More than 20 K-bentonite beds were discovered from the Wufeng Formation and the lowest Longmaxi Formation in two sections, both adjacent to the Ordovician-Silurian (O-S) bound-ary and located in Tongzi, Guizhou Provin...More than 20 K-bentonite beds were discovered from the Wufeng Formation and the lowest Longmaxi Formation in two sections, both adjacent to the Ordovician-Silurian (O-S) bound-ary and located in Tongzi, Guizhou Province and Yichang, Hubei Province, some 500 km apart from each other in South China. This indicates that many volcanic eruptions occurred near the southeast margin of the Yangtze Platform between the latest Ordovician and the earliest Silurian. Mainly through biostratigraphic and sequence stratigraphic studies, it was found that almost all these far-apart K-bentonite beds may be correlated with each other. This is the first time that a succession of volcanic ash deposits with high potential of correlation was discovered within a strata interval on the main platforms of China. Therefore, these K-bentonite beds may afford ex-cellent event-marker beds helpful to high-resolution research in integrated stratigraphy as well as other lines of research on the O-S boundary in South China.展开更多
描述了中国木兰科木兰属一新种———红花玉兰(Magnolia wufengensisL.Y.Ma et L.R.W ang),并绘了模式图。本种的主要特点是:落叶乔木,花被片9,近相等,整个花被片内外为均匀的红色,叶柄较长,叶片下面沿主脉密被白色柔毛,这些特征与武当...描述了中国木兰科木兰属一新种———红花玉兰(Magnolia wufengensisL.Y.Ma et L.R.W ang),并绘了模式图。本种的主要特点是:落叶乔木,花被片9,近相等,整个花被片内外为均匀的红色,叶柄较长,叶片下面沿主脉密被白色柔毛,这些特征与武当木兰(M.sprengeriPamp.)和玉兰(M.denudataDesr.)存在明显区别。展开更多
Objective The Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation is one of the priority interval for shale gas exploration in the Sichuan Basin and its peripheral areas, and commercial shale gas has b...Objective The Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation is one of the priority interval for shale gas exploration in the Sichuan Basin and its peripheral areas, and commercial shale gas has been discovered from this interval in Jiaoshiba, Changning and Weiyuan shale gas fields in Sichuan Province. However, there is no significant discovery in other parts of the basin due to the different quality of black shale and the differences of tectonic evolution. Based on the progress of shale gas geological theory and exploration discoveries, as well as the theory of ＂source rock and cap rock controls on hydrocarbon accumulation＂, of the Upper Ordovician the main controlling factors Wufeng Formation-Lower Silurian Longmaxi Formation shale gas enrichment in the Sichuan Basin and its peripheral areas were analyzed, and the source rock and cap rock controls on the shale gas were also discussed. The results can provide new insights for the next shale gas exploration in this area.展开更多
Through detailed analyses of the distribution characteristics of organic-rich shale, appearance features of high-quality shale, microscopic characteristics of shale reservoir rocks, fracability, and the relationship b...Through detailed analyses of the distribution characteristics of organic-rich shale, appearance features of high-quality shale, microscopic characteristics of shale reservoir rocks, fracability, and the relationship between preservation conditions and shale gas enrichment in Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in Sichuan Basin, theoretical understandings and specific suggestions with respect to the exploration and development of shale gas in China are summarized and proposed respectively. Important geological understandings in the exploration and development of shale gas of the Wufeng Formation–Longmaxi Formation in the Sichuan Basin can be summarized into the following aspects: depositional environment and depositional process control the distribution of organic-rich shale; high quality shale in "sweet spot segments" are commonly characterized by high content of organic carbon, high brittleness, high porosity and gas content; organic pores are important storage space for the enrichment of shale gas; preservation conditions are the key factor for the geological evaluation of shale gas in structurally complex regions; shale gas can be considered as "artificial gas reservoirs" and the fracability assessment is essential for high-production; nanoscale storage space and the mode of occurrence control the special seepage characteristics of shale gas. The following suggestions are proposed for the development of China's shale gas industry:(1) focus more on fundamental research to achieve new breakthrough in the geological theory of shale gas;(2) emphasize exploration practices to have all-round discoveries in multiple strata;(3) study the regularities of development and production to establish new models of shale gas development;(4) think creatively to invent new technologies to tackle key problems;(5) explore the management innovation to create new mechanisms in shale gas development.展开更多
As the hydrocarbon generation and storage mechanisms of high quality shales of Upper Ordovician Wufeng Formation– Lower Silurian Longmaxi Formation remain unclear, based on geological conditions and experimental mode...As the hydrocarbon generation and storage mechanisms of high quality shales of Upper Ordovician Wufeng Formation– Lower Silurian Longmaxi Formation remain unclear, based on geological conditions and experimental modelling of shale gas formation, the shale gas generation and accumulation mechanisms as well as their coupling relationships of deep-water shelf shales in Wufeng–Longmaxi Formation of Sichuan Basin were analyzed from petrology, mineralogy, and geochemistry. The high quality shales of Wufeng–Longmaxi Formation in Sichuan Basin are characterized by high thermal evolution, high hydrocarbon generation intensity, good material base, and good roof and floor conditions;the high quality deep-water shelf shale not only has high biogenic silicon content and organic carbon content, but also high porosity coupling. It is concluded that:(1) The shales had good preservation conditions and high retainment of crude oil in the early times, and the shale gas was mainly from cracking of crude oil.(2) The biogenic silicon(opal A) turned into crystal quartz in early times of burial diagenesis, lots of micro-size intergranular pores were produced in the same time;moreover, the biogenic silicon frame had high resistance to compaction, thus it provided the conditions not only for oil charge in the early stage, but also for formation and preservation of nanometer cellular-like pores, and was the key factor enabling the preservation of organic pores.(3) The high quality shale of Wufeng–Longmaxi Formation had high brittleness, strong homogeneity, siliceous intergranular micro-pores and nanometer organic pores, which were conducive to the formation of complicated fissure network connecting the siliceous intergranular nano-pores, and thus high and stable production of shale gas.展开更多
Stratigraphic division and correlation are crucial for the identification of sweet spots and drilling design of shale gas.In this study,a stratigraphic division and correlation was carried out for the Wufeng-Longmaxi ...Stratigraphic division and correlation are crucial for the identification of sweet spots and drilling design of shale gas.In this study,a stratigraphic division and correlation was carried out for the Wufeng-Longmaxi Formations in southern China from the prospective of lithostratigraphy,sea level changes,and biostratigraphy using data from seismic investigation,wells,and outcrops.The Wufeng and Longmaxi Formations were respectively divided into four members,Wu 1 and Wu2 for the former and Long 1 and Long 2 for the latter.Of the members,Long 1 was subdivided and its first subdivision(Long 11)was further divided into 4 layers(Long 1^(1)_(1),Long 1^(2)_(1),Long 1^(3)_(1),and Long 1^(4)_(1)).Three eustatic cycles were recognized in the Wufeng-Longmaxi Formations.Cycle I corresponds to the Wufeng Formation with the maximum flooding surface at the top ofWu 1.Cycle II corresponds to Long 1,with the maximum flooding surface at the top of Long 1^(3)_(1).CycleⅢⅢcorresponds to Long 2.Furthermore,4 graptolite biozones(WF1 to WF4)were identified in the Wufeng Formation and 9 graptolite biozones(LM1 to LM9)in the Longmaxi Formation.WF1-2 and WF3-4 correspond to Wu 1 and Wu 2,respectively;and LM1,LM2-4,LM5,LM6,and LM7-9 correspond to Long 1^(1)_(1),Long 1^(2)_(1),Long 1^(3)_(1),Long 1^(4)_(1),and Long 12 and Long 2,respectively.Highquality shales mainly occur in the Wufeng Formation and Long 11.The major intervals that should be investigated with regards to shale gas production include LM1eLM5(10m thick)in the Weiyuan Block and WF1eLM5(20e35m thick)in the Changning Block.Long 1^(1)_(1)is believed to be an optimal target for drilling due to its high TOC content,siliceous content,porosity,microfracture density,and horizontal/vertical permeability ratio.展开更多
