Compositional modification of crude oil during oil recovery

Ten crude oils from two recovery stages spanning 5–10-year interval of five productive wells in the Tarim Basin, northwest China were analyzed for their compositional modification during production process. Significant compositional changes in polar and nonpolar fractions between the previous oil samples and the latter ones were noted at both bulk and molecular level. The latter oil samples appear to contain more aromatic fraction and less asphaltenes and resin, and their gas chromatography (GC) data for whole oil show reduced alkanes with low molecular weight and enhanced high homologue relative to the previous oil samples. Compared with the oils collected from the previous recovery stage, the concentration of basic type of nitrogen-containing compounds and organic acids in oils from the latter recovery stage have a reducing trend, suggesting the occurrence of interaction between crude oil and reservoir rock.