichuan Basin

Through the field emission scanning electronic microscope(FESEM)and the nitrogen adsorption test,pore type and structure of shale reservoir in the Longmaxi Formation in the Sichuan Basin were well studied.Result showed that the pore type includes organic pore,intercrystalline pore,dissolution intracrystalline pore and interparticle pore,and the organic pore was one of major pore types;among the organic pore,the micropore had large pore volume and specific surface area,and was the main storage space of shale gas.Through study on effect of total organic carbon(TOC),organic matter maturity(Ro),diagenesis and tectonism on shale porosity,influence of TOC on porosity could be divided into four stages:the rapid increasing stage(TOC from 0 to 2%),the slow decreasing stage(TOC from 2 to 3%),the rapid increasing stage(TOC from 3 to 4%or 6%)and the rapid decreasing stage(TOC>4%or 6%);influence of the maturity on porosity of shale could be divided into three stages:the rapid decreasing stage(Ro from 1.5 to 2.2%),the rapid increasing stage(Ro from 2.2 to 2.7%)and the rapid decreasing stage(Ro>2.7%);during the high thermal evolution stage,the organic diagenesis was stronger than the inorganic diagenesis;the tectonism had a great impact on porosity,and the more intense the tectonism was,the smaller the porosity would be.The evolution of shale porosity of the Longmaxi Formation underwent five stages:the immature rapid compaction stage(Ro<0.7%),the mature hydrocarbon generation and dissolution stage(Ro from 0.7 to 1.3%),the high mature pore closed stage(Ro from 1.3 to 2.2%),the overmature secondary pyrolysis stage(Ro from 2.2 to 2.7%)and the overmature slow compaction stage(Ro>2.7%);among which the mature hydrocarbon generation and dissolution stage and the overmature secondary pyrolysis stage were the most favorable shale pore development stages。

High-performance organic electrolyte supercapacitors based on intrinsically powdery carbon aerogels

A novel class of powdery carbon aerogels（PCAs） has been developed by the union of microemulsion polymerization and hypercrosslinking, followed by carbonization. The resulting aerogels are in a microscale powdery form, demonstrate a well-defined 3D interconnected nanonetwork with hierarchical pores derived from numerous interstitial nanopores and intraparticle micropores, and exhibit high surface area（up to 1969 m^2/g）. Benefiting from these structural features, PCAs show impressive capacitive performances when utilized as electrodes for organic electrolyte supercapacitors,including large capacitances of up to 152 F/g, high energy densities of 37-15 Wh/kg at power densities of 34–6750 W/kg, and robust cycling stability.

Gas in place and its controlling factors of deep shale of the Wufeng–Longmaxi Formations in the Dingshan area, Sichuan Basin

Recently, deeply-buried shale (depth > 3500 m) has become an attractive target for shale gas exploration and development in China. Gas-in-place (GIP) is critical to shale gas evaluation, but the GIP content of deep shale and its controlling factors have rarely been investigated. To clarify this issue, an integrated investigation of deep gas shale (3740–3820 m depth) of the Lower Paleozoic Wufeng–Longmaxi Formations (WF–LMX) in the Dingshan area, Sichuan Basin had been carried out. Our results show that the GIP content of the studied WF–LMX shale in the Dingshan area ranges from 0.85 to 12.7 m^(3)/t, with an average of 3.5 m^(3)/t. Various types of pores, including organic matter (OM) pore and inorganic pore, are widely developed in the deep shale, with total porosity of 2.2 to 7.3% (average = 4.5%). The OM pore and clay-hosted pore are the dominant pore types of siliceous shale and clay-rich shale, respectively. Authigenic quartz plays a critical role in the protection of organic pores in organic-rich shales from compaction. The TOC content controls the porosity of shale samples, which is the major factor controlling the GIP content of the deep shale. Clay minerals generally play a negative role in the GIP content. In the Sichuan Basin, the deep and ultra-deep WF–LMX shales display the relatively high porosity and GIP contents probably due to the widespread of organic pores and better preservation, revealing great potentials of deep and ultra-deep shale gas. From the perspective of rock mechanical properties, deep shale is the favorable exploration target in the Sichuan Basin at present. However, ultra-deep shale is also a potential exploration target although there remain great challenges.

Reservoir characteristics and controlling factor of shale gas in Lower Cambrian Niutitang Formation, South China

Large-scale exploration and development of shale gas in Lower Cambrian Niutitang Formation of South China has been carried out in recent years,but the result is not good,only except some drilling wells in Jingyan-Qianwei area of Sichuan Basin and Yichang of Hubei obtain some commercial gas flows.In order to clarify reasons for failure of shale gas exploration and development in Niutitang Formation around Sichuan Basin and to provide reservoir geological parameters for subsequent efficient exploration and development,taking a case of shale gas reservoirs in Lower Cambrian Niutitang Formation in northeast Chongqing around Sichuan Basin,some experimental methods,such as analysis of organic carbon pyrolysis,determination of equivalent vitrinite reflectance,focused ion beam scanning electron microscope(FIB-SEM)and other are adopted in this study.The results show that the average TOC of shale samples in Niutitang Formation in northeast Chongqing is 3.1%,the equivalent vitrinite reflectance ranges from 3.0%to 4.0%,and the degree of thermal evolution reaches the post-matureemetamorphic stage.Due to excessively high degree of the thermal evolution,organic pores in shale samples are not developed in Lower Cambrian Niutitang Formation in the study area,instead,micro-nano pores dominated by intergranular pores and intragranular pores are developed in the shale.The degree of the thermal evolution controls the sustaining gas generation of kerogen and retained liquid hydrocarbons in the shale,it also controls the development of organic pores of the shale.The evolution of organic pores and hydrocarbon generation in the shale of Niutitang Formation in northeast Chongqing around Sichuan Basin do not match best with each other,that is,during development period of a large number of organic pores,thermal evolution degree of reservoirs is further enhanced because the strata are not uplifted in time,therefore,the quantity of organic pores is decreased sharply,the shale gas would be escaped due to the absence of organic pores as effective storage space after the shale gas generation.Therefore,the efficient exploration and development of the shale gas in Lower Cambrian Niutitang Formation in South China should be focused on the shale development area where the shale is characterized by moderate thermal evolution degree(2.0%<R_(o)<3.0%)and shallow buried depth,that is,the shale distribution area with paleo-uplift or paleo-buried hill.