Natural Gas Dissipation And Uranium Mineralization In The Northern Ordos Basin

Gas of Upper Paleozoic and uranium deposits of Middle Jurassic coexist and are closely related to occurrence,distribution and genesis in the northern Ordos Basin.In this study,I focus on the background of gas accumulation,dissipation and uranium mineralization,the process of gas-dominated hydrocarbon-rock interaction and ore-forming environment of the sandstone-hosted uranium deposits.These aspacts are beneficial to bulid the models of gas accumulation and uranium mineralization in the northern Ordos Basin,also to reveal the relationship between gas and uranium mineralization.Using a large number of field profiles,seismic refectance data,fission track thermochronology,drill core observation,mineralogy and geochemistry,I investigate the structural evolution during the formation of gas and uranium deposits,the characteristics of gas dissipation,gas and rock ineration in surficial conditions,fluid evolution and physical-chemical characteristics of the ore-forming fluids in the northern Orods Basin.The relationship between natural gas and uranium minerlization is also discussed.Stratigraphic observation and apatite fission track thermochronology indicate that the northern Ordos Basin experienced three significant uplift and exhumation events occurred during latest Triassic to early Juassic,late Jurassic and late Cretaceous to Miocene,repectively.The exhumation during latest Triassic to early Juassic mainly influenced the triangle aera(extending in Fugu-Dongsheng-Zhungeer)in the northeastern part of the Ordos Basin.Northwest-trending deformation structure developed during this tectonic event.During late Jurassic,the intensly titling and exhumation occurred mainly in the northern margin of the basin.The deformation during these two events mainly developed along and parallel to the paleo-margins of the Ordos Basin,but the deformation did not extend into the rigid basin inerior.The rapid uplift and cooling time of the northern Orods Basin was 100~20 Ma since late Cretaceous.The magnitude of the exhumation event decreased from northeast to southwest,which introduced a gentle monocline dipping to southwest in the northern basin.The latest uplift stage was the main stage of gas dissipation and uranium mineralization.There are obvious differences in gas reservoir type and accumulation models between the northern part of the Yishan Slope and the Yiming Uplift.The reservoirs of the Upper Paleozoic in the northern part of the Yishan slope are unconventional tight sandstones with extremely low permeability.The gas accumulation is dominated by short-distance and vertical migration.The gas geochemistry is mainly controlled by the source rock type and maturity.The porosity and permeability of Upper Paleozoic reservoirs in the Yimeng Uplift are obviously improved,and the gas generated from the southern part of the Yimeng Uplift can migrate to the northerest during accumulation and dissipation.The hydrocarbon traps are conventional structural and stratigraphic traps in the Yimeng Uplift.The source rocks,coal measures,generated a large amount of CO2 at the peak of gas generation.CO2 had easily dissolved into the formation water and migrated with natural gas to the northeast and caused large-scale carbonate cementation and veins during the latest uplift.Gas migration and dissipation were mainly along faults and unconformities with high permeability layers.Therefore,the bleached sandstones mainly developed in the weathered crust beneath unconformities.The lower part of the Middle Jurassic Zhiluo Fomation has also experienced significant gas-rock interaction due to its good porosity and permeability.Diagenesis,fluid inclusions,in-situ geochemistry of uranium minerals,cabon isotope of calcite cements,whole-rock geochemistry were intergrated to reveal the fluid evolution and genetic conditions of the uranium deposits in the northern Ordos Basin.The results show that the sandstone-hosted deposits are not typical meteoric related redox genesis.The ore-bearing fluids exhibit two distinct types:low-salinity cold meteoric waters and high-salinity hydrothermal fluids.Hydrothermal fluids from Upper Paleozoic gas reservoirs were enriched in carbonate ligands,and concentrated uranium during leaching and transport.With the formation of Hetao Graben to the north,the circulation of gas and hot brines increased and the deposits entered the hydrothermal mineralization stage.Natural gas from Upper Paleozoic coal measures had three important effects on the formation of the sandstone-hosted uranium deposits in the northern Ordos Basin:(1)as reductant for uranium reduction;(2)accomponied hydrothermal fluids can extract uranium during transport and caused hydrothermal mineralization(CO2 from thermal maturation of source rocks generated the carbonate ligands);(3)protected uranium ore from remobilization by later oxidizing fluids.