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.

Reservoir Characteristics of Lacustrine Shale and Marine Shale:Examples from the Songliao Basin,Bohai Bay Basin and Qiannan Depression

Lacustrine shale from the Qingshankou Formatin of Songliao basin and the Shahejie Formation of Bohai Bay basin, and marine shale from the lower Cambrian Jinmenchong Formation of Qiannan depression were analysed by using rock pyrolysis, TOC （total organic carbon）, XRD （X-ray diffraction）, SEM （scanning electron microscope）, FE-SEM （field emission scanning electron microscope）, high pressure mercury intrusion, and low pressure N2 and CO2 gas adsorption experiments, in aim to reveal their reservoir features. The results show that： （1） the width of micro-pores of all the studied samples mainly ranges from 0.45 to 0.7 nm indicated by CO2 isotherms, and the width of meso-pores is less than 10 nm, with type IV adsorption isotherms and type H2 hysteresis loop, indicative of ＂ink-bottle＂-shaped pores. Good correlations exist among pore volume, surface area and averaged pore diameter, and a good positive correlation exists between micro-pore volume and TOC content; however, there is no obvious correlation between meso-pore volume and TOC content; （2） interparticle pores, pores among the edge of mineral grains and organic matter pores were all identified in marine and lacustrine shale, among which the interparticle pores may be influence by dissolution effect. Not all bitumen develops organic matter pore, and only high to over mature bitumen present pores. Now the description methods of micrometer scale pores developed in shale are very lack. Micro- fractures developed in Jiyang depression and dissolution interparticle pores developed in Songliao Basin should be the accumulation sites for shale oil in lacustrine shale, and can be as sweet spots.

The Influence of Extractable Organic Matter on Pore Development in the Late Triassic Chang 7 Lacustrine Shales, Yanchang Formation, Ordos Basin, China

To investigate the influence of extractable organic matter （EOM） on pore evolution of lacustrine shales, Soxhlet extraction, using dichloromethane, was performed on a series of Chang 7 shale samples （Ordos Basin, China） with vitrinite reflectance of 0.64% to 1.34%. Low-pressure gas adsorption experiments were conducted on the samples before and after extraction. The pore structure parameters were calculated from the gas adsorption data. The results show complex changes to the pore volumes and surface areas after extraction. The pore development of both the initial and extracted samples is strongly controlled by total organic carbon （TOC） content. Micropores developed mainly in organic matter （OM）, while mesopores and macropores predominantly developed in fractions other than OM. The influence of EOM on micropores is stronger than on mesopores and macropores. Organic solvents with a higher boiling point should be used to explore the effect of EOM on pore structure in the future.

Nanoscale pore morphology and distribution of lacustrine shale reservoirs:Examples from the Upper Triassic Yanchang Formation,Ordos Basin

Pore structure plays an important role in the gas storage and flow capacity of shale gas reservoirs. Fieldemission environmental scanning electron microscopy（FE-SEM） in combination with low-pressure carbon dioxide gas adsorption（CO2GA）,nitrogen gas adsorption（N2GA）,and high-pressure mercury intrusion（HPMI） were used to study the nanostructure pore morphology and pore-size distributions（PSDs） of lacustrine shale from the Upper Triassic Yanchang Formation,Ordos Basin. Results show that the pores in the shale reservoirs are generally nanoscale and can be classified into four types： organic,interparticle,intraparticle,and microfracture. The interparticle pores between clay particles and organic-matter pores develop most often,l with pore sizes that vary from several to more than 100 nm. Mercury porosimetry analysis shows total porosities ranging between 1.93 and 7.68%,with a mean value of 5.27%. The BET surface areas as determined by N2 adsorption in the nine samples range from 10 to 20 m2/g and the CO2 equivalent surface areas（2 nm）vary from 18 to 71 m2/g. Together,the HPMI,N2 GA,and CO2 GA curves indicate that the pore volumes are mainly due to pores 100 nm in size. In contrast,however,most of the specific surface areas are provided by the micropores. The total organic carbon（TOC） and clay minerals are the primary controls of the structures of nanoscale pores（especially micropores and mesopores）. Micropores are predominantly determined by the content of the TOC,and mesopores are possibly related to the content of clay minerals,particularly the illite-montmorillonite mixed-layer content.

