On the origin, mixing and alteration of crude oils in the Tarim Basin

The origin of oil in the Tarim Basin, northwestern China, has been debated for decades. Here we present new insights into this challenging issue based on data from the Bachu-Maigaiti area, combined with a synthesis of data from other oil fields in the basin. Results of the organic geochemical characterization of 39 oil samples from various fields using GC-FID, Py-GC, GC-MS and GC-MS-MS revealed that the oils in the Tarim Basin can be generally classified into three families. The oils of the first family, whose representatives in our study come from the Luntaibei condensate field, are terrigenous as indicated by highest pristane/phytane (> 2.8) and methylcyclohexane/n-heptane (> 1.8), and lowest dibenzothiophene/phenanthrene (< 0.2) ratios. The oils of the second family, including the samples from the Bashituo and Yasongdi fields in the Bachu-Maigaiti area, are of marine origin and characterized by high pristane/phytane ratios (1.5-2.5) and the absence of 1,2,3,4-tetramethylbenzene and aryl isoprenoids. The third family includes the Yubei, Yakela, Shun9 and Tahe oils, which are also of marine origin but have lower contents of C28 regular steranes and gammacerane, a relatively high abundance of C31-C35 homohopanes as well as the presence of 1,2,3,4-tetramethylbenzene and aryl isoprenoids from C13 to C22 providing evidence for a contribution from green sulfur bacteria thriving under euxinic conditions. The occurrence of these two marine oil families implies hitherto unknown organic facies for the respective source rocks. Family 2 oils may have been generated from Cambrian shales deposited under suboxic conditions, while family 3 oils could have been generated from Cambrian to Ordovician sources deposited in euxinic environments. In addition, biodegradation and mixing is quite common for the oils from the Tarim Basin. Mixing is evident not only by the coexistence of an unresolved complex mixture and C7-C36n-alkanes in certain oils, but also by seemingly conflicting oil maturity parameters. The results presented in this communication confirm the complexity of Tarim Basin petroleum systems in terms of ages and organofacies of the source rocks, mixing of migrating and/or accumulating petroleum and concomitant biodegradation.