Abstract:

Crude oil and solid bitumen coexist in the volcanic reservoirs of the Well Ma25 in the Niuquanhu structure zone in the Malang
depression in the Santanghu Basin, Xingjiang. Through multistage separation and extraction, and geochemical analysis of the reservoirs,
the formation processes and exploration significance of these crude oil (or solid bitumen) have been discussed. The results show that the
crude oil / bitumen may mainly come from the source rocks of Lower Carboniferous, with lower Pr/Ph and gammacerane index, high
abundance of tricyclic terpane, high Ts/Tm ratio and the heavy aromatic component carbon isotope (δ13C>-28.0‰). Crude oil in the
reservoirs is now rich in biological degradation markers (25-norhopane) with no other significant biodegradation characteristics
(n-alkanes, steranes and hopanes is intact), reflecting the characteristics of the multi filling stages, that is, oil reservoirs suffered
biodegradation after accumulation in the Late Permian and were filled again in the Late Cretaceous. The existence of the Lower
Carboniferous source rocks in this area indicates that there are potentials for oil and gas exploration from reservoirs in the deep Lower
Carboniferous source rocks.

Key words: carbon isotope of crude oil, biomarker, biodegradation, multistage extraction, Malang sag, Santanghu Basin