Several issues worthy of attention in current lacustrine shale oil exploration and development

Based on the current research status of shale oil exploration and development at home and abroad,through field observations,dissection of typical shale oil regions,analysis and testing of organic-rich shale samples,etc.,we compare the differences in geological and engineering characteristics of shale oil reservoirs in marine and continental basins in China and the United States,put forward several issues worthy of attention in the exploration and development of lacustrine shale oil in typical basins of China,including the concept of tight oil and shale oil,vertical permeability and horizontal permeability,differences between continental and marine shale oil reservoirs,medium-low maturity and medium-high maturity,source-reservoir and source-caprock,geology and engineering,selection criteria of favorable areas and“sweet spots”,basic scientific research and application research.By comparing and analyzing organic-rich shales in the Triassic Yanchang Formation of the Ordos Basin,the Permian Lucaogou Formation in the Jimsar Sag of the Junggar Basin,the Permian Fengcheng Formation in the Mahu Sag,the Cretaceous Qingshankou&Nenjiang Formation in the Songliao Basin and the Paleogene Kongdian&Shahejie Formation in the Bohai Bay Basin,we believe that three key scientific issues must be studied in-depth from shale oil exploration to development in the future:(1)the physical,chemical and biological processes during the deposition of terrestrial fine-grained sediments and the formation mechanism of terrestrial organic-rich shale;(2)the dynamic evolution of diagenesis-hydrocarbon generation-reservoir formation,and the mechanisms of hydrocarbon formation and accumulation;(3)the fracturing mechanisms of terrestrial shale layers in different diagenetic stages and the multi-phase and multi-scale flow mechanism of shale oil in shale layers of different maturities.In addition,we should clarify the main controlling factors of shale oil reservoir characterization,oil-bearing properties,compressibility and fluidity of shale oil with different maturities,establish a lacustrine shale oil enrichment model and the evaluation methodology,to provide effective development methods,and ultimately to establish theoretical foundation and technical support for the large scale economical exploration and development of lacustrine shale oil resources in China.

Pore Characteristics and Factors Controlling Lacustrine Shales from the Upper Cretaceous Qingshankou Formation of the Songliao Basin,Northeast China:A Study Combining SEM,Low-temperature Gas Adsorption and MICP Experiments

To investigate pore characteristics and the factors controlling lacustrine shales,geochemical,mineralogical and petrophysical experiments were performed on 23 shale samples from the Qingshankou Formation of the Songliao Basin,China.A comparison of mercury injection capillary pressure(MICP)and low-temperature N2 adsorption pore-size distribution showed that MICP has a higher pore-size distribution(PSD)line in its overlapping pore diameter range,which may be elevated by the higher pressure of MICP.Therefore,in the overlapping range,low-temperature N2 adsorption data were preferred in pore characterization.Negative correlations were observed between pore volumes and TOC content,indicating organic matter pores are not well-developed in the studied samples.This may be related to their low grade of maturity and type I kerogens.There existed negative relationships between pore volumes and S1,which illustrated that liquid hydrocarbons occupied some pore space.Micropore volume had a better correlation with S1 than mesopore and macropore volumes,which suggests that liquid hydrocarbons preferentially occur in micropores.No obvious relationships between pore volumes and quartz or feldspar were observed,while pore volumes increased with the increasing clay mineral content.These relationships indicate that intraparticle pores in clay minerals represent the principal pore type.

Characterizing the Micropores in Lacustrine Shales of the Late Cretaceous Qingshankou Formation of Southern Songliao Basin, NE China

Micropores of shale are significant to the gas content and production potential of shale, which has been verified in the research of marine shale gas; while, few studies have been conducted on lacustrine shales. This study collected 42 samples from three wells in the Late Cretaceous Qingshankou Formation of the southern Songliao Basin, NE China, and investigated these samples by the focused ion beam-scanning electron microscope(FIB–SEM) and nitrogen adsorption analysis techniques. Four types of micropores were identified in the samples, i.e., intergranular pore, intracellular pore, organic matter pore and microfracture. The pore structure type is characterized by open slit pores and "ink type" pores which are mainly 1.5–5 nm in diameter with mesopores as the main pores. The mesopores account for 74.01% of the pore volume and 54.68% of the pore surface area. Compared with the lacustrine shales from the Triassic Yanchang Formation in the Ordos Basin and Xujiahe Formation in the Sichuan Basin, the intergranular clay mineral interlayer pores are considered to be the main reservoir space for shale gas storage in the study area, followed by intraparticle pores, organic matter pores and microfractures. Maturity and micropore are the key controlling factors which affect the shale gas content of the Qingshankou Formation in southern Songliao Basin.