The Upper Ordovician Wufeng-Lower Silurian Longmaxi shale is widely distributed in the Sichuan Basin and its periphery,which is the key stratum for marine shale gas exploration and development(E&D)in China.Based o...The Upper Ordovician Wufeng-Lower Silurian Longmaxi shale is widely distributed in the Sichuan Basin and its periphery,which is the key stratum for marine shale gas exploration and development(E&D)in China.Based on sedimentary environment,material basis,storage space,fracability and reservoir evolution data,the reservoir characteristics of the Wufeng-Longmaxi shale and their significance for shale gas E&D are systematically compared and analyzed in this paper.The results show that(1)the depocenter of the Wufeng(WF)-Longmaxi(LM)shale gradually migrates from east to west.The high-quality shale reservoirs in the eastern Sichuan Basin are mainly siliceous shales,which are primarily distributed in the graptolite shale interval of WF2-LM5.The high-quality reservoirs in the southern Sichuan Basin are mainly calcareous-siliceous and organic-rich argillaceous shales,which are distributed in the graptolite shale interval of WF2-LM7.(2)Deep shale gas(the burial depth>3500 m)in the Sichuan Basin has high-ultrahigh pressure and superior physical properties.The organic-rich siliceous,calcareous-siliceous and organic-rich argillaceous shales have suitable reservoir properties.The marginal area of the Sichuan Basin has a higher degree of pressure relief,which leads to the argillaceous and silty shales evolving into direct cap rocks with poor reservoir/good sealing capacity.(3)Combining shale gas exploration practices and impacts of lithofacies,depth,pressure coefficient and brittle-ductile transition on the reservoir properties,it is concluded that the favorable depth interval of the Wufeng-Longmaxi shale gas is 2200~4000 m under current technical conditions.(4)Aiming at the differential reservoir properties of the Wufeng-Longmaxi shale in the Sichuan Basin and its periphery,several suggestions for future research directions and E&D of shale gas are formulated.展开更多
The Ordovician Wufeng Formation-Silurian Longmaxi Formation organic-rich shales distributed widely and stably in Southern Sichuan Basin were investigated based on drilling data.Geological evaluation of wells show that...The Ordovician Wufeng Formation-Silurian Longmaxi Formation organic-rich shales distributed widely and stably in Southern Sichuan Basin were investigated based on drilling data.Geological evaluation of wells show that the shale reservoirs have good properties in the Yibin,Weiyuan,Zigong,Changning,Luzhou,Dazu areas,with key parameters such as TOC,porosity,gas content similar to the core shale gas production zones.Moreover,these areas are stable in structure,good in preservation conditions and highly certain in resources.The shale reservoirs have a burial depth of 4 500 m or shallow,a total area of over 2×10~4 km^2 and estimated resource of over10×10^(12) m^3,so they are the most resource-rich and practical areas for shale gas exploitation in China.Through construction of the Changning-Weiyuan national demonstration region,the production and EUR of shale gas wells increased significantly,the cost of shale gas wells decreased remarkable,resulting in economic benefit better than expected.Moreover,the localized exploration and development technologies and methods are effective and repeatable,so it is the right time for accelerating shale gas exploitation.Based on the production decline pattern of horizontal wells at present and wells to be drilled in the near future,at the end of the 13th Five Year Plan,the production of shale gas in southern Sichuan Basin is expected to reach or exceed 10 billion cubic meters per year.The resources are sufficient for a stable production period at 30 billion cubic meters per year,which will make the South Sichuan basin become the largest production base of shale gas in China.展开更多
Based on exploration and development results and evaluation of marine shale gas in South China in the past ten years, in view of the features of "high maturity, strong tectonic reformation and high shear stress&q...Based on exploration and development results and evaluation of marine shale gas in South China in the past ten years, in view of the features of "high maturity, strong tectonic reformation and high shear stress" of the shale in Zhaotong exploration zone in the Yunnan and Guizhou Plateau, as well as the key issues of long time diffusion and leakage of shallow shale gas, and the preservation conditions, the factors controlling shallow shale gas sweet spot and key zone selection evaluation technology of shale gas are investigated. From 2017 to 2018, the first significant exploration breakthrough was made in the Taiyang anticline at a buried depth of 700 to 2 000 m, discovering large-scale proved geological reserves of shallow shale gas. By examining the accumulation conditions and sweet spot control factors of the shallow shale gas in this area, it is found that the accumulation and productivity potential of shale gas in the mountainous area with complex structure outside basin are controlled by five factors:(1) The gas-rich area has weak tectonic reformation and good preservation conditions on the whole, taking on typical anticline trap occurrence mode.(2) The gas-rich area is in over-pressure state and high in shale gas content.(3) The gas-rich area has high quality shale and thus superior source rock condition.(4) The gas-rich area has high quality reservoirs dominated by class I.(5) The shale gas reservoir in the gas-rich area has high content of brittle minerals and small difference between maximum and minimum horizontal stresses which are conducive to hydraulic fracturing. The innovative practice and core technologies formed during the exploration and production capacity construction of shallow shale gas in the Zhaotong demonstration zone have great reference significance for shallow shale gas exploration and development in other areas.展开更多
The enrichment characteristics of deep shale gas in the Ordovician Wufeng-Silurian Longmaxi formations in the Sichuan Basin and its surrounding areas are investigated through experiments under high temperature and hig...The enrichment characteristics of deep shale gas in the Ordovician Wufeng-Silurian Longmaxi formations in the Sichuan Basin and its surrounding areas are investigated through experiments under high temperature and high pressure,including petrophysical properties analyses,triaxial stress test and isothermal adsorption of methane experiment.(1)The deep shale reservoirs drop significantly in porosity and permeability compared with shallower shale reservoirs,and contain mainly free gas.(2)With higher deviatoric stress and axial strain,the deep shale reservoirs have higher difficulty fracturing.(3)Affected by structural location and morphology,fracture characteristics,geofluid activity stages and intensity,deep shale gas reservoirs have more complicated preservation conditions.(4)To achieve the commercial development of deep shale gas reservoirs,deepening geological understanding is the basis,and exploring reservoir simulation technology befitting the geological features is the key.(5)The siliceous shale and limestone-bearing siliceous shale in the Metabolograptus persculptus-Parakidograptus acuminatus zones(LM1-LM3 graptolite zones)are the high-production intervals for deep shale gas and the most favorable landing targets for horizontal drilling.Deeps water areas such as Jiaoshiba,Wulong,Luzhou and Changning with deep shale reservoirs over 10 m thickness are the most favorable areas for deep shale gas enrichment.It is recommended to carry out exploration and development practice in deep-water shale gas areas deposited deep with burial depth no more than 5000 m where the geological structure is simple and the shale thickness in the LM1-LM3 graptolite zone is greater than 10 m.It is better to increase the lateral length of horizontal wells,and apply techniques including high intensity of perforations,large volume of proppant,far-field and near-wellbore diversions to maximize the stimulated deep reservoir volume.展开更多
