| Shale gas is a self-generated and self-storing unconventional gas reservoir,which integrate key accumulation factors of source rocks,reservoirs and caprocks in the same shale formation.Shale reservoirs are usually characterized by low porosity and low permeability,thus,it is important to find out the physical characteristics of shale reservoirs such as brittle characteristics and pore structure,in order to reduce the exploration risk.Quartz is the most important rock-forming mineral of shale.Large numbers of previous research results have confirmed that accurate evaluation of the characteristics of brittle minerals represented by quartz is of great significance to strategic selection for shale gas exploration area.The Lower Cambrian Niutitang Formation shale is the key stratum of shale gas exploration and development in southern China,and therefore we selected the Lower Cambrian Niutitang Formation shale in western Hunan and Hubei as the research object.By utilizing XRD,major and trace elements testing and SEM-CL-EDS,etc,we anatomized the type,distribution and formation mechanism of quartz in the Niutitang Formation,and discussed the effect of authigenic quartz on the physical properties of shale reservoirs.The main results achieved and understanding are as follows:1.Based on the samples observation of seven field profiles and one drilling well,shale samples from GCC section,ND1 well and YWT section were studied in this paper.The sedimentary environment,provenance and tectonic setting of Niutitang Fromation were investigated,and then the formation mechanism of organic-rich shale in Niutitang Formation was proposed:Redox environment,hydrothermal activity and upwelling ocean currents all affect the enrichment of organic matter in Niutitang Formation shale.In the early Cambrian,global sea level rise,which resulted in extensive transgression in Yangtze area,provided conditions for the formation of regional shale in the area.Submarine hydrothermal activities provide essential nutrients for biological life activities.Upwelling currents carry nutrients from deep seabed to surface seawater,resulting in the flourishing of plankton.At the same time,due to the decomposition of organic matter,a large amount of free oxygen is consumed,which aggravate ocean anoxia and in turn is conducive to the burial and preservation of a large number of organic matter.Under the combined action of hydrothermal activity and upwelling ocean currents,the organic-rich shale of Niutitang Formation was deposited in a stable anoxic reduction environment,forming a regional organic-rich black shale in Western Hunan and Hubei.2.The types of quartz in Niutitang Formation were identified by optical microscopy and SEM-CL-EDS.Based on the sedimentary environment of Niutitang Formation,the petrological evidence of quartz and geochemical data of shale samples,the genesis of different types of quartz were studied.Finally,the distribution of quartz and its controlling factors in the Niutitang Formation shale in the study area were concluded and the formation models of different types of quartz are established:?1?The type and origin of quartz:Based on the morphology of quartz,five different types of quartz are identified from Niutitang Formation shale.Type I occurs as angular to sub-rounded silt-size quartz grains with uniform luminescence,indicative of detrital origin.Type II appears as relatively large quartz aggregates composed of numerous microcrystalline quartz,which is usually filled in the spongy spicules.They are of authigenic origin,because they show dull luminescence in SEM-CL image.The petrological evidence and geochemical indicators show that type II quartz in GCC section and ND1 well was formed by local re-crystallization of biogenic silica during early diagenesis,but type II Quartz mixed with clay minerals filled in the spongy spicules in YWT section may be a hydrothermal source.Type III occurs as quartz overgrowths that are essentially non-luminescent at the periphery of high luminescence detrital quartz grains.Petrological and geochemical evidence indicate that this type quartz are derived from precipitation of silica released during the smectite to illite dissolution–precipitation reaction.Type IV quartz are dominated by sub-micron particles,some of which can reach several microns in size.This type quartz in GCC section represents microcrystalline quartz embedded in the illitized clay matrix as short chains or small clusters/nests;this type quartz in ND1 well mainly distributes among the detrital quartz grains,resulting in contiguous distribution of quartz grians.Type IV quartz in GCC section and ND1 well are derived from recrystallization of silicon released during transformation of clay minerals,but they may be of hydrothermal origin in YWT section.Type V quartz are composed of phantocrystalline microcrystalline quartz and cryptocrystalline nanocrystalline siliceous particles.Some of phantocrystalline microcrystalline quartz represent as solid or hollow spherical quartz or concentric circular quartz.Type V quartz is of hydrothermal origin and the spherical quartz and concentric quartz may be formed by siliceous hydrothermal deposits filling in algae cysts.?2?Distribution regularity of quartz and its controlling factors in Niutitang Formation shale:Compared with argillaceous shale,siliceous shale in GCC section has higher biogenic quartz content and lower diagenetic quartz content,but higher total authigenic quartz?biogenic quartz+diagenetic quartz?.The content of authigenic quartz in ND1 well increases first and then decreases