Formation conditions and enrichment model of retained petroleum in lacustrine shale: A case study of the Paleogene in Huanghua depression, Bohai Bay Basin, China

Compared with marine facies shale strata,lacustrine shale strata are more complicated in geological conditions,and thus more difficult to explore and develop.To realize economic exploration and development of lacustrine shale oil,the geological regularities of accumulation and high yield of retained movable petroleum in shale should be understood first.In this work,taking the shale strata of Kong 2 Member and Sha 3 Member in the Paleogene of Huanghua depression in the Bohai Bay Basin as examples,based on the previous joint analysis results of over ten thousand core samples and the latest oil testing,production test and geochemical data of more than 30 horizontal wells,accumulation conditions and models of retained movable petroleum in lacustrine shale were studied comprehensively.The study shows that at moderate organic matter abundance(with TOC from 2%to 4%),shale strata have the best match between oil content and brittleness,and thus are rich in oil and good in fracability.Moderate ancient lake basin size and moderate sediment supply intensity are the internal factors leading to best coupling of organic matter abundance and brittle mineral content in the shale formation.Moderate thermal evolution maturity of Ro of 0.7%–1.0%(at burial depth of 3200 to 4300 m)is the interval where oil generation from thermal evolution and oil adsorption by kerogen in shale layers match best,and retained movable petroleum is high in proportion.Moderate diagenetic evolution stage(3200 to 4300 m in the middle diagenetic stage A)is conducive to the formation of a large number of dissolved pores and organic matter pores,which provide storage space for shale oil enrichment.Moderate development degree of natural fractures(without damaging the shale oil roof and floor sealing conditions)is conducive to the storage,seepage and preservation of shale oil.The research results have overthrown the general understanding that high organic matter abundance,high maturity,and high development degree of natural fractures are conducive to shale oil enrichment,and have guided the comprehensive evaluation of shale oil and gas sweet spots and well deployment in the second member of the Kongdian Formation in the Cangdong sag and the Shahejie Formation in the Qikou sag.Industrial development of the shale oil in Kong 2 Member of the Cangdong sag has made major breakthrough,and important signs of shale oil have been found in Sha 3 Member of the Qikou sag,demonstrating huge exploration potential of lacustrine shale oil.

Exploration breakthrough and its significance of Gulong lacustrine shale oil in the Songliao Basin,Northeastern China

Lacustrine shale oil resources are abundant in many petroliferous basins in China.The shale oil formations are characterized by low API gravity,high viscosity,poor mobility,high clay content,low brittleness and etc.Exploration of lacustrine shale oil started relatively late in China,but its progress is very rapid and breakthroughs have been made successively.In this short communication,we introduced the most significant shale oil discovery which is made recently in the Qingshankou Formation of the Gulong sag in the Songliao Basin.Key exploratory wells including Guye 1H(GY1H),Yingye 1H(YY1H)and Guye 2HC(GY2HC)tested stable and high oil flow in shale reservoirs,revealing the relatively stable and high oil production capacity of shale in the Gulong sag of Daqing Oilfield.It marks a leap of petroleum theoretical recognition from lacustrine shale generating oil to producing oil and from the theory of traditional oil migration and accumulation to the theory of indigenous oil accumulation in organic-rich shale.Although lacustrine shale oil exploration and development still faces many challenges in China,its prospect is quite promising.