The difference in quartz types in shales not only affects the porosity and permeability of the rocks,but also reflects the difference in the sedimentary environments.We established the formation mechanism and numerica...The difference in quartz types in shales not only affects the porosity and permeability of the rocks,but also reflects the difference in the sedimentary environments.We established the formation mechanism and numerical model of quartz in shales of Wufeng and Longmaxi formations in the Wangjiawan Section,South China,based on thin-section studies using SEM(scanning electron microscope),SEM-CL(cathodoluminescence),XRD(X-ray diffraction)and geochemical analyses.There are two types of quartz in the shales:detrital quartz and authigenic quartz.Detrital quartz is mostly silt-size,typically ranging from 10 to 60μm in size and subangular to angular monocrystal in shape,and brighter than authigenic quartz by CL intensity;authigenic quartz is present in two phases in shape:grain overgrowths and crystallite grains.Overgrowth surfaces are subhedral.Crystallite grains are typically less than 10μm in size,euhedral or subhedral monocrystal in shape.Authigenic quartz can be subdivided into biogenic quartz and clay mineral transformed quartz according to the source of silicon.In the numerical model,the content of detrital quartz is relatively consistent(20%);the content of biogenic quartz ranges from 40%to 70%,with a sharp fall(0–30%)in the Guanyinqiao mudstone.During the Katian,a lower anoxic and dense water column make the dissolution of biogenic silica well preserved.Biogenic quartz is the major contributor to the sediment.During the early Hirnantian interval,due to the drop of sea level and the oxygenation of seafloor,the sediment is mainly composed of clay transformed quartz and detrital quartz.During the latest Hirnatian and Rhuddanian,rapid sea level rise and anoxic ocean enhance the preservation of the biogenic silica,thereby biogenic quartz re-emerges as the major contributors to the sediment.Authigenic crystallite grains and grain overgrowths have filled in primary pore space and have decreased the interparticle porosity,however,as a rigid framework,they can suppress compaction and maintain the internal pore structur展开更多
Acoustic wave velocity has been commonly utilized to predict subsurface geopressure using empirical relations.Acoustic wave velocity is, however, affected by many factors. To estimate pore pressure accurately, we here...Acoustic wave velocity has been commonly utilized to predict subsurface geopressure using empirical relations.Acoustic wave velocity is, however, affected by many factors. To estimate pore pressure accurately, we here propose to use elastic rock physics models to understand and analyze quantitatively the various contributions from these different factors affecting wave velocity. We report a closed-form relationship between the frame flexibility factor(γ) in a rock physics model and differential pressure, which presents the major control of pressure on elastic properties such as bulk modulus and compressional wave velocity. For a gas-bearing shale with abundant micro-cracks and fractures, its bulk modulus is much lower at abnormally high pore pressure(high γ values) where thin cracks and flat pores are open than that at normal hydrostatic pressure(low γ values) where pores are more rounded on average. The developed relations between bulk modulus and differential pressure have been successfully applied to the Upper Ordovician Wufeng and Lower Silurian Longmaxi formations in the Dingshan area of the Sichuan Basin to map the three-dimensional spatial distribution of pore pressure in the shale, integrating core, log and seismic data. The estimated results agree well with field measurements. Pressure coefficient is positively correlated to gas content. The relations and methods reported here could be useful for hydrocarbon exploration, production, and drilling safety in both unconventional and conventional fields.展开更多
It is essential to investigate shale lithofacies distribution and controlling factor of the shale for geological evaluation of shale gas exploration and development.Through comprehensive analysis of cores,thin section...It is essential to investigate shale lithofacies distribution and controlling factor of the shale for geological evaluation of shale gas exploration and development.Through comprehensive analysis of cores,thin sections,cathode luminescence,whole-rock X-ray diffraction,element capture spectroscopy,major/trace element and other data,three major types and eight sub-type shale lithofacies in the shale of Wufeng Formation-Member 1 of Longmaxi Formation in Jiaoshiba area are identified by the three-end-member method and shale lithological classification nomenclature,and the spatiotemporal distribution law and main development controlling factors of shale lithofacies are well studied.In the Jiaoshiba area,vertically,the marine shale develops siliceous shale,mixed shale and argillaceous shale from bottom to top.Besides,lateral distribution of the shale is different from north to south;the shale lithofacies in the north area changes rapidly,the mixed shale in the north area is much thicker than that in the south area,while the siliceous shale in the south area is relatively thicker.Difference in the shale lithofacies is controlled by special sedimentary geologic events;development of the siliceous shale is controlled by the Ordovician-Silurian global volcanic event to some extent,while the mixed shale is significantly influenced by effect of bottom current,and the argillaceous shale is mainly affected by supply of terrestrial clastic material.展开更多
For black shales,laminae and bedding are hard to identify,grain size is difficult to measure,and trace fossils do not exist.Taking the Ordovician Wufeng–Silurian Longmaxi shale in southern Sichuan Basin,China,as an e...For black shales,laminae and bedding are hard to identify,grain size is difficult to measure,and trace fossils do not exist.Taking the Ordovician Wufeng–Silurian Longmaxi shale in southern Sichuan Basin,China,as an example,the types,characteristics and models of microfacies in epicontinental shale are analyzed by means of full-scale observation of large thin sections,argon-ion polishing field emission-scanning electron microscopy(FE-SEM),and kerogen microscopy.The epicontinental sea develops delta,tidal flat and shelf facies,with black shale found in microfacies such as the underwater distributary channel and interdistributary bay under delta front facies,the calcareous and clayey flats under intertidal flat facies,the calcareous and clayey shelfs under shallow shelf facies,the deep slope,deep plain and deep depression under deep shelf facies,and the overflow under gravity flow facies.Basinward,silty lamina decreases and clayey lamina increases,the grain size changes from coarse silt to fine mud,the silica content increases from about 20%to above 55%,the carbonate and clay minerals content decreases from above 40%to around 10%,and the kerogen type changes from type II2 to type II1 and type I.Provenance and topography dominate the types and distribution of shale microfacies.The underwater distributary channel,interdistributary bay,clayey flat,clayey shelf,and overflow microfacies are developed in areas with sufficient sediment supply.The calcareous flat and calcareous shelf are developed in areas with insufficient sediment supply.The deep shelf shale area is divided into deep slope,deep plain,and deep depression microfacies as a result of three breaks.The formation of epicontinental shale with different microfacies is closely related to the tectonic setting,paleoclimate,and sea level rise.The relatively active tectonic setting increases the supply of terrigenous clasts,forming muddy water fine-grained sediment.The warm and humid paleoclimate is conducive to the enrichment of organic matter.The rapid sea level ri展开更多
