Characteristics Of Jurassic Coal-bearing Source Rocks And Its Shale Oil Significance In Medium And Small Basins Of Gansu Province

The Jurassic is the most important coal-bearing strata for Gansu Province,even the whole northwest China.In the Gansu Province,they distribute from the Ordos Basin(Yan'an Formation,J₂y)in southwest to the Dunhuang Basin(Zhonjianggou Formation,J₂z)in west,which usually developed in the continental faulted basins.The Jurassic coal-bearing sediments in the Gansu was characterized by the high organic carbon content and thick succession which always were main target areas for coal exploration.In addition,there are high-quality terrestrial source rocks in the adjacent horizons of these coal seams.Previously,due to the limitation of conventional hydrocarbon exploration,these coal-bearing source rocks have paid little attention.Recently,breakthroughs in shale oil exploration theory and development technology have made shale oil exploration a scientific focus.Therefore,finding the best quality source rock is an important prerequisite for shale oil exploration.The Chaoshui Basin and Minhe Basin are representative Jurassic continental coal-bearing sedimentary basins in Gansu Province.The high-quality source rocks are rich in the two basins which have high potential for the future shale oil exploration.Therefore,aim to probe the potential of shale oil exploration,this study conduct a comprehensive source rock assessment for the Jurassic coal-bearing sediments in the two basins using the some routine organic geochemical and petroleum reservoir methods.In the Chaoshui Basin,the biomarker characteristics of the source rock indicate that the hydrocarbon source of these coal-bearing source rocks is mainly mixed source input,and the thermal evolution is in the low-mature stage.A weakly oxidized-weakly reduced and shallow lake environment was reflected for the upper Qingtujing formation,while the lower was reflected as weakly saline and deep lake environment with slight water stratification;In the Minhe Basin,biomarker characteristics indicate that low-grade aquatic organisms such as algae were important hydrocarbon source for the upper Yaojie Formation,while the hydrocarbon source for the lower part is dominantly from terrestrial higher plants.Over all,a marsh environment and a mature evolution for source rock were indicated.Especially,the lacustrine planktonic algae,such as Botryococcus and leiosphaerid,with high hydrocarbon-generating potential were discovered in some horizon from the Chaoshui and Minhe basins.These algae fossils were also discovered in many main source rocks of petroliferous basin,such as the Ordos Basin,Jiuxi Basin which usually regarded as marks of high-quality source rocks.Lastly,compared of other organic geochemical data,coal-bearing source rocks in the Chaoshui and Minhe basins have hydrocarbon-generating potential and enter the threshold of petroleum generation.Besides,the source rocks of the Minhe Basin probably have higher potential than the Chaoshui Basin,for high aquatic organism input to the former.The source rock of the Chaoshui Basin contains higher quartz minerals with a brittleness index of 78%which usually contain poor pores structure;Relatively,high clay minerals composition and a brittleness index of 43%occurred in the Minhe Basin which likely contain good pores structure;The laminar structure for the coal-bearing source rocks in Chaoshui and Minhe basins are well identified,which are usually composed of organic clay lamina and silt lamina.The shale pores are mainly composed of nano-micro pores.Comprehensive analysis shows that there are high-quality source rocks for the Jurassic coal-bearing sedimentary succession in Gansu Province,evidence by the Chaoshui and Minhe basins.The low-to maturity stage and micro-pore structure were present for these source rocks.In comparison,the source rock quality of the Minhe Basin is better than that of the Chaoshui Basin,and the storage conditions are better.Both basins basically meet the high-quality shale oil resource standards.Therefore,these coal-bearing sedimentary basins may become important potential areas for continental shale oil resources.