Thermal Effect on the Distribution of Rearranged Hopanes in Hydrocarbon Source Rocks

Key words: 
• rearranged hopanes /
• genetic mechanism /
• thermal effect /
• hydrocarbon source rocks

Abstract: The molecular geochemical characteristics of a total 87 samples of lacustrine and coal measure hydrocarbon source rocks collected from the Songliao Basin and Ordos Basin are systematically analyzed in order to evaluate thermal effect on the distribution characteristics of rearranged hopanes in hydrocarbon source rocks. The results indicate that both lacustrine and coal measure hydrocarbon source rocks contain abundantly four series of rearranged hopanes and their distributions in geological bodies from different depositional environments or parent material sources show similar variation trends. The relatively lower abundant rearranged hopanes are distributed from low mature to high mature stages, whereas the abnormally high or higher abundant rearranged hopanes are mainly presented in mature stage. The linear correlations between the 17α(H)-diahopane series and other three series of rearranged hopanes including 18α(H)-neohopane series, the early-eluting rearranged hopane series and 21-methyl-28-nor-hopane series, are diverse clearly. In addition, the variations of relative abundance of rearranged hopanes from the two sets of samples show similar trend. For example, the relative abundances of 17α(H)-diahopane and 18α(H)-neohopane display an initial increase and a following decrease with increasing maturity and the highest amounts appear to be distributed in the range of maturity 0.8%-0.9% R_o. On the other hand, the absolute concentrations of rearranged hopanes have consistent variation for the hydrocarbon source rocks derived from different sedimentary environments with increasing maturity. Rearranged hopanes are formed at immature to mature stage corresponding to R_o=0.5%-0.7%. With the source rocks into mature stage (R_o=0.7%-0.8%), the absolute concentration decreases remarkably with the decreasing extent more than 90%. The consistency of variation of relative abundance and absolute concentration of rearranged hopanes in different depositional systems indicate that thermal effect has much more important impact on the distribution and composition of rearranged hopanes compared with sedimentary environments and biological sources.