A Novel Molecular Indicator for Sedimentary Environment in Pearl River Mouth Basin

Abstract

In bacteria, bacteriohopanepolyols (BHPs) are amphiphilic biochemicals that serve regulatory and rigidifying functions in cell membranes. It has been demonstrated that the cyanobacterial lineage currently comprises the only known quantitatively significant source of 2-methyl-BHPs, such that the presence of abundant 2α-methylhopanes (2-MHPs) derived from 2-MBHPs precursors is believed to be characteristic of cyanobacteria. Thus, abundant 2-MHPs seem to indicate that cyanobacteria could be live and develop very well under such an environment and related to the alkaline and anoxic source-rock deposition, which a vital element to indicate oil- source correlation and oil- oil correlation in Pearl River Mouth Basin. In this study, abundant and complete 2-MHPs series have been clearly identified and quantified in most of the studied crude oil samples from Pearl River Mouth Basin by using comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GC×GC/TOFMS). All target components were well separated with hopanes series under orthogonal conditions and almost spread throughout the whole region of the 2D-plane. Based the results, some related geochemical parameters were calculated. It shows that the 2-MHP index (C31-2-MHP×100/(C31-2-MHPs + C30-Hopane), where the hopane and its 2-methyl analog are quantified using m/z 191 and m/z 205, respectively) ranges from 41.59% to 79.19% for the crude oils from Wenchang Formation, implying that cyanobacteria could be rich under such an environment. At the same time, the 2-MHP index ranges from 5.4% to 45% for the crude oils from Enping Formation, and from 26.88% to 71.30% for the mixed oils from both formations. It can be seen that the values for the mixed oils are just between the two end number oils. Moreover, 2-methylhomohopane (2-MHHPs) index showed significant correlation with Pr/Ph and 4-methylsteranes index. It indicates that 2-MHPs might be as important as 4-methylsteranes, and 2-MHPs index could be used as a reliable parameter to rebuild the sedimentary environment as well as precise ascertainment of effective source rocks in Pearl River Mouth Basin. Thus, the proposed molecular indicator is beneficial to the evaluation of petroleum resource and the determination of exploration strategies in the Pearl River Mouth basin, and can be extended to other basins.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015