The relationship between fracture calcite veins and shale gas enrichment in the deep Ordovician Wufeng Formation-Silurian Longmaxi Formation (Wufeng-Longmaxi) shales in southern Sichuan Basin was investigated through ...The relationship between fracture calcite veins and shale gas enrichment in the deep Ordovician Wufeng Formation-Silurian Longmaxi Formation (Wufeng-Longmaxi) shales in southern Sichuan Basin was investigated through core and thin section observations, cathodoluminescence analysis, isotopic geochemistry analysis, fluid inclusion testing, and basin simulation. Tectonic fracture calcite veins mainly in the undulating part of the structure and non-tectonic fracture calcite veins are mainly formed in the gentle part of the structure. The latter, mainly induced by hydrocarbon generation, occurred at the stage of peak oil and gas generation, while the former turned up with the formation of Luzhou paleouplift during the Indosinian. Under the influence of hydrocarbon generation pressurization process, fractures were opened and closed frequently, and oil and gas episodic activities are recorded by veins. The formation pressure coefficient at the maximum paleodepth exceeds 2.0. The formation uplift stage after the Late Yanshanian is the key period for shale gas migration. Shale gas migrates along the bedding to the high part of the structure. The greater the structural fluctuation is, the more intense the shale gas migration activity is, and the loss is more. The gentler the formation is, the weaker the shale gas migration activity is, and the loss is less. The shale gas enrichment in the core of gentle anticlines and gentle synclines is relatively higher.展开更多
The Ordovician-Silurian Wufeng and Longmaxi Shale in the Sichuan Basin were studied to understand the genesis and diagenetic evolution of carbonate minerals and their effects on reservoir quality. The results of geoch...The Ordovician-Silurian Wufeng and Longmaxi Shale in the Sichuan Basin were studied to understand the genesis and diagenetic evolution of carbonate minerals and their effects on reservoir quality. The results of geochemical and petrological analyses show that calcite grains have a negative Ce anomaly indicating they formed in the oxidizing environment of seawater. The high carbonate mineral contents in the margin of basin indicate that calcite grains and cores of dolomite grains appear largely to be of detrital origin. The rhombic rims of dolomite grains and dolomite concretions with the δ^(13)C of –15.46‰ and the enrichment of middle rare earth elements were formed during the sulfate-driven anaerobic oxidation of methane. The calcite in radiolarian were related to the microbial sulfate reduction for the abundant anhedral pyrites and δ^(13)C value of –11.34‰. Calcite veins precipitated in the deep burial stage with homogenization temperature of the inclusions ranging from 146.70 ℃ to 182.90 ℃. The pores in shale are mainly organic matter pores with pore size mainly in the range of 1–20 nm in diameter. Carbonate minerals influence the development of pores through offering storage space for organic matter. When calcite contents ranging from 10% to 20%, calcite grains and cement as rigid framework can preserve primary pores. Subsequently, the thermal cracking of liquid petroleum in primary pores will form organic matter pores. The radiolarian were mostly partially filled with calcite, which combining with microcrystalline quartz preserved a high storage capacity.展开更多
Formation of organic-rich shale was controlled by depositional processes and environment condition.According to petrology,mineralogy,fossil composition and sedimentary structure,seven lithofacies of shale were identif...Formation of organic-rich shale was controlled by depositional processes and environment condition.According to petrology,mineralogy,fossil composition and sedimentary structure,seven lithofacies of shale were identified in the Wufeng Formation to Longmaxi Formation,e.g.,siliceous shale,silty shale,argillaceous shale,calcareous shale,shell-bearing argillaceous limestone/calcareous mudstone,siltstonefine sandstone and bentonite.For shale in the Wufeng and Longmaxi Formations,the depositional mode was not only the suspension deposition under the low-energy hydrodynamic condition,but also the biogenic deposition,the storm deposition and the bottom current deposition indicated by large amount of siliceous biological fossils,biological detritus with size graded structure,silt laminations,lenticular bedding,ripple cross-bedding and other sedimentary structures.Trace element analysis suggested that the siliceous shale in theWufeng Formation and the lower part of Longmaxi Formation was developed in the oxygen-dysoxic/anoxic depositional environment,which was rich in organic matters;then due to influence of the sea level gradually decline and bottom current,the silty and argillaceous shales were formed,and the oxygen-dysoxic/anoxic depositional environment was destroyed,gradually leading to the worse preservation condition of organic matter.展开更多
According to data of gas wells and typical sections of Wufeng Formation and Longmaxi Formation in Sichuan Basin,shale of various graptolite zones were analyzed to determine depositional environment,lithology and thick...According to data of gas wells and typical sections of Wufeng Formation and Longmaxi Formation in Sichuan Basin,shale of various graptolite zones were analyzed to determine depositional environment,lithology and thickness characteristics of the graptolite shale interval of WF2-WF3 in the lower part of Wufeng Formation,the graptolite shale interval of WF4 in Guanyinqiao Member of Wufeng Formation and the graptolite shale interval of LM1-LM4 in the bottom of Longmaxi Formation,and characteristics of shale horizontal distribution were also investigated.During the depositional period of the graptolite shale interval of WF2-WF3,the study area was less affected by the Guangxi movement,the depositional environment was the deep water of open sea,where black shale was mainly deposited;the sedimentation center was developed in northeast Guizhou-northeast Sichuan and south Sichuan,the maximum thickness was from 4 to 6 m in the sedimentation center.During the depositional period of the graptolite shale interval of WF4,the depositional environment in the study area changed greatly due to global sea level fall and enhanced Guangxi movement;the central Sichuan paleouplift,the central Guizhou paleouplift and the Jiangnan-Xuefeng palaeouplift were further expanded,and the area of the sedimentary basin decreased;the depositional environment was mainly carbonate bioclastic shoal of shallow sea,and partially deep sea which only was distributed in the Shizhu-Fuling-Wuxi area in east and northeast Sichuan and the Gongxian-Yongchuan area in south Sichuan;sediments of shallow water were dominated by limestone and argillaceous limestone with abundant Hirnantia,sediments of deep water were dominated by calcareous mudstone and shale with Hirnantia.During the depositional period of the graptolite shale interval of LM1-LM4,due to rise of global sea level and Guangxi movement,the sedimentary area was larger than that in the depositional period of Guanyinqiao Member,and the sedimentary environment mainly was stagnant deepwater;thickness of bla展开更多
基金the National Natural Science Foundation of China(Grant No.49802002)the State Key Project of Sequence Stratigraphy and Earth Rhythms the Lab of Earth Surface Systems of Hubei Province.
