The Distribution Characteristics And Significance Of Rearranged Hopanes In Hydrocarbon Source Rocks From Different Sedimentary Environment

17??H?-diahopanes and 18??H?-neohopanes,referring to the compound with identical carbon skeleton and distinct methyl side chain carbon position compared with regular hopanes,occur ubiquitously in source rocks and petroleum.Currently many scholars have discussed the geochemical characteristics of rearranged hopanes,which mainly investigates the effect of sedimentary environment,maturity and organic matter input on the the relative abundance of rearranged hopanes.However,the effects of clay mineral content,environmental redox and salinity in the sedimentary environment have not been studied systematically.Therefore,taking lacustrine rocks from the Songliao Basin,and coal measure rocks from the Ordos Basin and Tarim Basin as research objects,this paper explores their geochemical characteristics of rearranged hopanes systematically.Combined with the variation of the relative abundance and absolute concentration of rearranged hopanes with the clay content,pristane/phytane ratio and gammacerane index of sedimentary rocks,the effects of depositional environment on17??H?-diahopane and 18??H?-neohopane series from two sample sets were analyzed and their significance was discussed.Based on GC-MS analysis,lacustrine rocks from the Songliao Basin,and coal measure rocks from the Ordos Basin and Tarim Basin were found to contain abundant17??H?-diahopane and 18??H?-neohopane series.The tricyclic terpane in source rocks from the Songliao Basin were characterized by normal distribution patterns with the dominance of C23 tricyclic terpane,indicating typical lacustrine pattern.In contrast,the tricyclic terpane in source rocks from the Ordos Basin and Tarim Basin were characterized by gradually decrease distribution patterns of C₁₉-C₂₆ tricyclic terpane,indicating typical coal measure pattern.The relationships between same and different series of rearranged hopanes from two sample sets show that as for the same series,the relative abundance and absolute concentration of 17??H?-diahopanes and18??H?-neohopanes in lacustrine rocks display a better correlation than that in coal measure rocks;as for the different series,rearranged hopanes derived from the lacustrine rocks correlate with each other better compared with coal measure rocks.According to whole rock composition analysis of source rocks,it indicates that lacustrine rocks from the Songliao basin contains high content of chlorite,illite and illite/montmorillonite interlayer,and lack of kaolinite;however,coal measure rocks contains various content of chlorite,illite,illite/montmorillonite interlayer,and kaolinite.As for the total clay content,the relative abundance and absolute concentration of17??H?-diahopanes and 18??H?-neohopanes in lacustrine rocks demonstrate an initial increase and subsequent decrease with increasing clay content,whose largest values appear to present at clay content in the range of 30-40%;the relative abundance and absolute concentration of rearranged hopanes in coal measure rocks consistently increase with increasing clay content,and their largest values tend to be at clay content in the range of 80-90%.In addition,as for the clay mineral composition,17??H?-diahopanes and 18??H?-neohopanes in lacustrine rocks correlate closely with chlorite,whereas the distribution characteristics of two series rearranged hopanes in coal measure rocks show a close correlation with kaolinite.Based on the distribution of Pr/Ph ratio,it indicates that lacustrine source rocks from the Songliao basin were deposited in reducing and suboxic-weak reduction conditions,whereas coal measure rocks from the Ordos Basin and Tarim Basin were mainly distributed in partial oxic and reducing conditions.Through exploring the relationships between rearranged hopanes and Pr/Ph ratio,it is found that the relative abundance and absolute concentration of 17??H?-diahopanes and 18??H?-neohopanes in lacustrine source rocks display an initial increase and subsequent decrease trend with increasing Pr/Ph ratio,and their largest amounts appear to be generated with Pr/Ph ranging from 0.8 to 3.0 indicating suboxic-weak reduction condition;in contrast,the absolute concentrations of 17??H?-diahopanes and 18??H?-neohopanes in coal measure rocks increase with increasing Pr/Ph ratio,and their relative abundance increase at first,followed by a decrease at the largest value in suboxic-weak reduction condition.Some molecular parameters,such as C₃₅/C₃₄ homohopane index and OF/?F+OF+SF?,were found to change as a function of depositional environment redox,which could further conform the correlation of 17??H?-diahopanes and 18??H?-neohopanes with Pr/Ph ratio.In general,abundant rearranged hopanes in lacustrine source rocks appear to be distributed in suboxic-weak reduction,while abundant rearranged hopanes in coal measure rocks appear to be present in oxic condition.It suggests that at diagenesis stage oxygen content controls the relative amounts of rearranged precursors and promotes acid catalyzed rearrangement processes,and then the exclusion of oxygen suppresses the acidic catalysis in lacustrine source rocks;as for coal measure rocks the more oxygen content tends to facilitates the formation of rearranged hopanes.According to element geochemistry analysis,lacustrine source rocks from the Songliao basin were deposited in fresh-brackish and brackish conditions,whereas coal measure rocks from the Ordos Basin and Tarim Basin were mainly distributed in fresh or fresh-brackish conditions.It is consistent with the results of gammacerane index;the gammacerane/C₃₀ hopane of lacustrine source rocks is mainy more than 0.08 indicating fresh-brackish and brackish conditions,and the gammacerane/C₃₀ hopane of coal measure rocks is less than 0.08 and in the range of 0.08-0.30,indicating fresh or fresh-brackish conditions.Through exploring the relationships between rearranged hopanes and gammacerane/C₃₀ hopane ratio,it suggests that the relative abundance and absolute concentration of 17??H?-diahopanes and 18??H?-neohopanes in lacustrine source rocks show an initial increase and subsequent decrease trend with increasing gammacerane/C₃₀ hopane ratio,and their largest amounts appear to be generated with gammacerane/C₃₀ hopane ratio ranging from 0.08 to 0.3 indicating fresh-brackish condition;in contrast,as for coal measure rocks,the relative abundance of17??H?-diahopanes and 18??H?-neohopanes is similar with that in coal measure rocks,whereas the absolute concentrations disply an initial increase and subsequent decrease,and then an following increase and decrease with gammacerane/C₃₀ hopane ratio.In the whole,abundant rearranged hopanes in lacustrine source rocks appear to be distributed in fresh-brackish condition,while abundant rearranged hopanes in coal measure rocks appear to be present in fresh condition.