| The coexistence of oil,gas,coal and uranium in the same basin is universal.The Central-East Asian Multi-Energy Minerals Metallogenetic Domain?C-EAMD?is a typical multi-energy resources metallogenic belt.The Northern China is located in the middle and eastern part of this belt.The peripheral provenance areas within Northern China contain uranium-rich acid rocks that have sourced mass uranium into adjacent basins leading to the formation of typical sandstone-type uranium deposits and the infrequent coal-type and mudstone-type uranium deposits.In addition,large amounts of uranium enter the sedimentary basin with mudstone and the underground water during the depositional stage and afterwards making the source rocks in some basins rich in uranium?such as the source rocks of Chang 7 member in the Ordos Basin and the Jurassic source rocks in the Turpan-Hami Basin?.As a kind of element with strong oxidative catalysis and radioactivity,lots of researches have been done about the oxidative catalytic effects of uranium on the hydrocarbon generation in source rocks,while the impacts of radioactivity on source rocks and hydrocarbon generation is studied weakly.The physicochemical reactions between radioactivity of uranium and organic matter and coexisting fluids in source rock system rich in organic matter and uranium will happen unavoidably.In order to focus on the radioactivity effects of urnaiumn and the exogenous hydrogen on source rocks,gamma rays source(60Co)?radiation dose of gamma rays is related to uranium content?was selected to radiate samples.The selected samples are representative hydrogen-containing materials in source rock system.Samples including water,gypsum,aqueous solutions with different kinds of salts,decane,pitch,and kerogen were selected by considering the main types of hydrogen-containing materials and the possible factors influencing the hydrogen output in source rocks.Through the detection and comparative analysis of radiation products?gas,liquid and solids?,the experiment results show that:?1?The hydrogen-containing materials in source rock system,including organic matter and compounds,can produce enough hydrogen?H2?under irradiation and the amount of produced hydrogen is generally related to H-X bond energy?X represent O,C or other element?.?2?Under irradiation,hydrogen production is controlled by the total radiation dose.In the geological environment,long-time irradiation at low dose rate can also produce huge radiation dose,which in turn affects hydrocarbon generation.The total radiation dose can be used as a parameter to evaluate the contribution of subsurface radiation to hydrocarbon formation.?3?The amount of hydrogen produced by solutions irradiation is influenced by the types and concentration of ions contained in it.The addition of Na+,K+,Ca2+and Cl-ions and the increase of their concentration can greatly promote the hydrogen production of water,while oxygen-rich ion,SO42-,will suppress the product of hydrogen.The underground water is abundant in Na+,K+,Ca2+and Cl-ions,which is benefit for hydrogen formation.?4?Radiation is a chemical reaction under physical action.In a certain volume of close environment,as the hydrogen production and the pressure increases,the yield of hydrogen decreases gradually,indicating that the closed environment is not conducive to the hydrogen generation.The source rock system is semi-closed to open environment which is in favor of the hydrogen generation.?5?The ratio of H2/O2 is controlled by the concentration of the solution and the irradiation dose.When the dose is less than 1WGy,the H2/O2 ratio increase sharply with the dose;when the dose is more than 1WGy,the H2/O2 ratio is mainly influenced by concentration and with the concentration increases,the H2/O2 ratio increases.The hydrogen isotope composition is controlled by the irradiation dose rate,the salt solution concentration and the pressure above the solution.The hydrogen isotope composition becomes heavier with the increase of irradiation dose rate and the increase of gas pressure above solution,and becomes lighter with the addition and the increasing concentration of salt ions.The H2/O2 ratio and hydrogen isotope composition can be used as references for identifying the radiogenesis hydrogen in underground environments.?6?Cracking reaction and polymerization will happern in decane under irradiation leading to the production of large amounts of hydrogen and n-alkane,iso-paraffin and unsaturated hydrocarbons of C1-C20.And,the carbon isotope composition of hydrocarbon gas becomes heavier with the increase of carbon number.The irradiation of dry kerogen and pitch is a process in which the H/C and O/C ratios decrease,the branches are reduced and aromatization increases.The irradiation of kerogen is similar with the thermal maturation process.There are several factors conducive to the product and preservation of exogenous hydrogen in uranium-rich source rocks in the Ordos Basin under irradiation,including huge irradiation dose and enough hydrogen-containing materials,and high concentration ions,high content of clay minerals and small porosity,reduced and non-enclosed environment,etc.According to calculation,50ppm uranium in Chang 7 member has provided about 26 L exogenous hydrogen for per m3 source rocks since the deposition of Chang 7 member?about 200 Ma?,which must has influenced the hydrocarbon generation in source rocks.Radiation can be another energy source for hydrocarbon generation in source rocks in addition to heat.Cracking,polymerization and hydrogenation reactions will happen in source rocks rich in radioactive substance promoting the hydrocarbon generation and the production of low-mature oil and gas.This experiment verified the radiogenic exogenous hydrogen in geological environments which can guide the evaluation and exploration of oil and gas resources;it also shows that except heat,radiation can radiate source rocks directly to generate hydrocarbon in source rocks which complements and perfects the traditional theory of hydrocarbon generation.It is worth mentioning that this research also provides valuable materials for the industrial production of hydrogen. |
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