Quantitative Gas-in-place Comparison of Original and Bitumen-free Lacustrine Shale

Residual bitumen in organic-rich shale of oil windows exists widely, and its effect on the gas storage capacity of shale could be two-fold. Bitumen could occupy and block the nanopores of shale, thereby reducing the gas storage capacity. On the other hand, gas could be dissolved in bitumen in shale gas reservoirs, leading to enhanced gas storage capacity. To quantify the effect of bitumen on the gas-in-place(GIP) estimation of lacustrine organic-rich shale, the micropore characteristics and methane sorption capacity of original and bitumen-free shale from the Triassic Yangchang Formation of the Ordos Basin, combined with the methane dissolution capacity for the isolated bitumen, were analyzed and compared. GIP for the original and bitumen-free shale in the depth range of 500–2500 m was evaluated. The results show that micropores in the shale samples were mainly related to organic matter. Clay mineral-hosted pores contributed slightly to microporosity. Bitumen significantly reduced the micropore surface area and volume of the original shale, with average percentages of 28.09% and 51.26%, respectively. The methane sorption capacity decreased after bitumen removal. When normalized to the original shale mass, the sum of the methane sorption capacity for bitumen-free shale and the methane dissolution capacity for isolated bitumen was similar to the methane sorption capacity of the original shale, indicating that the lack of methane absorbed on bitumen is the main reason for the decrease in methane sorption capacity after bitumen removal. The contribution of absorbed methane on bitumen to sorbed methane in shale could be higher than 36.23%. Dual effects of bitumen on shale GIP were observed. A high content of bitumen(1.12%) increased the GIP of the shale samples, with an average percentage of 23.5% in the depth range of 500–2500 m, while a low content of bitumen(0.06%) decreased the GIP, with an average percentage of 13.6%.

Hydrocarbon generation from lacustrine shales with retained oil during thermal maturation

Thermal maturation in the shale oil/gas system is inherently complex due to the competitive interplays between hydrocarbon generation and retention processes.To study hydrocarbon generation characteristics from shales within different stages of thermal maturation under the influence of retained oil,we performed Micro-Scale Sealed Vessels(MSSV)pyrolysis on a set of artificially matured lacustrine shale s amples from the Shahejie Formation in the Dongpu Depression in Bohai B ay Basin,China.Experimental results show that hydrocarbon yields of shale samples with or without retained oil at various thermal maturities follow different evolution paths.Heavy components(C15+)in samples crack at high temperatures and generally follow a sequence,where they first transform into C6-14 then to C2-5 and C1.Methane accounts for most of the gaseous products at high temperatures in all samples,with different origins.The cracking of C2-5 is the main methane-generating process in samples with retained oil,whereas the source of methane in samples without retained oil is kerogen.In the studied shales,retained oils at early-mature stage retard the transformation of liquid to gaseous hydrocarbon and prompt the cracking of C2-5 to C1 to some extent.TSR reaction related to gypsum in the studied samples is the primary reason that can explain the loss of hydrocarbon yields,especially at high temperatures.In addition,transformation of volatile hydrocarbons to gas and coke also accounts for the loss of generated hydrocarbon,as a secondary factor.

Effect of petroleum chemical fraction and residual oil content in saline lacustrine organic-rich shale: A case study from the Paleogene Dongpu Depression of North China

Halite and gypsum minerals in saline shale make the retention mechanism and chemical fractionation of residual oil unique. The Dongpu Depression in North China is a typically saline lacustrine basin with developing halite and gypsum. The effect of gypsum minerals on residual oil content and chemical fractionation remains unclear. In this study, shale samples with different gypsum contents were used in organic geochemical experiments, showing that the high total organic matter (TOC) content and type II kerogen leads to a high residual oil content, as shown by high values of volatile hydrocarbon (S1) and extractable organic matter (EOM). XRD and FE-SEM result indicate that the existence of gypsum in saline shale contributes to an enhanced pore space and a higher residual oil content in comparison to non-gypsum shale. Additionally, the increase in the gypsum mineral content leads to an increase in the saturated hydrocarbon percentage and a decrease in polar components percentage (resins and asphaltene). Furthermore, thermal simulation experiments on low-mature saline shale show that the percentage of saturated hydrocarbons in the residual oil is high and remains stable and that the storage space is mainly mesoporous (> 20 nm) in the oil expulsion stage. However, the saturated hydrocarbons percentage decreases rapidly, and oil exists in mesopores (> 20 nm and < 5 nm) in the gas expulsion stage. In general, gypsum is conducive to the development of pore space, the adsorption of hydrocarbons and the occurrence of saturated hydrocarbon, leading to large quantities of residual oil. The data in this paper should prove to be reliable for shale oil exploration in saline lacustrine basins.

