Occurrence States And Resource Abundance Of Shale Oil In Chang 7 Member In The Southern Ordos Basin

Based on identifying the spatial distribution and formation conditions of shale and laminated mudstone with good shale oil resource potential in Chang 7 member in the southern Ordos Basin,this study analyzed the oil content,occurrence state,and mobility of these two types of fine-grained sedimentary rocks,and classified and graded the shale oil resources of these two types of fine-grained sedimentary rocks.In addition,the proportion of shale oil in different occurrence states and the main controlling factors of shale oil mobility are also discussed.The results show that high organic carbon shale is mainly developed in the Chang7³ sub-Member of Jinghe district,while the laminar mudstone of the Luohe district is mainly developed in the Chang7³ sub-Member.The thickness of the laminated mudstone in the Chang7³ sub-Member shale in the eastern Jinghe district and in most parts of the Luohe district is greater than 10m,which meets the thickness standard of the shale oil exploration aera.The measured TOC of Chang 7black shale in Jinghe district is generally greater than 6%,the highest is 33.84%,and the average is 20.72%;the measured TOC of Chang 7 Member laminated mudstone in Luohe district is mostly less than 6%,the highest is 6.82%,and the average is 3.73%.The types of organic matter in the organic-rich fine grained sedimentary rocks of Chang 7³ sub-Member in Jinghe and Luohe districts are mainlyⅠ―Ⅱ₁.Low aquatic organisms such as green algae,botryophyllum and chrysochroma were the main sources of organic matter in fine-grained sedimentary rocks of this period.The Chang 7 shale in Jinghe district is in the stage of low maturity to maturity,and the R_o is less than 0.70%in most areas,while the central and western parts of Luohe district enter the maturity stage.From the top of Chang 8 Member to the end of Chang 7³ shale deposition,the climate changed from subtropical warm and humid to temperate dry and cold.During the sedimentary period of Chang 7³ shale,the water body of the lake basin was fresh-brackish water.The Chang 7³shale in Jinghe district was formed in anoxic reduction environment,while the laminated mudstone in Luohe district was deposited in oxic―suboxic environment.The nutrients brought by volcanic ash weathering promoted the increase of the paleoproductivity of the lake basin,resulting in the early Botryococcus braunii eruption,the late green algae species-Botryococcus braunii combination and Chrysophyta alternately bloom.The oil content of Chang 7³ black shale in Jinghe district is higher than that of laminated mudstone in Luohe district.The oil content of Chang 7³ shale in Jinghe district ranges from 6.65 to 25.45mg/g,with an average value of 16.61mg/g.The shale with high oil content is mainly distributed in the northeast of the district.The oil content of laminated mudstones in the Chang 7³ sub-Member of Luohe district is 1.46～18.61mg/g,with an average value of 9.83mg/g.The laminated mudstones with high oil content are mainly distributed in the middle of the district.The percentage of free oil in Chang 7 shale in Jinghe district ranges from 27.05%to 46.95%,with an average of 38.20%.The free hydrocarbon percentage of Chang 7 laminated mudstone in Luohe district ranges from53.38%to 68.78%,with an average of 60.74%.For the fine-grained sedimentary rocks rich in typeⅠ―Ⅱ₁ organic matter,the abundance and maturity of organic matter are the main factors controlling the ratio of free hydrocarbon to adsorbed hydrocarbon.The higher the abundance of organic matter,the lower the free hydrocarbon/adsorbed hydrocarbon ratio,and the higher the maturity of organic matter,the higher the free hydrocarbon/adsorbed hydrocarbon ratio.In the oil-generating window,the organic pores in the fine-grained sedimentary rocks of Chang 7 Member in Jinghe and Luohe districts are underdeveloped,and the inorganic minerals are the main contributors of the pore space,and the diameter of the hydrothermal alteration pores of the minerals is more than200nm.The higher the TOC of the fine-grained sedimentary rocks in Chang 7 Member,the lower the porosity,which is not conducive to the enrichment of free hydrocarbons.The mobility of shale oil in Luohe district is better than that in Jinghe district.The OSI range of Chang 7 laminated mudstone in Luohe district is 57.23～218.42mg/g,with an average of 95.05mg/g.The OSI range of Chang 7 high organic carbon shale in Jinghe district is 24.83～67.82mg/g,with an average value of 43.36mg/g.The mobility of shale oil is controlled by the type of organic matter,maturity,type of pore space,and composition of fine-grained sedimentary rocks.Shale oil in fine-grained sedimentary rocks rich in saturated hydrocarbons and rich in typeⅠ―Ⅱ₁ organic matter,medium and low organic carbon content,inorganic pores larger than 200nm developed,and shale oil in fine-grained sedimentary rocks rich in saturated hydrocarbons generally has good mobility.OSI increases with the increase of percentage of saturated hydrocarbons,and when the relative percentage of saturated hydrocarbons is greater than 50%,OSI reaches more than 100mg/g.Hydrothermal activities led to the thermal anomaly during the Chang7 sedimentary period in Luohe district,which promoted the hydrocarbon generation of sedimentary organic matter in Luohe district,transformed the composite laminae containing morphoorganic matter into asphalt laminae rich in saturated hydrocarbons,and improved the mobility of shale oil in the laminated mudstone of Chang 7³ sub-Member in Luohe district.The shale oil resources of Chang 7³ sub-Member in Jinghe district range from 7.24×10⁸ to 10.78×10⁸t,with an average value of 9.01×10⁸t,in which the free oil resources range from 2.85×10⁸ to 6.34×10⁸t,and the mobile oil resources range from0×10⁸ to 0.28×10⁸t.The shale oil resources in Luohe district range from 11.33×10⁸ to20.84×10⁸t,with an average value of 16.08×10⁸t.The free oil resources ranged from5.81×10⁸ to 15.47×10⁸t,and the mobile oil resources ranged from 0.19×10⁸ to 8.87×10⁸t.According to OSI differences,the shale oil resources in the study area can be divided into three levels,among which the gradeⅠ―Ⅲshale oil resources are developed in Luohe district,and theⅡ-grade shale oil resources are the main ones.Only grade III shale oil resources are developed in Jinghe district.