| At present,global energy consumption is growing rapidly,and conventional oil and gas can no longer meet the growing demand.Shale gas is an important part of unconventional oil and gas,and its resource reserves are equal to those of conventional natural gas.easy to find and difficult to exploit,low oil recovery,long production cycle and so on.China has become the third country to successfully commercialize shale gas after the United States and Canada.Although the exploration and development of shale gas in China started relatively late,it has entered a golden period of commercial exploitation after more than ten years of development.Chinese had experienced multi-stage complex tectonic evolution and formed three types of multi-layer shale:Marine facies,continental facies and marine-continental transitional facies.Marine organic-rich shale is widely distributed in Yangtze plate(Sichuan basin and its periphery),Tarim plate and North China plate,with the characteristics of old age,wide distribution,large stratigraphic thickness,high TOC content,high maturity and so on.Four sets of Paleozoic high quality marine source rocks are developed in South China: lower Cambrian,Upper Ordovician-Lower Silurian,Lower Permian and Upper Permian.The Niutitang Formation in the early Cambrian is one of the high-quality marine hydrocarbon source rocks in southern China,and it is also the target stratum for shale gas exploration,which is widely distributed in Sichuan,Guizhou,Hunan,Chongqing and other places.Predecessors have made important progress in the study of shale distribution,paleontology,lithofacies Palaeogeography,polymetallic deposits,sedimentary environment,shale gas exploration and so on.The study of shale gas exploration in the Niutitang Formation mainly focuses on the controlling factors of organic matter enrichment and the characteristics of shale reservoir(porosity,permeability,mineral composition,physical properties,etc.).However,little is known about the type of organic matter,the composition of hydrocarbon-generating organisms and the role of silica in the process of organic matter enrichment in the Niutitang formation.Therefore,this paper takes the Niutitang Formation in Weng'an,Guizhou Province as the main research object.the types of organic matter,hydrocarbon-generating organisms,the relationship between silica and organic matter and the role of silica in the enrichment process of organic matter were studied and discussed by means of palynofacies,geochemistry and scanning electron microscope.The main conclusions of the study are as follows:1.Amorphous organic matter(AOM)is the main type of organic matter in the Niutitang Formation,and its relative content is more than 90%.According to palynofaciese analysis and scanning electron microscope observation,AOM can be further divided into porous AOM,other organic matter particles,flocs particles,bitumen and AOM mixed with minerals.AOM mixed with minerals is the most common and abundant type in protolith,including organic matter-clay polymer and organic matter-mineral polymer,among which organic matter-quartz polymer is the most abundant.2.The results of CONISS clustering and principal component analysis(PCA)show that the Niutitang Formation can be divided into three palynofacies zones:Leiosphaeridia-Micrhystridium-fungal zone,algae development zone and fungal zone.Leiosphaeridia and Micrhystridium are the main contributors to organic matter.Combined with previous studies,it is inferred that the hydrocarbon-forming organisms of the Niutitang Formation are micro-planktonic algae such as green algae and dinoflagellate,which are related to Leiosphaeridia and Micrhystridium.3.Palynofaciese analysis and geochemical analysis show that the deposition of the Niutitang formation has experienced three stages: transgression,high water level and retrogression.The siliceous rock in the lower part of the Niutitang Formation was deposited in the anoxic transgression stage affected by hydrothermal solution;the shale in the middle part was deposited in the anoxic,low-energy and high-water stage where algae flourished,and the organic matter was enriched;in the upper shale,due to the decrease of sea level,the input of terrigenous clastic increased,the oxygen content in water also increased,and the content of organic matter showed a downward trend.4.Four types of quartz,namely,terrigenous clastic quartz,siliceous organisms,tiny quartz crystals and quartz polymers,can be observed in the Niutitang Formation shale under rock slices and scanning electron microscopy,among which quartz polymers are often mixed with organic matter.it's also the most abundant type.The input of terrigenous clastic quartz is relatively large in the upper part of Niutitang Formation(zone III)in Weng'an,Guizhou,and the silicon comes from terrigenous detritus and authigenic quartz.,while the tiny quartz crystals and quartz polymers in the middle(zone II)and the lower part zone I are derived from radiolarians and sponges.5.The statistics of 20 published profiles of Niushitang formation show that the TOC of Niutitang Formation in transitional facies zone and deep-water basin facies zone is higher than that in shallow-water platform facies belt.There is a linear positive correlation between biogenic silicon and TOC in most profiles,but parabola in Weng'an section of Guizhou Province.When the content of biological silicon is more than 65%,TOC decreases with the increase of biological silicon content,indicating that in the process of organic matter enrichment,the higher the biological silicon,the better.Through the integrated propulsion tree(ABT)analysis,it is found that the importance of biogenic silicon in the Niuzhitang formation in Weng'an,Guizhou is higher than that in the sedimentary environment and terrigenous clastic input.6.The mechanism of the effect of biological silicon on organic matter deposition includes biological protection mechanism,biological loading mechanism and polymerization mechanism.In the biological protection mechanism,the solution-resistant sponge may be more beneficial to protect its own organic matter than radiolaria;in the biological loading mechanism,radiolaria can be used as the loading biological mineral or algae host of organic matter aggregate.by increasing density and gravity,organic matter can be rapidly deposited on the seafloor and promote the enrichment of organic matter on the seafloor.In the polymerization mechanism,the interaction between biological silica colloid and organic matter can form larger colloidal particle deposition.However,the deposition of a large amount of biological silicon will not only dilute the content of organic matter,but also destroy the formed polymer of organic matter and play a role of depolymerization.Therefore,when the content of biological silicon exceeds the threshold range,it will not be conducive to the enrichment of organic matter.7.Clay minerals have a large surface area,so organic matter can be preserved in interlayers or pores by adsorption,and finally deposited to form organic-clay polymer;silica gel formed by silica biodissolution and organic matter polymerization deposition can form organic-quartz polymer.The organic-mineral polymer is characterized by the scattered distribution of organic matter with quartz,clay minerals and other particles,which may be formed by the deposition of various particles bonded by algae extracellular polymer(TEP)or polysaccharides. |
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