Thermal Effect On Rearranged Hopanes Composition In Source Rocks And Study On Its Formation Mechanism

Rearranged hopanes,as an important part of biomarker,occur ubiquitously in petroleum and source rocks.Recently,the thermal effect on the distribution of rearranged hopanes is widely investigated by most geochemists.Taken the lacustrine source rocks from the Songliao Basin and coal measure rocks from the Ordos Basin as examples,immature source rocks of different kerogens were chosen to perform the thermal simulation experience.Based on the macro and micro,qualitative and quantitative,geological and geochemical analysis methods,variation regularity of rearranged hopanes in source rocks at different thermal evolution stages is studied systematically to discuss the thermal effect on the formation mechanism of rearranged hopanes.The difference of abundance,type and depositional environment of organic matter demonstrates that lacustrine and coal measure rocks are derived from totally distinct sedimentary facies.On the whole,suboxic-weak reduction condition facilitates the formation of rearranged hopanes,which provides a basis of organic facies to explore the influence of thermal action on the rearranged hopanes in source rocks form different organic input and depositional environment.The maturity of lacustrine source rocks and coal measure rocks from two basins are over a wide range from immature to high mature stage,corresponding to Ro=0.4%~1.8%,which provides maturation basis for research on thermal effect on the distribution of rearranged hopanes.The variation of rearranged hopanes from lacustrine source rocks from the Songliao Basin influenced by thermal affect is similar with that in coal measure rocks from the Ordos Basin.Low abundant rearranged hopanes are distributed at immature to high mature stage;high abundant rearranged hopanes are distributed at mature stage;abnormal high abundant rearranged hopanes are distributed in oil-generation window.That source rocks have been into mature stage is the essential requirement.The 17?(H)-diahopane parameter show a normal distribution with Ro,and the largest value of 17?(H)-diahopane parameter appears in oil-generation window corresponding toRo = 0.8%-0.9%.In addition,the 18?(H)-neohopane parameter increase with the maturity at low mature to mature stage,and remains constant at Ro>0.9%.The change trends of absolute concentrations of 17?(H)-diahopane and 18?(H)-neohopane in lacustrine and coal measure rocks from two basins with different sedimentary environment and organic input show similar profiles.At low mature stage corresponding to Ro=0.5%~0.7%,the absolute concentration of rearranged hopanes increases with maturity,and reach the largest amount.Then,at mature stage,the absolute concentration of rearranged hopanes shows a sharp decrease.It suggests that low mature stage is the main period of rearranged hopane formation.The correlations of rearranged hopane series are diverse.It reveals that compounds in the same series of rearranged hopanes covary with each other closely,whereas in different series the 17?(H)-diahopane series show an excellent correlation to the early-eluting rearranged hopane series.Both the 17?(H)-diahopane seriesand the 18?(H)-neohopane series exhibit a general correlation with the 21-methyl-28-nor-hopane series.The 21-methyl-28-nor-hopane series and the early-eluting rearranged hopane series are not closely correlated with the 18?(H)-neohopane series.Due to the limitation of maturity parameters of saturated hydrocarbon biomarkers,the maturity parameters such as terpanes and steraes could not reflect the real variation of rearranged hopanes with maturity,which show a poor correlation with the rearranged hopanes.On the whole,high abundant rearranged hopanes appear at the equilibrium value of maturity parameters,indicating that high to abnormal high abundant rearranged hopanes are formed at mature stage.Aromatic maturity parameters display a positive correlation with Ro,which suggests that aromatic maturity parameters could present the real thermal evolution stage.The rearranged hopane parameters exhibit a good normal distribution with aromatic maturity parameters,suggesting that high to abnormal high abundant rearranged hopanes are generated at oil-generation window stage.The result of thermal simulation experience shows that thermal affect has an important influence on the formation of rearranged hopanes.In the process of generation and expulsion of hydrocarbons,there are two main stages for the formation of rearranged hopanes in source rocks,namely,immature to low mature stage for the precursor generation of rearranged hopanes at and mature stage for the methyl rearrangement of regular hopanes.In thermal simulation experience,the thermal effect on 17?(H)-diahopanein source rocks of different kerogens is similar,but the thermal effect on 18?(H)-neohopaneis distinct,suggesting that 17?(H)-diahopanes are mainly controlled by maturity,whereas 18?(H)-neohopane are controlled by maturity and sedimentary environment.The distributions of rearranged hopanes in source rocks are mainly controlled by thermal effect,followed by depositional environment and organic input.Thermal effect is the first controlling factor.Mature stage is the period of formation of high to abnormal high abundant rearranged hopanes,which illustrates that high to abnormal high rearranged hopanes is the effective maturity parameter for the source rocks at mature stage.