Geochemical characteristics and correlation of extracts from Silurian bituminous sandstones and Carboniferous oil sands in well Ha6, northern Tarim Basin

Five Silurian bituminous sandstone and Carboniferous oil sand samples were collected from well Ha6 in the Halahatang Sag of the Tarim Basin, and geochemical analyses including extraction, stable carbon isotope composition, saturate fraction gas chromatography and biomarkers were performed. The δ¹³C (‰) values for extracts from the Silurian bituminous sandstone and the Carboniferous oil sand samples are very close. The CPI values range from 0.95 to 1.06, the OEP values from 0.94 to 1.00, the Pr/Ph values from 0.34 to 0.76, the C₂₁/C₂₃ tricyclic terpane values from 0.37 to 0.47, the C₂₉/C₃₀ hopane values from 0.91 to 0.97, the C₃₅S/C₃₄S hopane values from 0.91 to 1.00, the gammacerane/C₃₀ hopane values from 0.69 to 0.79, the Ts/(Ts+Tm) values from 0.39 to 0.43. Besides, the relative concentration of C₂₇, C₂₈ and C₂₉ regular steranes for extracts from all samples is significantly similar and so does the relative concentration of fluorene, dibenzofurans and dibenzothiophene. Extracts from five samples all contain 25-norhopanes and n-alkanes and acyclic isoprenoids with different abundance and the saturated fraction gas chromatograms show baseline humps called UCM, i.e., n-alkanes and acyclic isoprenoids co-exist with UCM and 25-norhopanes. Based on the above analyses, it was concluded that Silurian and Carboniferous oils were derived from same source rocks and had undergone multiple charges and different degrees of biodegradation.