| Long to thinking of the first,the third and the five members of Xujiahe Formation only as hydrocarbon source rocks or sandstone reservoir caprock,lacking of the perspective from of shale gas evaluation.This thesis aimed at southwestern Sichuan of black shale geology,petrology characteristics,reservoir characteristics and the organic matter enrichment mechanism of these scientific problems,from the surface by the only one Well in southwestern Sichuan basin fron Jurassic drill through to Xujaihe Formation well LD-1 with emphasis on the research object,and combining with surface profilels,the center of southwest of sichuan shale center of petrology characteristics,shale reservoir characteristics and organic matter enrichment mechanism are analyzed.Xujaihe Formation shales were divided into clayey parallel layered mudstone,rich bioclastic shale,biological disturbance clayey mudstone,fine grained argillaceous hyperpycnal flow sedimentary facies,reveals the rich organic matter mudstone of the Xujaihe Formation's particle size change is big,sedimentary structure by the traction current process,biological disturbance and benthic macro biological form,presents the characteristics of shallow water oxygen enriched the aquatic environment.The microscopic chip collophane and siderite environmental implications and mineral.The third and five members of Xujiahe Formation's organic matter type is given priority to with?type,the first member?2 is given priority to,the organic matter in the maturity to high mature stage of a lot of,majority of over 2.0%,organic carbon content to shale gas accumulation of organic matter abundance lower standard.Siliceous shale content is higher,are all over 45%,is almost entirely from terrigenous clastic silicon,including the fifth member of Xujiahe Formation more than an average of 54%,followed by third member,as high as 48%,the higher the brittleness of altering the shale reservoir.On clay minerals content must be terminating five section of the core center of overall and clay minerals content of distribution of the outcrop are similar,both in the illite-smectite mixed layer absolute,all outcrop samples and core sample did not detect the existence of montmorillonite.By using scanning electron microscopy?SEM?,argon ion polishing,mineral composition and physical property analysis,pointed out that shale mainly inorganic pore,mineral dissolution pore,followed by a small amount of organic matter pore and intergranular pore,pyrite pore and organic matter are mainly from the first member.Much of clay minerals and mineral dissolution hole clastic particles intergranular and intragranular dissolution pore,irregular shape,poor connectivity.Reservoir physical properties show the characteristics of low permeability reservoirs,shale brittleness is good,have to form shale gas reservoir conditions.With the increase of buried depth,different samples of shale pore structure parameters of sedimentary environment changed significantly,characterized by shale pore BET specific surface area increased gradually,BJH average pore size decreases and the characteristics of the BJH total pore volume increased,and compared to two layer covering the multiplication of orders of magnitude.Xujiahe Formation shale deposit on the relatively shallow waters are oxide as the main background,bottom water didn't reach even exuinl of hypoxia.Use of Ba/Al,biologic barium,Sr/Al ratio and P/Ti four characterization of paleoproductivity,these indexes are linked to the TOC showed high positive correlation,suggesting that surface waters to improve primary production of organic carbon enrichment mechanism model.Using the Sr/Ba ratio,TS/TOC ratio,B/Ga ratio and Ca/?Ca+Fe?than qualitative and semi-quantitative paleosalinity recovery,the third and the fifth members are mainly freshwater sediment,but often in freshwater area-salt water area,could be the result of intermittent transgression.Sr/Cu and C value of combination of late Triassic global paleoclimate and palaeogeographic reconstruction results,show that three shale are deposited on the warm climate background.Cd/Mo-TOC discriminant diagrams(Mn,Co,Al chart?Mn[EF]*Co[EF]-Al chart and Cd/Mo-Co*Mn intersection diagram and key enrichment coefficient of trace metal elements can distinguish the degree of water retention.Shale deposited on the bottom of the second oxidation center of water environment,lead to the variety of chart effectiveness is poorer,more in the upwelling and limitations basin at the intermediate condition of two end meta model,combined with foreland basin evolution history,that the first member linked to open sea,half limitations or connected to the sea limited?third member?and limited?five member?all three stages of evolution.Shale depositional period must be family has high deposition rate,rapid deposition contribute to rapid supply and burial organic matter and be able to quickly and overlying water oxidation or sedimentary substances isolated,thus eliminating the was oxidation and degradation of organic matter.The first member of Xujiahe Formation shale's organic matter enrichment by upwelling improve primary productivity level and salinity stratification cause vertical exchange or circulation of water be blocked similar to the estuary envirorment,high deposition rate and suboxic environment togher control.The third and fifth members of Xujiahe Formation shale organic matter enrichment in addition to high levels of primary productivity,high deposition rate and the common factor such as suboxic environment,plenty of refractory carbide plant fossils such as organic carbon of vitrinite and matrix vitrinite maceral?may be even more important,so to some extent,these two shale may be independent from the ancient environmental evolution of organic matter enrichment.So to some extent,organic matter enrichment of these two shale members are likely to be independent from the paleoenvironmental evolution.TOC of mainly reflect the change of terrestrial plants and/or continental weathering,and has nothing to do with the paleoproductivity. |
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