文摘More than 20 K-bentonite beds were discovered from the Wufeng Formation and the lowest Longmaxi Formation in two sections, both adjacent to the Ordovician-Silurian (O-S) bound-ary and located in Tongzi, Guizhou Province and Yichang, Hubei Province, some 500 km apart from each other in South China. This indicates that many volcanic eruptions occurred near the southeast margin of the Yangtze Platform between the latest Ordovician and the earliest Silurian. Mainly through biostratigraphic and sequence stratigraphic studies, it was found that almost all these far-apart K-bentonite beds may be correlated with each other. This is the first time that a succession of volcanic ash deposits with high potential of correlation was discovered within a strata interval on the main platforms of China. Therefore, these K-bentonite beds may afford ex-cellent event-marker beds helpful to high-resolution research in integrated stratigraphy as well as other lines of research on the O-S boundary in South China.
文摘描述了中国木兰科木兰属一新种———红花玉兰(Magnolia wufengensisL.Y.Ma et L.R.W ang),并绘了模式图。本种的主要特点是:落叶乔木,花被片9,近相等,整个花被片内外为均匀的红色,叶柄较长,叶片下面沿主脉密被白色柔毛,这些特征与武当木兰(M.sprengeriPamp.)和玉兰(M.denudataDesr.)存在明显区别。
基金supported by the National Natural Science Foundation of China(grant No.41202103)
文摘Objective The Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation is one of the priority interval for shale gas exploration in the Sichuan Basin and its peripheral areas, and commercial shale gas has been discovered from this interval in Jiaoshiba, Changning and Weiyuan shale gas fields in Sichuan Province. However, there is no significant discovery in other parts of the basin due to the different quality of black shale and the differences of tectonic evolution. Based on the progress of shale gas geological theory and exploration discoveries, as well as the theory of ＂source rock and cap rock controls on hydrocarbon accumulation＂, of the Upper Ordovician the main controlling factors Wufeng Formation-Lower Silurian Longmaxi Formation shale gas enrichment in the Sichuan Basin and its peripheral areas were analyzed, and the source rock and cap rock controls on the shale gas were also discussed. The results can provide new insights for the next shale gas exploration in this area.
基金Supported by the Chinese Academy of Engineering Consultancy Project(2018-xz-09-01)
文摘Through detailed analyses of the distribution characteristics of organic-rich shale, appearance features of high-quality shale, microscopic characteristics of shale reservoir rocks, fracability, and the relationship between preservation conditions and shale gas enrichment in Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in Sichuan Basin, theoretical understandings and specific suggestions with respect to the exploration and development of shale gas in China are summarized and proposed respectively. Important geological understandings in the exploration and development of shale gas of the Wufeng Formation–Longmaxi Formation in the Sichuan Basin can be summarized into the following aspects: depositional environment and depositional process control the distribution of organic-rich shale; high quality shale in "sweet spot segments" are commonly characterized by high content of organic carbon, high brittleness, high porosity and gas content; organic pores are important storage space for the enrichment of shale gas; preservation conditions are the key factor for the geological evaluation of shale gas in structurally complex regions; shale gas can be considered as "artificial gas reservoirs" and the fracability assessment is essential for high-production; nanoscale storage space and the mode of occurrence control the special seepage characteristics of shale gas. The following suggestions are proposed for the development of China's shale gas industry:(1) focus more on fundamental research to achieve new breakthrough in the geological theory of shale gas;(2) emphasize exploration practices to have all-round discoveries in multiple strata;(3) study the regularities of development and production to establish new models of shale gas development;(4) think creatively to invent new technologies to tackle key problems;(5) explore the management innovation to create new mechanisms in shale gas development.
基金Supported by the China National Science and Technology Major Project(2017ZX05036,2017ZX05036001).
文摘As the hydrocarbon generation and storage mechanisms of high quality shales of Upper Ordovician Wufeng Formation– Lower Silurian Longmaxi Formation remain unclear, based on geological conditions and experimental modelling of shale gas formation, the shale gas generation and accumulation mechanisms as well as their coupling relationships of deep-water shelf shales in Wufeng–Longmaxi Formation of Sichuan Basin were analyzed from petrology, mineralogy, and geochemistry. The high quality shales of Wufeng–Longmaxi Formation in Sichuan Basin are characterized by high thermal evolution, high hydrocarbon generation intensity, good material base, and good roof and floor conditions;the high quality deep-water shelf shale not only has high biogenic silicon content and organic carbon content, but also high porosity coupling. It is concluded that:(1) The shales had good preservation conditions and high retainment of crude oil in the early times, and the shale gas was mainly from cracking of crude oil.(2) The biogenic silicon(opal A) turned into crystal quartz in early times of burial diagenesis, lots of micro-size intergranular pores were produced in the same time;moreover, the biogenic silicon frame had high resistance to compaction, thus it provided the conditions not only for oil charge in the early stage, but also for formation and preservation of nanometer cellular-like pores, and was the key factor enabling the preservation of organic pores.(3) The high quality shale of Wufeng–Longmaxi Formation had high brittleness, strong homogeneity, siliceous intergranular micro-pores and nanometer organic pores, which were conducive to the formation of complicated fissure network connecting the siliceous intergranular nano-pores, and thus high and stable production of shale gas.
基金Preparation of this manuscript was supported by the National Natural Science Foundation of China(No.41572079)National Major Science and Technology Project(No.2017ZX05035).
