Effect of water washing on hydrocarbon compositions of petroleum sandstone reservoir rocks in Tarim Basin, NW China

Gross compositions and distribution of saturated and aromatic hydrocarbons in Carboniferous sandstone reservoir rocks in oil and water zones for Tazhong-10 well of the Zhongyang Uplift in the Tarim Basin were studied in detail by means of Rock-Eval pyrolysis, thin-chromatograph-flame ionization detection (TLC-FID), gas chromatography, gas chromatography-mass spectrometry. The results suggest that the gross composition of reservoir hydrocarbons between the oil zone and the water zone show significant differences. Water washing has a dramatic effect on saturated hydrocarbon biomarkers, especially drimane series compounds. Drimane series compounds in the water zone have been depleted completely. However, the contents of tricyclic diterpanes and pentacyclic triterpanes tend to decrease slightly, but the water-zone reservoir hydrocarbons contain a large amount of gammacerane. This suggests that gammacerane be more resistant to water washing than diterpanes and homohopanes. The contents of pregnane, homopregrane, diasteranes relatively decrease as a result of water washing. Water washing has a noticeable effect on polycyclic aromatic hydrocarbon compounds, especially aromatic sulfur compounds, and the contents of dibenzothiophene series compounds and benzonaphiothiophene decrease significantly as a result of water washing. However, the contents of bicyclic and tricyclic aromatic hydrocarbons decrease slightly and those of tetracyclic and pentacyclic aromatic hydrocarbons, especially benzofluoranthene and benzopyrenes, increase markedly owing to a decrease in light aromatic hydrocarbons as a result of water washing.