Investigation on the biodegradation levels of super heavy oils by parameter-striping method and refined Manco scale: a case study from the Chepaizi Uplift of Junggar Basin

Abstract

The Carboniferous volcanic reservoir in the Chepaizi Uplift became an exploration hot target in recent years for its substantial amount of oils discovered. However, most of the Carboniferous heavy oils were biodegraded to PM7 or higher with orders of magnitude variation in oil viscosities. Two oil groups (I and II) exactly corresponding to the western and eastern Chepaizi Uplift were distinguished according to their source diagnose. Furthermore, three oil families (II1, II2 and II3), with the biodegradation level of PM7, PM8–8+, PM9+, respectively, were classified based on molecular compositions and parameter-stripping method of strongly bioresistant parameters. Allowing for this extremely high biodegradation case, more biodegradation refractory compound class were added to establish a refined Manco scale to quantitatively evaluate the biodegradation extent. Refined Manco number (RMN2) positively correlated with the oil density, NSO contents, and absolute concentrations of diasteranes and gammacerane, negatively correlated with the absolute concentrations of diahopane, summed tricyclic terpanes and pentacyclic terpanes. This refined scale showed higher resolution than the PM one to differentiate the biodegradation extent of Carboniferous heavy oils from the Chepaizi Uplift, especially those with same PM values but different oil viscosities.

Characterization and Origin Analysis of Heavy Oil in the Western Sag of the Liaohe Basin

“…A complete series of normal alkanes and isoalkanes are developed and the distribution characteristics of steranes, terpanes, and sesquiterpenes are normal. The steranes are mainly C 27 –C 29 regular steranes and 4-methylsteranes, except for the rearrangement of 13α (β) (H), 17α (β) (H), and C 21 and C 22 steranes (Figure A). ,, This kind of heavy oil is the product of short-distance migration and accumulation of crude oil with high colloid and asphaltene content.…”

“…The steranes are mainly C 27 –C 29 regular steranes and 4-methylsteranes, except for the rearrangement of 13α (β) (H), 17α (β) (H), and C 21 and C 22 steranes ( Figure 7 A). 17 , 32 , 49 This kind of heavy oil is the product of short-distance migration and accumulation of crude oil with high colloid and asphaltene content.…”

Origin of Mesozoic Biodegraded Crude Oils from the Eastern Chepaizi Uplift (Junggar Basin) Constrained by the Biomarker Recovery Method

“…Chang et al 11 proposed a parameter-striping method to define four stages of biodegradation based on the overall trend in the PM scheme. 11,30,31 In this study, different biomarkers were selected according to the PM scale, and their resistance to biodegradation increased in turn (Table 3). Ideally, biodegradation sequentially removed biomarkers in a stepwise fashion and can be described by the 10-point scales.…”

“…29−33 However, many case investigations suggest that the sequential microbial degradation of oil constitutes does not occur in a true stepwise fashion and strictly is a quasi-stepwise process with consumption of biomarkers in a preferential order. 8,11 Therefore, this study adopts the parameter-striping method to define the biodegradation levels of the above three oil families. Stage 3 covers approximately ranges from PM 6 to PM 7, and stage 4 contains PM 8 to PM 9+.…”

“…4,9 Indeed, from the severe to extreme level, variations in the molecular composition of monomer compounds are not strictly consistent with their stepwise fashion in established schemes. 8,10,11 Hence, the commonly used indicators for oil− source correlation cannot effectively define oil sources in this case. Thus, there is an urgent need to find a new method that is suitable for the case of severe to extreme biodegradation.…”