Experimental study of water imbibition characteristics of the lacustrine shale in Sichuan Basin

Through the stimulation method of large-scale hydraulic fracturing,the spontaneous imbibition capacity of the water phase in the shale reservoir has great influence on the effect of stimulation.Generally,the lacustrine shale has the characteristics of high clay minerals content,strong expansibility,development of nanopores and micro-pores,and underdevelopment of fractures,which leads to the unclear behavior of spontaneous imbibition of aqueous phase.The lacustrine shale of Da'anzhai Member and marine shale of Longmaxi Formation in Sichuan Basin were selected to prepare both the shale matrix sample and fractured shale sample,and the spontaneous imbibition experiment of simulated formation water was carried out.By means of an XRD test,SEM observation,nuclear magnetic resonance test and linear expansion rate test,the mineral composition,the structure of pores and fractures,the capacity of hydration and expansion of both lacustrine and marine shale are compared and analyzed.The results show that the average spontaneous imbibition rate of lacustrine shale is 60.8%higher than that of marine shale within the initial 12 hours of imbibition.The lacustrine shale has faster imbibition rate than the marine shale in the initial stage of spontaneous imbibition.However,the lacustrine shale has underdeveloped pores and fractures,as well as poor connectivity of pores.Besides,the strong hydration and expansion of clay minerals can easily lead to dispersion and migration of clay minerals on the fracture surface,which will plug up the seepage channels,resulting in poor capacity of spontaneous imbibition.The spontaneous imbibition rate in the middle and late stage of Lacustrine shale is obviously lower than that of the marine shale.The overall spontaneous imbibition rate ability of the lacustrine shale is less than that of the marine shale.According to the characteristics of water imbibition of lacustrine shale,considering the dual effects of hydration expansion of clay minerals on the effective reconstructed volume,the microfractures can be initiated and extended by fully utilizing the hydration of shale.Acidification treatment,oxidation treatment or high temperature treatment can be used to expand pore space,enhance water phase imbibition capacity and improve multi-scale mass transfer capacity of the lacustrine shale.

Differences in and factors controlling organic matter enrichment in the Ziliujing Formation shale in the Sichuan Basin

Lacustrine shale oil and gas are important fields for unconventional exploration and development in China,and organic-rich shale deposition lays down the critical foundation for hydrocarbon generation.There are two sets of shale,the Dongyuemiao and Da’anzhai Members,in the Ziliujing Formation in the Sichuan Basin.To identify the differential enrichment characteristics of organic matter and clarify its controlling factors,geochemical analyses of organic and inorganic geochemical analyses were performed.The results showed that the total organic carbon content of the Dongyuemiao shale(1.36%)is slightly higher than that of the Da’anzhai shale(0.95%).The enrichment of organic matter in the two shales resulted from the comprehensive controls of paleoproductivity,paleoenvironment,and terrigenous input,but different factors have different effects.In addition,driven by climate,the change in the sulfate concentration in the bottom water further led to the different intensities of bacterial sulfate reduction in early diagenesis.This made a great difference regarding organic matter accumulation in the two members.In general,climate may have played a dominant role in organic matter enrichment in the two sets of shale.

Lacustrine Shale Deposition and Variable Tectonic Accommodation in the Rift Basins of the Bohai Bay Basin in Eastern China

Organic-rich lacustrine shales are widely distributed in China and have significant potential for unconventional shale gas and oil production although the primary factors controlling the deposition of lacustrine shale are disputed. This work clarifies the different characteristics of tectonic evolution and shale among sub-basins in the Bohai Bay Basin in eastern China as a case study by studying basal subsidence, tectonic subsidence rate, basin extensional proportions and shale chemical characteristics. The paper summarizes the correlation between structure and shale deposition, and concludes that tectonic activity is the primary controlling factor for shale development. Episodic tectonic activity controls not only the timing of shale deposition（with the greatest shale deposition occurring primarily during the peak period of basin tectonic activity） but also the spatial distribution of shale（located mainly in areas of maximum subsidence）, the migration pattern of shale（conforming to that of the basin subsidence center）, and shale strata thickness. Tectonic activity also affects the total organic carbon content and organic matter type in shale. When the tectonic activity was the most active and basal subsidence was the maximum, the total organic carbon content of the shale reached its highest value with organic matter type mainly Type I. As tectonic activity weakened, the total organic carbon content decreased, and the organic matter type changed from Type I to Type I-III.

