Elemental characteristics of lacustrine oil shale and its controlling factors of palaeo-sedimentary environment on oil yield: a case from Chang 7 oil layer of Triassic Yanchang Formation in southern Ordos Basin

摘要

As an important unconventional resource,oil shale has received widespread attention.The oil shale of the Chang 7 oil layer from Triassic Yanchang Formation in Ordos Basin represents the typical lacustrine oil shale in China.Based on analyzing trace elements and oil yield from boreholes samples,characteristics and paleo-sedimentary environments of oil shale and relationship between paleo-sedimentary environment and oil yield were studied.With favorable quality,oil yield of oil shale varies from 1.4％ to 9.1％.Geochemical data indicate that the paleo-redox condition of oil shale's reducing condition from analyses of V/Cr,V/(V + Ni),U/Th,δU,and authigenic uranium.Equivalent Boron,Sp,and Sr/Ba illustrate that paleosalinity of oil shale is dominated by fresh water.The paleoclimate of oil shale is warm and humid by calculating the chemical index of alteration and Sr/Cu.Fe/Ti and (Fe + Mn)/Ti all explain that there were hot water activities during the sedimentary period of oil shale.In terms of Zr/Rb,paleohydrodynamics of oil shale is weak.By means of Co abundance and U/Th,paleo-water-depth of oil shale is from 17.30 to 157.26 m,reflecting sedimentary environment which is mainly in semi deep-deep lake facies.Correlation analyses between oil yield and six paleoenvironmental factors show that the oil yield of oil shale is mainly controlled by paleo-redox conditions,paleoclimate,hot water activities,and depth of water.Paleosalinity and paleohydrodynamics have an inconspicuous influence on oil yield.