Crude-oil hydrocarbon composition characteristics and oil viscosity prediction in the northern Songliao Basin

Crude oil hydrocarbon composition characteristics and oil viscosity prediction are important bases in petroleum exploration. A total of 54 oil/heavy-oil samples and 17 oil sands were analyzed and quantified using both comprehensive 2D gas chromatography (GC×GC) and comprehensive 2D gas chromatography/time-of-flight mass spectrometry (GC×GC/TOFMS). The results show that crude oil in the West slope is mainly heavy oil and its hydrocarbon composition is characterized overall by paraffins > mono-aromatics > naphthenes > non-hydrocarbons > di-aromatics > tri-aromatics > tetra-aromatics. Aromatics are most abundant and non-hydrocarbons are least abundant, whilst content differences among paraffins, naphthenes, aromatics, and non-hydrocarbons are less than 15%. There are two types of heavy oil, secondary type and mixing type. Biodegradation is the main formation mechanism of heavy oil. Biodegradation levels cover light biodegradation, moderate biodegradation, and severe biodegradation. With increasing biodegradation, paraffin content decreases while contents of aromatics and non-hydrocarbons increase. In contrast, naphthene content increases first and then decreases with increasing biodegradation. In severe biodegradation stage, naphthenes decrease more quickly than aromatics and non-hydrocarbons. This provides a new method for studying oil/heavy-oil biodegradation mechanism and biodegradation resistance of different hydrocarbons at different biodegradation stages. In the Longhupao-Daan terrace and Qijia-Gulong depression, most crude oil is conventional oil. Its composition is dominated by paraffins with the lowest content of aromatics. In some casual oil wells from the Longhupao-Daan terrace, crude oil from Saertu oil reservoirs is moderately biodegraded whereas crude oil from Putaohua oil reservoir is lightly biodegraded. Chemical parameters using saturate hydrocarbons and aromatics are usually not suitable for determining organic type and thermal maturity of biodegraded oil, especially of moderately or severely biodegraded oil, whilst Ts/(Ts+Tm) ratio can be used to determine thermal maturity of both conventional crude oil and heavy oil.