Complex origins of naphthenic oils in the Junggar Basin, China: Biodegradation vs. non-biodegradation

Abstract

Naphthenic crude oils are scarce resources of great value. Their origins are generally considered to be related to biodegradation, which consumes paraffins and enriches naphthenes. However, other possible controls on the origins of the naphthenic oils remain insufficiently clear. To fill the knowledge gap, we conducted a case study in the Junggar Basin by using comprehensive two-dimensional gas chromatography–mass spectrometry for semi-quantitative analysis of naphthenes in combination with relevant organic geochemical data, and address its global implications by synthesizing the typical naphthenic oils discovered worldwide. It was found that the occurrence of naphthenic crude oils in the basin was not caused by biodegradation alone, while sedimentary paleoenvironments and source-rock biological inputs (i.e., indicative of organofacies) are important controlling factors. Naphthenic crude oils are mainly derived from dolomitic source rocks of the Permian Fengcheng Formation in the Mahu Sag of the northwestern basin, where the average naphthene content is 43.8 %, with an average naphthene/paraffin (N/P) ratio of 1.24. Source rocks were deposited in a reducing, hypersaline and stratified alkaline lake (0.75 < pristane/phytane (Pr/Ph) < 1.46, gammacerane/C30αβ hopane > 0.23), with source-rock biological inputs being dominated by halophilic algae (steranes/hopanes > 1, C28/C27-29-steranes > 40 %). In contrast, crude oils in the Jimusaer Sag of the southeastern basin are paraffinic–naphthenic with an average naphthene content of 24.8 % and an average N/P ratio of 0.75. Source rocks here were deposited in an anoxic–oxic lake of low salinity (1.05 < Pr/Ph < 1.59, 0.15 < gammacerane/C30αβH < 0.18), and organic matter consists of mixed bacteria, algae, and terrigenous higher plants (steranes/hopanes < 1, C28/C27-29-steranes < 40 %). Thermal maturity and biodegradation also control the composition of naphthenic oils. High thermal maturity promotes cracking of naphthenic groups, whereas moderate biodegradation (rank PM < 6) promotes naphthenic groups by preferentially catabolizing paraffins. Our results suggest that halophilic algae in strongly-reducing alkaline lakes provide important material for the generation of naphthenic oils in the Mahu Sag and the occurrence of naphthenic crude oils is not controlled solely by biodegradation; influencing factors are complex and vary between regions without universality. This may be the reason for their limited and sparse distribution worldwide.