Features and significance of rearranged hopanes in pyrolyzates of hydrocarbon source rocks

Pyrolysis experiments were conducted on immature hydrocarbon source rocks from the well PL-1 in the Bohai Bay Basin to study the influence of thermal evolution on the formation of rearranged hopanes as well as the relationships among rearranged hopane parameters and thermal evolution. The experimental results showed that rearranged hopanes displayed various characteristics during different thermal evolution stages. The absolute concentration of rearranged hopanes in pyrolysis at 250℃ increased relative to that of the original sample, and it gradually reduced as experimental temperature increased from 250℃ to 400℃. The higher the degree of thermal evolution was, the greater the proportion of absolute concentration of rearranged hopanes decreased. When the experimental temperature was between 300-400℃, the reduced proportions of the absolute concentration of compounds showed a pattern of 17α(H)-hopane>18α(H)-neohopane>17α(H)-diahopane. In addition, the rearranged hopane parameters (17α(H)-diahopane/17α(H)-hopane) were nearly unchanged at the stage of low mature to mature, nevertheless it gradually increased at the stage of mature to highly mature. The immature to low mature stage is the main stage of the formation of rearranged hopanes. Higher thermal evolution has an influence on the formation of rearranged hopanes to some degree. Moreover, rearranged hopane parameters can be used as an effective maturity index in the stage of mature to highly mature.