Oil-source correlations and petroleum accumulation in oil sands in the Carboniferous Yanghugou Formation in the Shajingzi area, Ordos Basin, China

The south-central part of the western Ordos Basin is an important structural belt. Recent drilling of Well YT2 in the Shajingzi area of this belt has led to the discovery of oil sands in the Carboniferous Yanghugou Formation. In this study, Paleozoic source rocks and oil sands were sampled from the Well YT2 and analyzed for oil-source correlations. Geochemical data including saturate and aromatic biomarker compositions were used to investigate the genetic origins of oil sands. Saturated hydrocarbon biomarkers, such as n-alkanes, isoprenoids, tricyclic terpanes and C27-C28-C29 regular steranes, indicate that the oil and source rocks in the Ordovician Wulalike Formation are similar in terms of maturity, provenance and sedimentary environment. Aromatic parameters, such as dibenzothiophene/phenanthrene and alkyldibenzothiophene/alkyldibenzofuran ratios in the oil are similar to those in source rocks in the Carboniferous Yanghugou Formation and the Permian Taiyuan Formation. Moreover, retene is direct evidence that the oil has originated in part from the Yanghugou and/or Taiyuan formations. In addition, the modelled accumulation ages indicate that that the maturity of the two types of oil inclusions is consistent with the source rocks of the Wulalike, Yanghugou and Taiyuan formations during the charging times. The thrust faults runs through the Wulalike, Yanghugou and Taiyuan formations can provide pathways for oil migration. Therefore, the oil from the Yanghugou Formation has a mixed source from the Wulalike, Yanghugou and/or Taiyuan formations. This is the first documented discovery of Ordovician marine oil in the Ordos Basin.