Organic geochemistry of Carboniferous source rocks and their generated oils from the Eastern Junggar Basin, NW China

Carboniferous source rocks have been gaining increasing attention after the discovery of the Kelameili gasfield in the Eastern Junggar Basin in 2005. Two sets of source rocks within the Lower Carboniferous Dishuiquan Formation (C1d) and Upper Carboniferous Batamayineishan Formation (C2b), respectively, have been identified in this area. In this paper, clear differences between these two source rocks are demonstrated in molecular and carbon isotopic compositions. The C1d source rocks, in comparison with the C2b source rocks, have remarkably lower Pr/n-C17 ratio, higher Ts/(Ts + Tm) and C30 diahopane/(C30 diahopane + C30 hopane) ratios, lower concentration of C24 tetracyclic terpane relative to C23 and C26 tricyclic terpanes and higher gammacerane/C31 hopane ratio. δ13C values of individual n-alkanes decrease with carbon number for C1d source rocks while the opposite is true for C2b source rocks. the 10 oils collected from the studied area, four were generated exclusively from C1d source rocks while the others were mainly derived from C1d source rocks but contaminated by a small or trace amount of oil components which were derived from the C2b, Middle Permian Pingdiquan Formation (P2p) and Lower Jurassic source rocks. ee, adsorbed and inclusion oils from three oil-containing volcanic reservoir rocks within the C2b formation generally correlate with C1d source rocks based on molecular and carbon isotopic data. However, the three types of oil from the same reservoir rock vary significantly in molecular and carbon isotopic compositions, reflecting facies and maturity changes of charging oil during the reservoir filling.