Organic-rich formation and hydrocarbon enrichment of lacustrine shale strata:A case study of Chang 7 Member

Lacustrine shale is an important target for the exploration of unconventional oil and gas in China beyond marine shale gas.However,the formation environment of lacustrine shale differs from that of marine shale,resulting in a different reservoir composition,organic matter,oil and gas content,and hydrocarbon mobility.In this study,the Chang 7 shale of the Yanchang Formation in the Ordos Basin was used to analyze the effect of volcanic activity on the paleoproductivity and preservation conditions during the formation of lacustrine shale.The results show that algae and bacteria were developed before the eruption.After the eruption,the number of bacteria declined,but the increased prosperity of algae reflects that the volcanic activity enhanced ancient productivity.The sulfate generated by volcanic activity promotes bacterial sulfate reduction,and the produced H_(2)S leads to a strong reducing environment in the waterbody,which is conducive to the preservation of organic matter.Organic geochemical analysis shows that the black shale in the shale strata has a high total organic carbon(TOC)content and strong hydrocarbon generation potential,whereas the tuff has a low TOC content and can scarcely generate hydrocarbons,indicating that the tuff deposited by volcanic activity cannot be considered as effective source rock.In terms of storage space,shale is mainly laminar and dispersed,and it includes organic and inorganic pores.The development of organic pores is affected by thermal maturity,whereas inorganic pores mainly occur between detrital particles and crystals.Tuff is mainly supported by heterogeneous matrix and associated with alteration.Its pores include inter-and intragranular mineral pores.The development of tight sandstone pores is affected by compaction,cementation,and dissolution,which mainly consist of intra-and intergranular pores.The Chang 7 lacustrine shale generally contains oil,but different lithologies have different oil drainage efficiencies.Sandstone and shale exhibit the best and worst oil drainage efficiency,respectively.It is mainly affected by the pore size distribution,fluid properties,and rock wettability.Therefore,the development of shale oil should mainly focus on lacustrine shale formations with interbeds.The mutual dissolution of organic matter and hydrocarbons in the shale section leads to the poor mobility and difficult development of hydrocarbons.

Sedimentary environment and model for lacustrine organic matter enrichment:lacustrine shale of the Early Jurassic Da’anzhai Formation,central Sichuan Basin,China

Based on the analysis of element geochemistry and total organic carbon(TOC),this study investigates the main factors controlling organic matter(OM)enrichment,reconstructs the evolution process of the sedimentary environment,and proposes a dynamic OM enrichment model of the Jurassic Da’anzhai(D)Formation,Sichuan Basin.The results indicate that the Sichuan Basin was generally dominated by a warm and oxidizing sedimentary environment,but with some peculiarities,such as a hotter climate in the D1 member and more anoxic lake water in the D2a member.The sedimentary evolution of the Da’anzhai Formation can be divided into a fluctuating sedimentary stage,a stable sedimentary stage and a reef-building stage.The D2a member showed the strongest hypoxia,the weakest weathering,the largest amount of terrestrial inputs,and the highest TOC content.The TOC is positively correlated with reducing conditions and terrestrial inputs,negatively correlated with weathering.Based on these findings,it is suggested that the global climate in the Early Jurassic period had a complex regional effect and the global oceanic anoxic events of the Toarcian did not spread to the Sichuan Basin.Thus,the anoxic deep water,high terrestrial inputs,and weak weathering were conducive to rapid deposition and preservation of lacustrine OM.

