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.
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