Application of Rare-Earth Elements and Comparison to Molecular Markers in Oil-Source Correlation of Tight Oil: A Case Study of Chang 7 of the Upper Triassic Yanchang Formation in Longdong Area, Ordos Basin, China

The biomarker features of 10 Chang 7 crude oil samples were investigated by gas chromatography-mass spectrometry (GC-MS), and the rare-earth element (REE) compositions of 16 Chang 7 and Chang 8 crude oil samples were determined by inductively coupled plasma-mass spectrometry (ICP-MS) for the first time in Longdong area. Oil-source correlation analysis was improved by biomarkers and REEs. The distribution and relative ratios of a series of biomarker parameters in saturated hydrocarbons and aromatic hydrocarbons of crude oil samples indicate that Chang 7 tight oil has already reached the mature stage. The organic matter mainly comes from lower aquatic organisms of algae, with some contribution of micro-organisms and bacteria, while the forming environment of tight oil is mainly the transitional environment of sub-oxidizing to sub-reducing. The V/(V + Ni) and Ni/Co ratios of crude oil samples suggest that the specific redox conditions of Chang 7(1) and Chang 7(2) samples were slightly oxic, while Chang 7(3) and Chang 8 samples were formed under an anoxic environment. The results of both biomarker-based and REE-based oil-source correlation analysis indicate that Chang 7(1) and Chang 7(2) tight oils come from Chang 7 mudstone, while most of the Chang 7(3) tight oils are from Chang 7 oil shale, with part of mixed from Chang 7 mudstone. This recognition may indicate that Chang 7 mudstone and oil shale are two relatively independent hydrocarbon self-generation and near-storage systems. The analysis results demonstrate that the REE composition in crude oil is an efficient and accurate tool for oil-source correlation in the petroleum system.