| Through comparative studies of petrology,mineralogy,geochemistry and multi-scale pore structure analysis of organic-matter-rich black shales from core and outcrop samples from the Wufeng-Longamxi Formation in the Sichuan Basin,the following conclusions and understanding are obtained:(1)The thickness of the organic-matter-rich black shales(TOC>2.0%)of the Wufeng-Longmaxi Formation in the Sichuan Basin is controlled by the uplift-depression framework,with a large depositional thickness in the depression center and thins towards the ancient uplift.In the Weiyuan and Changning areas,the organic-matter-rich black shales were deposited in the depression zone between the Guizhou uplift and the central Sichuan uplift.The main lithology is black calcareous shales,black silty shales and a small amount of black siliceous shales.In Jiaoshiba area,the organic-matter-rich black shales were deposited in the depression between the Xufeng uplift and the central Sichuan uplift.The main lithology is black siliceous shales and black silty shales.(2)There are variations in the mineral composition,spatial and temporal distribution and thickness of the black shales of the Wufeng-Longamxi Formation.In the Changning and Weiyuan areas,there is a relatively higher carbonate mineral content.In the Jiaoshiba area,the quartz content is relatively high and appears to be positively correlated with the TOC.Such a correlation is not found in the black shales in the Weiyuan and Changning areas.In the Weiyuan area,the organic-matter-rich black shales mainly occur in the middle and lower part of the Longmaxi Formation,with a thickness around 25 m,and are not developed in the Wufeng Formation.In the Changning and Jiaoshiba areas,the organic-matter-rich black shales mainly occur in the Wufeng Formation and the lower section of the Longmaxi Formation,with thicknesses around 30 m and 40 m respectively.These differences are mainly a result of the migration of the sedimentary center caused by the migration of the ancient uplift.(3)The development and migration of the ancient uplift during the sedimentary period of the Wufeng-Longmaxi Formation have an important influence on the sedimentary pattern of the Upper Yangtze Oceanic Basin.This resulted in different geochemical characteristics of the organic-matter-rich black shales of the Wufeng-Longmaxi Formation in different regions,indicating differences in the paleoproductivity,redox condition,water restriction and burial efficiency.The primary productivity,redox environment and burial efficiency of the Weiyuan area were less optimal,but the water restriction was stronger than in the Changning and Jiaoshiba areas.These differences led to the different organic-matter enrichment modes in different areas.In the Weiyuan area,organic-mater enrichment was mainly due to water retention,which led to the anoxia of the bottom water,thus the increase of the burial efficiency of organic matter.The organic-matter enrichment in the Jiaoshiba area was mainly due to high biological productivity in the semi open-open marine environment,coupled with bottom anoxia of deep water.The Changning area is the intermediate of the two modes.(4)The black shales in the Wufeng-Longmaxi Formation mainly developed organic pores,inorganic pores and fractures,among which the organic pore is the main pore type.The pore characteristics are closely related to the rock facies.The inorganic pores are less developed across different shale lithofacies,with most original intergranular pores filled with organic matter.The difference of organic matter occurrence status affects the distribution of organic matter pores: the organic matter of calcareous shales is mainly in the form of granules and have less migrated organic matter.The organic matter of siliceous shales is mainly composed of mass granules and intergranular fillings.The organic matter of silty shales is mainly composed of intergranular fillings.Micropores and mesopors are developed in both the black calcareous shales and the black siliceous shales,and the total pore volume of mesopores is slightly larger than that of the micropores.In silty shales,the total pore volume of mesopores is much larger than that of micropores and macropores.(5)The diagenesis of the black shales influences the original pore state prior to kerogen maturity,thus affecting the distribution of migrated organic matter.The thermal evolution of organic matter mainly influences the formation and distribution of organic pores.There are four characteristics of the organic pores:(1)The organic pore is round and pore diameter is large;(2)The organic pore is flat;(3)The organic pore is dry and fissured;(4)The organic pore is small and numerous.(6)The organic-matter-rich black shales in the Wufeng-Longmaxi Formation have experienced substantial early stage burial,followed by significant late stage uplift and erosion and strong deformation.Changes of temperature and pressure of the shales caused changes in shale pore structure and diameter.In black shales that has undergone tectonic deformation,organic pores are smaller,pore volume and pore specific surface area of isothermal nitrogen adsorption is small,and mesopores are not developed.(7)The pore structure is controlled by shale lithofacies,thermal evolution of organic matter and tectonic uplift.Shale lithofacies and diagenesis affect the original pore morphology and distribution before the maturity of rich organic shale,thus affecting the distribution and structure of the migrated organic matter.Thermal evolution of the organic matter mainly influences the formation and distribution of organic pores.The significant uplift and erosion and strong deformation affect the pore size and porosity of the organic matter.The pore size and porosity of the organic pores could to some extent reflect the preservation condition of the initial pressure.The sooner the original abnormal pressure in the process of oil cracking is released,the smaller is the organic pore size and the lower the porosity.Therefore,the size of the organic pores can to certain extent reflect the quality of the shale preservation condition.(8)The inorganic pores of black shale in the Jiaoshiba,Weiyuan and Changning area are similar,which are mainly intergranular pores between clastic particles and intragranular pores in the carbonate particles.Because of the differences of the carbonate mineral content,intragranular pores are relatively developed in the Weiyuan and Changning area.The thermal evolution process of Weiyuan,Changning and Jiaoshiba area is similar and have all reached an overmature stage,therefore organic pores were developed.The differences among them are the pore morphology and pore size of the organic matter,which were caused by the differences of tectonic deformation in the uplift process. |
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