Distribution and geochemical significance of phenylphenanthrenes and their isomers in selected oils and rock extracts from the Tarim Basin, NW China

Twenty-two oil samples and eight source rock samples collected from the Tarim Basin, NW China were geochemically analyzed to investigate the occurrence and distribution of phenylphenanthrene (PhP), phenylanthracene (PhA), and binaphthyl (BiN) isomers andmethylphenanthrene (MP) isomers in oils and rock extracts with different depositional environments. Phenylphenanthrenes are present in significant abundance in Mesozoic lacustrine mudstones and related oils. The relative concentrations of PhPs are quite low or below detection limit by routine gas chromatography-mass spectrometry (GC-MS) in Ordovician oils derived from marine carbonates. The ratio of 3-PhP/3-MPwas used in this study to describe the relative abundance of phenylphenanthrenes to their alkylated counterparts-methylphenanthrenes. The Ordovician oils in theTabeiUplift have quite low3-PhP/3-MP ratios (<0.10), indicating their marine carbonate origin, associating with low Pr/Ph ratios (pristane/phytane), high ADBT/ADBF values (relative abundance of alkylated dibenzothiophenes to alkylated dibenzofurans), low C-30 diahopane/C-30 hopane ratios, and low Ts/(Ts + Tm) (18 alpha-22, 29, 30-tris-norneohopane/(18 alpha-22, 29, 30-trisnorneohopane + 17 alpha-22, 29, 30-trisnorhopane)) values. In contrast, the oils from Mesozoic and Paleogene sandstone reservoirs and related Mesozoic lacustrine mudstones have relatively higher 3-PhP/3-MP ratios (>0.10), associating with high Pr/Ph, low ADBT/ADBF, high Ts/(Ts + Tm), and C-30 diahopane/C-30 hopane ratios. Therefore, the occurrence of significant amounts of phenylphenanthrenes in oils typically indicates that the organic matter of the source rocks was deposited in a suboxic environment with mudstone deposition. The phenylphenanthrenes may be effective molecular markers, indicating depositional environment and lithology of source rocks.