文摘Stratigraphic division and correlation are crucial for the identification of sweet spots and drilling design of shale gas.In this study,a stratigraphic division and correlation was carried out for the Wufeng-Longmaxi Formations in southern China from the prospective of lithostratigraphy,sea level changes,and biostratigraphy using data from seismic investigation,wells,and outcrops.The Wufeng and Longmaxi Formations were respectively divided into four members,Wu 1 and Wu2 for the former and Long 1 and Long 2 for the latter.Of the members,Long 1 was subdivided and its first subdivision(Long 11)was further divided into 4 layers(Long 1^(1)_(1),Long 1^(2)_(1),Long 1^(3)_(1),and Long 1^(4)_(1)).Three eustatic cycles were recognized in the Wufeng-Longmaxi Formations.Cycle I corresponds to the Wufeng Formation with the maximum flooding surface at the top ofWu 1.Cycle II corresponds to Long 1,with the maximum flooding surface at the top of Long 1^(3)_(1).CycleⅢⅢcorresponds to Long 2.Furthermore,4 graptolite biozones(WF1 to WF4)were identified in the Wufeng Formation and 9 graptolite biozones(LM1 to LM9)in the Longmaxi Formation.WF1-2 and WF3-4 correspond to Wu 1 and Wu 2,respectively;and LM1,LM2-4,LM5,LM6,and LM7-9 correspond to Long 1^(1)_(1),Long 1^(2)_(1),Long 1^(3)_(1),Long 1^(4)_(1),and Long 12 and Long 2,respectively.Highquality shales mainly occur in the Wufeng Formation and Long 11.The major intervals that should be investigated with regards to shale gas production include LM1eLM5(10m thick)in the Weiyuan Block and WF1eLM5(20e35m thick)in the Changning Block.Long 1^(1)_(1)is believed to be an optimal target for drilling due to its high TOC content,siliceous content,porosity,microfracture density,and horizontal/vertical permeability ratio.
基金granted by the National Science and Technology Major Project of the Ministry of Science and Technology of China (Grant No. 2017ZX05036002–001)National Natural Science Foundation of China (No. 41202103, 41872124)SINOPEC Ministry of Science and Technology Project (Grant No. P17027–2)
文摘The Upper Ordovician Wufeng-Lower Silurian Longmaxi shale is widely distributed in the Sichuan Basin and its periphery,which is the key stratum for marine shale gas exploration and development(E&D)in China.Based on sedimentary environment,material basis,storage space,fracability and reservoir evolution data,the reservoir characteristics of the Wufeng-Longmaxi shale and their significance for shale gas E&D are systematically compared and analyzed in this paper.The results show that(1)the depocenter of the Wufeng(WF)-Longmaxi(LM)shale gradually migrates from east to west.The high-quality shale reservoirs in the eastern Sichuan Basin are mainly siliceous shales,which are primarily distributed in the graptolite shale interval of WF2-LM5.The high-quality reservoirs in the southern Sichuan Basin are mainly calcareous-siliceous and organic-rich argillaceous shales,which are distributed in the graptolite shale interval of WF2-LM7.(2)Deep shale gas(the burial depth>3500 m)in the Sichuan Basin has high-ultrahigh pressure and superior physical properties.The organic-rich siliceous,calcareous-siliceous and organic-rich argillaceous shales have suitable reservoir properties.The marginal area of the Sichuan Basin has a higher degree of pressure relief,which leads to the argillaceous and silty shales evolving into direct cap rocks with poor reservoir/good sealing capacity.(3)Combining shale gas exploration practices and impacts of lithofacies,depth,pressure coefficient and brittle-ductile transition on the reservoir properties,it is concluded that the favorable depth interval of the Wufeng-Longmaxi shale gas is 2200~4000 m under current technical conditions.(4)Aiming at the differential reservoir properties of the Wufeng-Longmaxi shale in the Sichuan Basin and its periphery,several suggestions for future research directions and E&D of shale gas are formulated.
基金Supported by the China National Science and Technology Major Project(2016ZX05062)
文摘The Ordovician Wufeng Formation-Silurian Longmaxi Formation organic-rich shales distributed widely and stably in Southern Sichuan Basin were investigated based on drilling data.Geological evaluation of wells show that the shale reservoirs have good properties in the Yibin,Weiyuan,Zigong,Changning,Luzhou,Dazu areas,with key parameters such as TOC,porosity,gas content similar to the core shale gas production zones.Moreover,these areas are stable in structure,good in preservation conditions and highly certain in resources.The shale reservoirs have a burial depth of 4 500 m or shallow,a total area of over 2×10~4 km^2 and estimated resource of over10×10^(12) m^3,so they are the most resource-rich and practical areas for shale gas exploitation in China.Through construction of the Changning-Weiyuan national demonstration region,the production and EUR of shale gas wells increased significantly,the cost of shale gas wells decreased remarkable,resulting in economic benefit better than expected.Moreover,the localized exploration and development technologies and methods are effective and repeatable,so it is the right time for accelerating shale gas exploitation.Based on the production decline pattern of horizontal wells at present and wells to be drilled in the near future,at the end of the 13th Five Year Plan,the production of shale gas in southern Sichuan Basin is expected to reach or exceed 10 billion cubic meters per year.The resources are sufficient for a stable production period at 30 billion cubic meters per year,which will make the South Sichuan basin become the largest production base of shale gas in China.
基金Supported by the China National Science and Technology Major Project(2017ZX05063).
文摘Based on exploration and development results and evaluation of marine shale gas in South China in the past ten years, in view of the features of "high maturity, strong tectonic reformation and high shear stress" of the shale in Zhaotong exploration zone in the Yunnan and Guizhou Plateau, as well as the key issues of long time diffusion and leakage of shallow shale gas, and the preservation conditions, the factors controlling shallow shale gas sweet spot and key zone selection evaluation technology of shale gas are investigated. From 2017 to 2018, the first significant exploration breakthrough was made in the Taiyang anticline at a buried depth of 700 to 2 000 m, discovering large-scale proved geological reserves of shallow shale gas. By examining the accumulation conditions and sweet spot control factors of the shallow shale gas in this area, it is found that the accumulation and productivity potential of shale gas in the mountainous area with complex structure outside basin are controlled by five factors:(1) The gas-rich area has weak tectonic reformation and good preservation conditions on the whole, taking on typical anticline trap occurrence mode.(2) The gas-rich area is in over-pressure state and high in shale gas content.(3) The gas-rich area has high quality shale and thus superior source rock condition.(4) The gas-rich area has high quality reservoirs dominated by class I.(5) The shale gas reservoir in the gas-rich area has high content of brittle minerals and small difference between maximum and minimum horizontal stresses which are conducive to hydraulic fracturing. The innovative practice and core technologies formed during the exploration and production capacity construction of shallow shale gas in the Zhaotong demonstration zone have great reference significance for shallow shale gas exploration and development in other areas.