Accumulation contribution differences between lacustrine organic-rich shales and mudstones and their significance in shale oil evaluation

The differences in organic matter abundance and rock composition between shale and mudstone determine the discrepancy of their contributions to the formation of conventional and shale oil/gas reservoirs.The evaluation criteria of source rocks are different in the future exploration in self-sourced petroleum systems.Shales are deposited in deep/semi-deep lacustrine,with low sedimentation rate and chemical depositions of various degrees,while mudstones are mostly formed in shallow lacustrine/lakeside,with high deposition rate and density flow characteristics.Three factors contribute to the enrichment of organic matter in shales,including the"fertility effect"caused by volcanic ash deposition and hydrothermal injection,excessive and over-speed growth of organisms promoted by radioactive materials,and deep-water anaerobic environment and low sedimentation rate to protect the accumulation of organic matter from dilution.Lamellations in shales are easy to be stripped into storage space,and acid water produced during hydrocarbon generation can dissolve some particles to generate new pores.The massive mudstones with high clay content are of poor matrix porosity.Shales with high total organic carbon,developed laminations,relatively good reservoir property,and high brittle mineral content,are the most favorable lithofacies for shale oil exploration and development.It is necessary to conduct investigation on the differences between shale and mudstone reservoirs,to identify resources distribution in shale and mudstone formations,determine the type and standard of"sweet-spot"evaluation parameters,optimize"sweet-spot areas/sections",and adopt effective development technologies,which is of great significance to objectively evaluate the total amount and economy of shale oil resources,as well as the scale of effective exploitation.

Micropore structure characteristics and quantitative characterization methods of lacustrine shale-A case study from the member 2 of Kongdian Formation, Cangdong sag, Bohai Bay Basin

To better understand the micropore characteristics of lacustrine shale and develop quantitative methodsfor characterizing lacustrine shale, the reservoir space types, structures and spatial distribution patternsof effective pores in the shale of Member 2 of Kongdian Formation in Cangdong sag are studied usingcores, thin sections, scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM),nuclear magnetic resonance (NMR), whole-rock X-ray diffraction (XRD) data, etc. Various shale porositycalculation methods are evaluated. The study results show that the reservoir spaces of the shale mainlyinclude three types, i.e., matrix pores, organic pores and fractures. Flaky pore throats formed bydissolution-induced pores and mould pores can improve the pore connectivity. NMR effective porosity ofthe shale varies from 0.59% to 4.42% with an average of 2.38%, accounting for 49.54% of the total NMRporosity and 34.53% of the gas porosity. The shale is divided into the unimodal-type felsic shale, bimodaltype felsic shale, carbonate shale and mixed shale. The different lithologies exhibit linear correlationsamong NMR, gas and effective porosities. The shale has effective porosity of 0.56%-4.53% with an averageof 2.12%. Furthermore, the shale reservoir is divided into four classes: Class I reservoir with effectiveporosities of more than 2.5%;Class II1 reservoir with effective porosity of 2.0%-2.5%;Class II2 reservoirwith effective porosity of 1.5%-2%;and Class III reservoir with effective porosity of less than 1.5%.

Pore structure evolution of lacustrine organic-rich shale from the second member of the Kongdian formation in the Cangdong Sag,Bohai Bay Basin,China

Pyrolysis experiments were conducted on lacustrine organic-rich shale from Cangdong Sag in Bohai Bay Basin,China,to investigate the impact of hydrocarbon generation on shale pore structure evolution.Thermal evolution is found to control the transformation of organic matter,hydrocarbon products characteristics,and pore structure changes.Furthermore,pore volume and specific surface area increase with increasing maturity.In low-mature stage,the retained oil content begins to increase,pore volumes show slight changes,and primary pores are occluded by the generated crude oil of high molecular weight and density.In the oil-window stage,the retained oil content rapidly increases and reaches maximum,and pore volumes gradually increase with increasing thermal maturity.At high mature stage,the retained oil content begins to decrease,and the pore volume increases considerably owing to the expulsion of liquid hydrocarbon.In over mature stage,natural gas content significantly increases and kerogen transforms to asphalt.Numerous organic pores are formed and the pore size gradually increases,resulting from the connection of organic pores caused the increasing thermal stress.This study lays a foundation for understanding variation of hydrocarbon products during the thermal evolution of lacustrine shales and its relationship with the evolution of shale reservoirs.