Effects of Maturity on the Pr/Ph Ratio of the Soluble Organic Matters in the Terrestrial Source Rocks

Key words: 
• maturity /
• the Pr/Ph ratio /
• hydrocarbon generation-expulsion /
• organic phase

Abstract: The Pr/Ph ratio has been used widely as a significant biomarker. Except for the sources and palaeosedimentary environments, the Pr/Ph ratio is also affected by the maturity. In this paper, the terrestrial source rocks of different organic phases, taken from the western Pearl River Mouth Basin, were thermally simulated by the hydrocarbon generation-expulsion experiments and the soluble organic matters were obtained, then the tendency of the Pr/Ph ratio following the maturity was relatively systematically researched in the generated oils/extracts and the samples from strata. The results showed that, from Ro=0.6％ to Ro=1.8％, the Pr/Ph ratios in both the medium-deep (terrestrial kerogen of type Ⅱ) and the shallow lacustrine (terrestrial kerogen of type Ⅲ) source rocks were decreasing with the increase of maturity and the latter decreased more obviously than the former; However, the Pr/Ph ratios in the two source rocks were almost similar in high maturity phases. This research indicated that the effects of maturity on the Pr/Ph ratio should be considered in identifying palaeosedimentary environments and the correlations of oil-oil and oil-source rocks and it was necessary to combine with other geochemical parameters for the applications of the Pr/Ph ratio to the basins containing terrestrial Ⅲ source rocks.