基金Supported by the National Natural Science Foundation of China(41872124,42130803)Sinopec Key Scientific and Technological Project(P20046)。
文摘The enrichment characteristics of deep shale gas in the Ordovician Wufeng-Silurian Longmaxi formations in the Sichuan Basin and its surrounding areas are investigated through experiments under high temperature and high pressure,including petrophysical properties analyses,triaxial stress test and isothermal adsorption of methane experiment.(1)The deep shale reservoirs drop significantly in porosity and permeability compared with shallower shale reservoirs,and contain mainly free gas.(2)With higher deviatoric stress and axial strain,the deep shale reservoirs have higher difficulty fracturing.(3)Affected by structural location and morphology,fracture characteristics,geofluid activity stages and intensity,deep shale gas reservoirs have more complicated preservation conditions.(4)To achieve the commercial development of deep shale gas reservoirs,deepening geological understanding is the basis,and exploring reservoir simulation technology befitting the geological features is the key.(5)The siliceous shale and limestone-bearing siliceous shale in the Metabolograptus persculptus-Parakidograptus acuminatus zones(LM1-LM3 graptolite zones)are the high-production intervals for deep shale gas and the most favorable landing targets for horizontal drilling.Deeps water areas such as Jiaoshiba,Wulong,Luzhou and Changning with deep shale reservoirs over 10 m thickness are the most favorable areas for deep shale gas enrichment.It is recommended to carry out exploration and development practice in deep-water shale gas areas deposited deep with burial depth no more than 5000 m where the geological structure is simple and the shale thickness in the LM1-LM3 graptolite zone is greater than 10 m.It is better to increase the lateral length of horizontal wells,and apply techniques including high intensity of perforations,large volume of proppant,far-field and near-wellbore diversions to maximize the stimulated deep reservoir volume.
基金This study was supported by the National Natural Science Foundation of China(Nos.41690131,41572327,4127300)the Natural Science Foundation of Hubei Province(No.2019CFA028)the Program of Introducing Talents of Discipline to Universities of China(No.B14031).
文摘The difference in quartz types in shales not only affects the porosity and permeability of the rocks,but also reflects the difference in the sedimentary environments.We established the formation mechanism and numerical model of quartz in shales of Wufeng and Longmaxi formations in the Wangjiawan Section,South China,based on thin-section studies using SEM(scanning electron microscope),SEM-CL(cathodoluminescence),XRD(X-ray diffraction)and geochemical analyses.There are two types of quartz in the shales:detrital quartz and authigenic quartz.Detrital quartz is mostly silt-size,typically ranging from 10 to 60μm in size and subangular to angular monocrystal in shape,and brighter than authigenic quartz by CL intensity;authigenic quartz is present in two phases in shape:grain overgrowths and crystallite grains.Overgrowth surfaces are subhedral.Crystallite grains are typically less than 10μm in size,euhedral or subhedral monocrystal in shape.Authigenic quartz can be subdivided into biogenic quartz and clay mineral transformed quartz according to the source of silicon.In the numerical model,the content of detrital quartz is relatively consistent(20%);the content of biogenic quartz ranges from 40%to 70%,with a sharp fall(0–30%)in the Guanyinqiao mudstone.During the Katian,a lower anoxic and dense water column make the dissolution of biogenic silica well preserved.Biogenic quartz is the major contributor to the sediment.During the early Hirnantian interval,due to the drop of sea level and the oxygenation of seafloor,the sediment is mainly composed of clay transformed quartz and detrital quartz.During the latest Hirnatian and Rhuddanian,rapid sea level rise and anoxic ocean enhance the preservation of the biogenic silica,thereby biogenic quartz re-emerges as the major contributors to the sediment.Authigenic crystallite grains and grain overgrowths have filled in primary pore space and have decreased the interparticle porosity,however,as a rigid framework,they can suppress compaction and maintain the internal pore structur
文摘Acoustic wave velocity has been commonly utilized to predict subsurface geopressure using empirical relations.Acoustic wave velocity is, however, affected by many factors. To estimate pore pressure accurately, we here propose to use elastic rock physics models to understand and analyze quantitatively the various contributions from these different factors affecting wave velocity. We report a closed-form relationship between the frame flexibility factor(γ) in a rock physics model and differential pressure, which presents the major control of pressure on elastic properties such as bulk modulus and compressional wave velocity. For a gas-bearing shale with abundant micro-cracks and fractures, its bulk modulus is much lower at abnormally high pore pressure(high γ values) where thin cracks and flat pores are open than that at normal hydrostatic pressure(low γ values) where pores are more rounded on average. The developed relations between bulk modulus and differential pressure have been successfully applied to the Upper Ordovician Wufeng and Lower Silurian Longmaxi formations in the Dingshan area of the Sichuan Basin to map the three-dimensional spatial distribution of pore pressure in the shale, integrating core, log and seismic data. The estimated results agree well with field measurements. Pressure coefficient is positively correlated to gas content. The relations and methods reported here could be useful for hydrocarbon exploration, production, and drilling safety in both unconventional and conventional fields.
基金The work was supported by supported by the National Science and Technology Major Project of China(2016ZX05060)the National Natural Science Foundation of China(No.41602147,No.41702149)the China Postdoctoral Science Foundation(2017M622559).
文摘It is essential to investigate shale lithofacies distribution and controlling factor of the shale for geological evaluation of shale gas exploration and development.Through comprehensive analysis of cores,thin sections,cathode luminescence,whole-rock X-ray diffraction,element capture spectroscopy,major/trace element and other data,three major types and eight sub-type shale lithofacies in the shale of Wufeng Formation-Member 1 of Longmaxi Formation in Jiaoshiba area are identified by the three-end-member method and shale lithological classification nomenclature,and the spatiotemporal distribution law and main development controlling factors of shale lithofacies are well studied.In the Jiaoshiba area,vertically,the marine shale develops siliceous shale,mixed shale and argillaceous shale from bottom to top.Besides,lateral distribution of the shale is different from north to south;the shale lithofacies in the north area changes rapidly,the mixed shale in the north area is much thicker than that in the south area,while the siliceous shale in the south area is relatively thicker.Difference in the shale lithofacies is controlled by special sedimentary geologic events;development of the siliceous shale is controlled by the Ordovician-Silurian global volcanic event to some extent,while the mixed shale is significantly influenced by effect of bottom current,and the argillaceous shale is mainly affected by supply of terrestrial clastic material.
基金Supported by the"14th Five-Year Plan"Major Special Project of the Science and Technology Management Department of RIPED(2021DJ1901).
文摘For black shales,laminae and bedding are hard to identify,grain size is difficult to measure,and trace fossils do not exist.Taking the Ordovician Wufeng–Silurian Longmaxi shale in southern Sichuan Basin,China,as an example,the types,characteristics and models of microfacies in epicontinental shale are analyzed by means of full-scale observation of large thin sections,argon-ion polishing field emission-scanning electron microscopy(FE-SEM),and kerogen microscopy.The epicontinental sea develops delta,tidal flat and shelf facies,with black shale found in microfacies such as the underwater distributary channel and interdistributary bay under delta front facies,the calcareous and clayey flats under intertidal flat facies,the calcareous and clayey shelfs under shallow shelf facies,the deep slope,deep plain and deep depression under deep shelf facies,and the overflow under gravity flow facies.Basinward,silty lamina decreases and clayey lamina increases,the grain size changes from coarse silt to fine mud,the silica content increases from about 20%to above 55%,the carbonate and clay minerals content decreases from above 40%to around 10%,and the kerogen type changes from type II2 to type II1 and type I.Provenance and topography dominate the types and distribution of shale microfacies.The underwater distributary channel,interdistributary bay,clayey flat,clayey shelf,and overflow microfacies are developed in areas with sufficient sediment supply.The calcareous flat and calcareous shelf are developed in areas with insufficient sediment supply.The deep shelf shale area is divided into deep slope,deep plain,and deep depression microfacies as a result of three breaks.The formation of epicontinental shale with different microfacies is closely related to the tectonic setting,paleoclimate,and sea level rise.The relatively active tectonic setting increases the supply of terrigenous clasts,forming muddy water fine-grained sediment.The warm and humid paleoclimate is conducive to the enrichment of organic matter.The rapid sea level ri
基金Supported by the PetroChina Science and Technology Project(2022KT1205).
