Geochemistry of severely biodegraded oils in the Carboniferous volcanic reservoir of the Chepaizi Uplift, Junggar Basin, NW China

Youde Xu,Bingbing Shi,Xiangchun Chang,Yue Wang,Yang Li

doi:10.1177/0144598718770692

Abstract

A substantial amount of petroleum was recently discovered in the Carboniferous volcanic reservoir of the Chepaizi Uplift in the western Junggar Basin, yet the source is still indefinitive. Geochemical investigation indicates that the Carboniferous oils from the eastern and western Chepaizi Uplift are characterized by different source facies, although they are all typically of lacustrine origin. The eastern oils exhibit a restricted, clastic starved, highly reducing hypersaline condition during source rock deposition, which is distinct from the western oils. The Carboniferous oils were subjected to biodegradation ranging from rank 6 to rank 9, as indicated by the presence of 25-norhopane, evident depletion of hopanes and regular steranes, and even selective reduction of tricyclic terpanes. The maturities for the Carboniferous oils correspond to the onset of oil generation. The eastern oils contain lower (C19 + C20)TT/(C23 + C24)TT and C19TT/C21TT, and lighter stable carbon isotopes than the western ones, correlating well with the Middle Permian Wuerhe (P2w) source rocks and the Jurassic source rocks, respectively. The good correlation of tricyclic terpanes source-related parameters further implies less contribution to the eastern oils by the Carboniferous source rocks.

Highlights

With increasing difficulties in prospecting conventional clastic and carbonate petroleum reservoirs, volcanic reservoirs gradually have become a new exploration target (Zou et al, 2011)
We report here an investigation into the geochemistry of the Carboniferous oils through a systematic biomarker and isotope geochemical characterization of the 10 oils obtained from the well test, involving helpful and supplementary information from different perspectives
The Carboniferous oils from the eastern Chepaizi Uplift are characterized by high oil density (0.9285–0.9590g/cm3) and viscosity (154–8968 mPaÁs), so they can be classified as heavy crude oils (Table 1)

Summary

Introduction

With increasing difficulties in prospecting conventional clastic and carbonate petroleum reservoirs, volcanic reservoirs gradually have become a new exploration target (Zou et al, 2011). Oil and gas reserves in volcanic provinces are found on all continents and many future petroleum exploration areas will be located in volcanic margins (Senger et al, 2017; Sydnes et al, 2018; Zou et al, 2011). Commercial volumes of oil were obtained from Cretaceous, Jurassic, Palaeogene, and Neogene clastic reservoirs in the Chepaizi Uplift by the Sinopec Company (Zhang et al, 2014). Until 12 December 2014, 11 of the 32 wells penetrated to the Carboniferous interval in the eastern Chepaizi Uplift obtained commercial volumes of oils with proven in-place reserves of up to 61.01 Â 106t (Dong, 2015)

Results

Discussion

Conclusion