Characteristics And Formation Mechanism Of Fibrous Mineral Veins In Shale

Mineral veins,such as calcite,gypsum and quartz veins with special crystal morphology are common in low-permeability argillaceous rocks in sedimentary basins and foreland belts.The mineral crystals filling in veins generally show fibrous morphology and parallel-aligned growth.This type of mineral veins is called antitaxial fibrous veins（referred to as fibrous veins）.Antitaxial fibrous veins are an important carrier for fluid activity in low-permeability shale,recording and preserving precious information on the formation and evolution of fluids in shale.In this thesis,the fibrous calcite veins in the Lower Cambrain shale of the Dabashan Belt and fibrous gypsum veins in the Neogene Qingshuiying Formation of the Ningnan Basin were selected to study the opening and dilation mechanisms,material sources and migration pattern of antitaxial fibrous veins,and the relationship between their formation in organic-rich shale and hydrocarbon generation by using detailed field survey,petrographic observations,mineralogy,elemental geochemistry and isotopic geochemistry.A comparative study on the formation mechanisms of typical antitaxial fibrous veins that were formed under different tectonic contexts was carried out.This study is of scientific significance for revealing the formation and evolution of hydrocarbon-bearing fluids,and for exploring tectonic activities and fluid-rock interaction mechanisms.Fibrous calcite veins in the Lower Cambrian organic-rich shale in the Dabashan Belt are structurally composed of a median zone and two fibrous zones.The median zone is the original location where the veins were opened,and is mainly composed of host rock fragments and minor solid bitumen.S-shaped and C-shaped fibrous calcite crystals indicate that fibrous calcite veins were subjected tectonic compressive stress during their growth.Integrated with regional tectonic evolution and burial-thermal history of the Lower Cambrian shale,it can be inferred that the opening of fibrous calcite veins is due to overpressure by hydrocarbon generation（gas generation）.The fibrous zone consists of parallel-aligned fibrous crystals that are generally inclined to the vein-wall interface.Individual fibrous calcite crystals show optical and compositional continuity without significant growth competition between neighouring calcite fibres.The rare earth elements of both fibrous calcite veins and wall rocks are characterized by deficit of light rare earth elements（LREE）,enrichment of heavy rare earth elements（HREE）,significantly positive Eu anomaly and slightly negative Ce anomaly.Theδ¹³C values of fibrous calcite veins and wall rocks are in range of-8.34‰～-2.68‰and 5.91‰～-3.03‰,with averages of-5.50‰and-4.86‰,respectively.The ⁸⁷Sr/⁸⁶Sr values of fibrous calcite veins and wall rocks range from 0.710908 to 0.715002 and 0.711544 to0.71428,averaging of 0.713112 and 0.713227,respectively.The rare earth elements,carbon and oxygen isotopes,strontium isotope and petrographic observations of both fibrous calcite veins and wall rocks show that the veining materials of fibrous calcite veins are dominantly derived from the dissolution-reprecipitation of calcite in wall rocks,and contain minor organic carbon during vein formation.The fibrous gypsum veins of the Neogene Qingshuiying Formation in the Ningnan Basin are mainly oriented to the bedding,and composed of a median zone and two fibrous zones with quite different petrographic features.The median zone consists of blocky gypsum crystals and host rock inclusions,indicating that the median zone of fibrous gypsum veins was formed by crack-seal mechanism.In the Late Miocene,the Ningnan Basin was subjected to tectonic inversion,leading to horizontal compression in the southwest to the northeast direction due to the remote effect of the uplift of the Tibet Plateau,and the fluid overpressure was gradually established in low-permeability shale of the Qingshuiying Formation,resulting in bedding-parallel hydrofracturing.At the same time,blocky gypsum crystals were precipitated in hydraulic fractures,and much host rock inclusions were trapped,forming the median zone of fibrous gypsum veins.The fibrous zone consists of parallel-aligned fibrous gypsum crystals with individual gypsum crystals containing optical continuity and lack of growth competition between adjacent gypsum fibres,indicating the absence of open fractures during gypsum fibre growth.The interaction of adjacent veins,vein tips and wall rock deformation indicate the force exerted by gypsum fibre growth plays an important role during vein dilation.The bending of median zone,difference in thickness of fibrous zones on both sides and wall rock plastic deformation occurred due to the force of crystallization during gypsum fibre growth.The Sr and S isotopic compositions of the fibrous gypsum veins and wall rocks show obvious similarity.The ⁸⁷/⁸⁶Sr values of fibrous gypsum veins and wall rocks range from 0.710242 to 0.711375 and 0.710287 to 0.711327,with averages of 0.711019 and0.711057,respectively.The?³⁴S values of fibrous gypsum veins and wall rocks are in range of 7.8‰～11.2‰and 8.6‰～11.8‰,averaging of 10.1‰and 10.4‰,respectively.The isotopic results indicate the material of fibrous gypsum veins was mainly sourced from wall rocks,and the veining material was migrated from wall rocks to the vein-wall interface by diffusion.The distribution of fibrous gypsum veins shows vertical heterogeneity,and gypsum veins mainly occur in the middle and lower parts of the Qingshuiying Formation with relatively high gypsum content,ranging from 6.7%to 42.1%with an average of 21.5%;whereas the fibrous gypsum veins in the upper part of the Qingshuiying Formation are rare,and the gypsum content of wall rocks is between 0 and 9.8%with an average of 5.4%,indicating that the distribution of fibrous gypsum veins is affected by the mineral composition of wall rocks.Based on a comparative study of two different types of antitaxial fibrous veins that formed in extensional and compressional tectonic contexts,it can be concluded that the opening of antitaxial fibrous veins in low-permeability shale is due to fluid overpressure,and hydrocarbon generation and tectonic compression are the two dominant factors for the overpressure.In the extensional context,fibrous mineral crystals are generally perpendicular to the vein wall in bedding-parallel fibrous veins,and the force of crystallization and fluid pressure are the main driving forces to drive vein dilation.In the compressional context,the mineral fibres are generally inclined to the vein wall,commonly showing S-shaped or C-shaped morphologies which indicate shear components.The main driving forces for the vein dilation are fluid pressure and force of crystallization,and tectonic compressional stress also contributes to the vein dilation.In conclusion,the foramation of antitaxial fibrous veins in shale is the result of the combined action of regional tectonic stress,fluid pressure and force of crystallization.