文摘The relationship between fracture calcite veins and shale gas enrichment in the deep Ordovician Wufeng Formation-Silurian Longmaxi Formation (Wufeng-Longmaxi) shales in southern Sichuan Basin was investigated through core and thin section observations, cathodoluminescence analysis, isotopic geochemistry analysis, fluid inclusion testing, and basin simulation. Tectonic fracture calcite veins mainly in the undulating part of the structure and non-tectonic fracture calcite veins are mainly formed in the gentle part of the structure. The latter, mainly induced by hydrocarbon generation, occurred at the stage of peak oil and gas generation, while the former turned up with the formation of Luzhou paleouplift during the Indosinian. Under the influence of hydrocarbon generation pressurization process, fractures were opened and closed frequently, and oil and gas episodic activities are recorded by veins. The formation pressure coefficient at the maximum paleodepth exceeds 2.0. The formation uplift stage after the Late Yanshanian is the key period for shale gas migration. Shale gas migrates along the bedding to the high part of the structure. The greater the structural fluctuation is, the more intense the shale gas migration activity is, and the loss is more. The gentler the formation is, the weaker the shale gas migration activity is, and the loss is less. The shale gas enrichment in the core of gentle anticlines and gentle synclines is relatively higher.
基金supported by the National Natural Science Foundation of China(Nos.42172148,41830431,and 41902127).
文摘The Ordovician-Silurian Wufeng and Longmaxi Shale in the Sichuan Basin were studied to understand the genesis and diagenetic evolution of carbonate minerals and their effects on reservoir quality. The results of geochemical and petrological analyses show that calcite grains have a negative Ce anomaly indicating they formed in the oxidizing environment of seawater. The high carbonate mineral contents in the margin of basin indicate that calcite grains and cores of dolomite grains appear largely to be of detrital origin. The rhombic rims of dolomite grains and dolomite concretions with the δ^(13)C of –15.46‰ and the enrichment of middle rare earth elements were formed during the sulfate-driven anaerobic oxidation of methane. The calcite in radiolarian were related to the microbial sulfate reduction for the abundant anhedral pyrites and δ^(13)C value of –11.34‰. Calcite veins precipitated in the deep burial stage with homogenization temperature of the inclusions ranging from 146.70 ℃ to 182.90 ℃. The pores in shale are mainly organic matter pores with pore size mainly in the range of 1–20 nm in diameter. Carbonate minerals influence the development of pores through offering storage space for organic matter. When calcite contents ranging from 10% to 20%, calcite grains and cement as rigid framework can preserve primary pores. Subsequently, the thermal cracking of liquid petroleum in primary pores will form organic matter pores. The radiolarian were mostly partially filled with calcite, which combining with microcrystalline quartz preserved a high storage capacity.
基金The work was supported by the China Postdoctoral Science Foundation(No.2016M591350)the National Natural Science Foundation of China(No.41202103)Sinopec Scientific and Technological Project(G5800-14-ZS-KJB158).
文摘Formation of organic-rich shale was controlled by depositional processes and environment condition.According to petrology,mineralogy,fossil composition and sedimentary structure,seven lithofacies of shale were identified in the Wufeng Formation to Longmaxi Formation,e.g.,siliceous shale,silty shale,argillaceous shale,calcareous shale,shell-bearing argillaceous limestone/calcareous mudstone,siltstonefine sandstone and bentonite.For shale in the Wufeng and Longmaxi Formations,the depositional mode was not only the suspension deposition under the low-energy hydrodynamic condition,but also the biogenic deposition,the storm deposition and the bottom current deposition indicated by large amount of siliceous biological fossils,biological detritus with size graded structure,silt laminations,lenticular bedding,ripple cross-bedding and other sedimentary structures.Trace element analysis suggested that the siliceous shale in theWufeng Formation and the lower part of Longmaxi Formation was developed in the oxygen-dysoxic/anoxic depositional environment,which was rich in organic matters;then due to influence of the sea level gradually decline and bottom current,the silty and argillaceous shales were formed,and the oxygen-dysoxic/anoxic depositional environment was destroyed,gradually leading to the worse preservation condition of organic matter.
基金The work was supported by National Science and Technology Major Project of China(2016ZX05061-001)the National Natural Science Foundation of China(No.41202103).
文摘According to data of gas wells and typical sections of Wufeng Formation and Longmaxi Formation in Sichuan Basin,shale of various graptolite zones were analyzed to determine depositional environment,lithology and thickness characteristics of the graptolite shale interval of WF2-WF3 in the lower part of Wufeng Formation,the graptolite shale interval of WF4 in Guanyinqiao Member of Wufeng Formation and the graptolite shale interval of LM1-LM4 in the bottom of Longmaxi Formation,and characteristics of shale horizontal distribution were also investigated.During the depositional period of the graptolite shale interval of WF2-WF3,the study area was less affected by the Guangxi movement,the depositional environment was the deep water of open sea,where black shale was mainly deposited;the sedimentation center was developed in northeast Guizhou-northeast Sichuan and south Sichuan,the maximum thickness was from 4 to 6 m in the sedimentation center.During the depositional period of the graptolite shale interval of WF4,the depositional environment in the study area changed greatly due to global sea level fall and enhanced Guangxi movement;the central Sichuan paleouplift,the central Guizhou paleouplift and the Jiangnan-Xuefeng palaeouplift were further expanded,and the area of the sedimentary basin decreased;the depositional environment was mainly carbonate bioclastic shoal of shallow sea,and partially deep sea which only was distributed in the Shizhu-Fuling-Wuxi area in east and northeast Sichuan and the Gongxian-Yongchuan area in south Sichuan;sediments of shallow water were dominated by limestone and argillaceous limestone with abundant Hirnantia,sediments of deep water were dominated by calcareous mudstone and shale with Hirnantia.During the depositional period of the graptolite shale interval of LM1-LM4,due to rise of global sea level and Guangxi movement,the sedimentary area was larger than that in the depositional period of Guanyinqiao Member,and the sedimentary environment mainly was stagnant deepwater;thickness of bla
