| The black shale of the Upper Ordovician and Lower Silurian in the Southern Sichuan,with high maturity and total organic matter content,is a significant target for shale gas exploration.Few studies have investigated the origin of fragile mineral and its effect on shale gas reservoirs.The study of fine grain deposit process is not enough,and hinders the exploration and development of shale gas.In this thesis,several methods including field sections,geochemical analysis,core observation,Ar-ion-beam-SEM,logging data high-pressure mercury intrusion,gas expansion methods were used to investigate the petrology,geochemistry,reservoir geology of black shale in Southern Sichuan.The black shale is predominantly composed of quartz,clay,calcite and dolomite with a small amount of feldspar and pyrite.The quartz is of detrital and authigenic origin.Detrital quartz shows clear boundaries,monocrystals and subrounded to subangular outlines,and high-medium luminescence,its ?30Si values range from-0.34‰ to-0.64‰ and ?18O values range from 18.09‰ to 19.43‰.Authigenic quartz show a blurred edge and weak-cluminescence in scanning mono-CL electron microscopy,its ?30Si values range from-0.09‰ to 0.31‰ and ?18O values range from 18.75‰ to 22.14‰.Major elements,trace elements and Si-O isotope anylasis show authigenic quartz are biogenic source and it is maybe come from Radiolaria.Carbonate type include bioclast carbonate,authigene carbonate during diagensis and microbiogenic carbonate.Microbiogenic carbonate is closely related to sulfate reducting bacteria and it contain intraP pores which could improved storage conditions.Based on shale mineral composition and their origin,taking terrigenous silt,authigenic quartz,clay mineral,intraclast carbonate and authigenic carbonate as end member,black shale can be divided into carbonate,muddy carbonate,siliceous rocks,muddy siliceous rocks,siliceous claystone,intraclast carbonate siltstone,muddy intraclast carbonate siltstone,muddy mixed fine-grained rocks(FGR),calcareous FGR,siliceous FGR and intraclast carbonate FGR.Six kinds of sedimentary structure was identified in core and outcrops:massive struture,horizontal laminae,upward-fining laminae,wavy laminae,lenticular structure.Massive struture was developed in intraclast carbonate siltstone,carbonate,mixed FGR,siliceous rocks and claystone.Lenticular structure was found in the siliceous rocks and siliceous mixed FGR of Daozhen and Changing area.Horizontal laminae was developed in siliceous rocks,mixed FGR and intraclast carbonate siltstone.Upward-fining laminae was developed in claystone and siliceous rocks.Wavy laminae are developed in siltstone.Truncated structure are rarely found in the Well B1 which is close to Qianzhong Uplifts.Qualitative and quantitative methods were used to investigate the the reservoir space and pore structure of black shale.The SEM observations show that various types of pores occur in the shales,and the pore sizes range from several nanometers to several hundred nanometers.Based on the classification of Loucks et al.(2012),organic matter(OM)pores,intraparticle(intraP)pores and interparticle(interP)pores have been identified in the samples.InterP pores occur primarily around rigid particles or crystals,such as those of quartz,dolomite,feldspar and calcite.Their sizes range from tens of nanometers to hundreds of nanometer.IntraP pores occur primarily in carbonates,including pores formed via partial dissolution or fluid inclusions remaining after the formation of calcite and dolomite crystals,pores within framboidal pyrite,and cleavage-sheet intraP pores within clay minerals.The strong correlations between TOC and the BET surface area on the one hand and the BJH pore volume on the other indicate that the OM pores are the predominant pore system within the Wufeng and Longmaxi shales.OM pores tend to have a pore size less than 6.5nm.Intrap pores associated with carbonates could provide few mesorepores and micropores.Intrap pores associated with quartz are the main source of macropores.Siltstone deposited from a gravity flow was firstly identified in Wufeng-Longmaxi black shale of Southern Sichuan Basin,it is composed of dolomite intraclast and terrigenous quartz.Terrigenous quartz show angular-subangular shape with grain size ranging from 50 to 110 micrometer.Dolomite intraclast show similar grain size with terrigenous quartz and seriously affected by diagenesis.Grading bedding was developed in siltstone deposited from a gravity flow.Compared with the adjacent shale,a obvious increase of total gas content and free gas content was found in logging curves.According to the TOC,gas content,pore type,the quality physical property into account,shale reservoir was believed to be mainly controlled by organic matter,lithofacies,sedimentary environment and diagensis.OM contain abundent OM pore and could produce organic acid which could improve the quality of reservoir.Free gas content serious influence on the shale gas production.Siliceous rocks,muddy siliceous rocks and Siliceous mixed FGR with high TOC and silica are favorite reserviors in study area.The shale pores are strongly affected by compaction,cementation and dissolution.Compaction and cementation destroy pores and dissolution could produce secondary porosity.Mesopore volumes increase with increasing carbonate content while macropore volumes decrease due to the "pore-size controlled solubility" effect,which causes dissolved calcium carbonate to precipitate in larger macropores. |
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