Thermal maturity of the Permian Lucaogou Formation organic-rich shale at the northern foot of Bogda Mountains, Junggar Basin (NW China): Effective assessments from organic geochemistry

•The thermal maturity indexes from hopanoids and steriods of Lucaogou Formation show the relatively lower maturity.•The thermal maturity indexes in the lower member of Lucaogou Formation mainly show the early oil window.•The alkylnaphthalene ratios (TMNr, TeMNr and TeMNr-1) can well describe the maturity features of Lucaogou Formation.•The organic-rich shale is immature in the upper member and has generated oil in lower member (early oil window) of Lucaogou Formation.

The northern foot of Bogda Mountains in Junggar Basin, Northwest China hosts thick organic-rich shale deposits in the Permian Lucaogou Formation. The Lucaogou Formation consists of the lower member and upper member. To assess the thermal maturity of Lucaogou Formation organic-rich shale in this area, the maturity parameters from organic geochemistry of twenty-four samples selected from JZK well are analyzed. The analytical results from hopanoids (Ts/(Ts + Tm), 29Ts/(29Ts + norhopane), C31-C32 22S/(22S + 22R) homohopanes, C29-C30 αβ/(αβ + βα) hopanes) and steroids (C27 diasterane/ααα sterane, C29 αββ/(αββ+ααα) sterane and C29ααα20S/(20S + 20R) sterane) indicate that the Lucaogou Formation has reached the oil window. But these parameters can not distinguish the maturity differences between the lower and upper members of Lucaogou Formation. The aromatic hydrocarbons such as alkylnaphthalene ratios (methylnaphthalene ratio (MNR), trimethylnaphthalene ratio-2 (TNR-2), dimethylnaphthalene ratio-1 (DNR-1), trimethylnaphthalene ratio (TMNr), tetramethylnaphthalene ratio (TeMNr), tetramethylnaphthalene-1 ratio (TeMNr-1)), alkylphenanthrene ratios (methylphenanthrene index (MPI), methylphenanthrene distribution fraction (MPDF), methylphenanthrene ratio (MPR)) and alkylbiphenyl ratios (methylbiphenyl ratio (MBpR), dimethylbiphenyl ratio-x (DMBpR-x), dimethylbiphenyl ratio-y (DMBpR-y)) are also used to assess the thermal maturity of Lucaogou Formation organic-rich shale. These aromatic indicators show that the lower member is mainly in the early mature stage (early oil window), which is consistent with the results from Rock-Eval pyrolysis and Ro. But the evaluation effects of alkylphenanthrene ratios (MPI, MPDF, MPR) and alkylbiphenyl ratios (MBpR, DMBpR-x, DMBpR-y) are not as clear and therefore not as useful as alkylnaphthalene ratios (TMNr, TeMNr, TeMNr-1). Above analytical results suggest that the alkylnaphthalene ratios (TMNr, TeMNr and TeMNr-1) can act as effective maturity indicators to assess the Lucaogou Formation organic-rich shale in Junggar Basin. Additionally, with the evidences of thermal maturity from organic geochemistry and shale energy phenomena observed in the JZK well, it can be concluded that the organic-rich shale is immature in the upper member and has generated oil in lower member (early oil window). However, shale gas is not a producible resource in the Lucaogou Formation, because the maturity hasn't reached the gas generation stage.

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