from bottom to top.The content of authigenic quartz is the highest in YWT section.The distribution of authigenic quartz is controlled by the distance from terrigenous provenance and hydrothermal activity.?3?The formation models of different types of quartz:After sponges died,its skeletons quickly disintegrate into scattered spicules,as the result of sponge body decomposition in the sedimentary stage.In the early diagenetic stage,opal-A was transformed into opal-CT,and finally into microcrystalline quartz aggregate?biogenic quartz?,which filled in the sponge spicules.In the late diagenetic stage,with the increase of burial depth and temperature,smectite transforms into illite and releases a large amount of silica into pore water.The extremely low permeability in mudstone limits fluid-flow transport and diffusion,which favors local precipitation of quartz.In addition,during the late diagenetic stage,a large amount of silica imported by hydrothermal activity recrystallized into microcrystalline quartz?hydrothermal quartz?,filling between clastic minerals.The formation of authigenic quartz blocked the primary pore,but as a rigid framework,it was beneficial to the preservation of organic pore and clay mineral pore and the formation of secondary intergranular pore.3.The pore types and pore structure parameters of siliceous shale and argillaceous shale in Niutitang Formation are investigated,by utilizing argon ion polishing-scanning electron microscope,low pressure N2 and CO2 Adsorption,etc:?1?High-resolution SEM images show that The siliceous shale in shallow water section?GCC section?is dominated by intergranular pore,while the siliceous shale in deep water section?YWT section?is dominated by intergranular pore and organic pore.As for argillaceous shale,the intraparticle pores within clay minerals are dominated with few organic matter pores and interparticle pores are not so developed since the shale is condensed.Reasons for more organic matter pores in deep water profile than shallow water profile are related to its sedimentary environment and authigenic quartz content.Due to the influence of hydrothermal activity,the deep water profile has high organic matter content and a large number of authigenic quartz.Authigenic quartz is benefical to the preservation of organic pore and clay mineral pore and the formation of secondary intergranular pore.Therefore,organic pore is developed in deep water profile.The reasons that the argillaceous shale is more compact and more cemented is related to its sedimentary environment,mineral composition and diagenesis.?2?According to different theoretical models,shale pore structure parameters?pore specific surface area,pore volume and pore size distribution?are obtained based on N2 and CO2 adsorption.For mesopore size distribution,the pores with diameter range from 2nm to 10nm are most abundant in siliceous shale and argillaceous shale.For micropore size distribution,two peaks are observed and it presents similar distribution in siliceous shale and argillaceous shale.Compared with argillaceous shale,the pore volume and pore specific surface area of siliceous shale is higher except for the sample ND1-28 because of its low TOC content.The contribution of micropore and mesoporous to total pore specific surface area are similar,but the contribution of mesoporous to total pore volume is much higher than that of micropore.4.The brittleness of Niutitang Formation shale in the study area was evaluated based on mineral components.The author clarifies that the precipitation of authigenic quartz can greatly improve the brittleness of shale:The brittleness index BI1(W?quartz+feldspar+carbonate?/W total)indicates that siliceous shale in GCC profile is more brittle than argillaceous shale,and The brittleness index of siliceous shale in ND1 well and YWT profile is much higher.The comprehensive analysis of the brittle of Niutitang Formation shale in ND1well show that the content of authigenic quartz is consistent with the trend of the log curves GR,AC,DEN and total gas content,indicating that the shale with high content of authigenic quartz has high brittleness.Therefore,this paper point out that a new method to evaluate the shale brittleness,calculating another brittleness index BI2 based on the percentage content of authigenic quartz.The results show that shale brittleness is closely related to authigenic quartz content,and the section with high authigenic quartz content has high brittleness index.5.Based on the comprehensive research of the genesis and pore development characteristics of the Niutangtang Formation shale in the study area,it is clear that the precipitation of authigenic quartz has a positive effect on the pore development of the Niutitang Formation shale:The structure parameters of mesoporous and microporous,and the porosity in siliceous shale are positively correlated with the content of authigenic quartz,indicating that the precipitation of authigenic quartz is conducive to the preservation of primary pores and the formation of secondary pores.In addition,the microporous structure parameters of siliceous shale are also positively correlated with the organic carbon content,suggesting that the enrichment of organic matter plays an important role in the formation of shale micropores.The mesoporous and microporous structural parameters of argillaceous shale are positively or negatively correlated with illite content,which suggests that the pore structure of argillaceous shale is mainly controlled by illite content. |
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