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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

Investigation on the biodegradation levels of super heavy oils by parameter-striping method and... The Carboniferous volcanic reservoir in the Chepaizi Uplift became an exploration hot target in recent years for its sub- stantial 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 (II , II and II ), 1 2 3 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 (RMN ) positively correlated with the oil density, NSO contents, and absolute concentrations of diasteranes and gammacerane, negatively correlated with the absolute concentrations of diaho- pane, 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. Keywords Super heavy oil · Biodegradation · Parameter-striping method · Refined Manco scale · Junggar Basin 1 Introduction degradation of the conventional oils over geological time- scales (Larter et al. 2012). With the proceeding of biodegra- Heavy oil and oil sand bitumen dominate the world’s oil dation, the oil will exhibit decreases in mass and net volume, inventory (Marcano et al. 2013), even exceed the normal increases in oil density, oil viscosity, oil acidity and sulfur oil in quantity (Wang et al. 2016; Li and Huang 2020). Bio- content, enrichments in nitrogen, sulfur and oxygen-contain- degradation may be responsible for these unconventional ing organic compounds, and trace mental elements, which resources, which primarily formed from the microbial heavily affect the oil’s chemical compositions and physical properties (Larter et al. 2006; Zhou et al. 2008; Forsythe et al. 2019). Even within a single petroleum reservoir, bio- Edited by Jie Hao and Chun-Yan Tang degradation may create orders of magnitude variation in oil viscosity and fluid property laterally and vertically (Adams * Xiang-Chun Chang et  al. 2012). Allowing for their great impact on the eco- xcchang@sina.com nomic value and predictability, knowledge of the petroleum College of Earth Science and Engineering, Shandong biodegradation is critical in selecting exploitation strategies University of Science and Technology, Qingdao 266590, (Marcano et al. 2013; Yin et al. 2013), evaluating the alter- China ing extent (Bautista et al. 2015) and forecasting the oil com- Laboratory for Marine Mineral Resources, Pilot National positional characteristics and viscosity (Larter et al. 2012; Laboratory for Marine Science and Technology, Zhang et al. 2014a, b). Qingdao 266071, China Experimental examinations and case studies indicate that Research Institute of Petroleum Exploration relative sensitivity to microbial attack exist among differ - and Development, Shengli Oil Company, ent compound class, even among the individual compound Sinopec, Dongying 257001, China Vol:.(1234567890) 1 3 Petroleum Science (2021) 18:380–397 381 in each class. Therefore, biodegradation is a quasi-stepwise attracted much more attention. Using semi-quantitative process with consume of biomarkers in a preferential order parameter-stripping method and refined Manco scale, this (Peters et  al. 2005). Based on the alteration/removal of paper aims to deeply understand and quantitatively deter- saturated biomarkers, i.e., normal alkanes, acyclic isopre- mine the biodegradation extent of heavy oils in the Chepaizi noids, terpanes and steranes, and selected aromatic com- Uplift, Junggar Basin, NW China. pounds (mainly aromatic steroids), Peters and Moldowan (1993) developed a classical scale to assess the extent of oil biodegradation, which is typically abbreviated as PM 2 Geological setting scale. Five terms, i.e., light (PM1–3), moderate (PM4–5), heavy (PM6–7), very heavy (PM8–9) and severe (PM10) The Chepaizi Uplift, covering an area of 10,500 km , is a are assigned to ranges of the PM scale. Another scheme was petroliferous target of the northwestern Junggar Basin (Zhao established by Wenger et al. (2002) fundamentally based on et al. 2019), which is surrounded by two hydrocarbon gen- the alteration extent with a compound class and the presence erative source kitchens, i.e., the Changji Sag to the east and or depletion of single key compound class, only with the Sikeshu sag to the south (Fig.  1). The early stage of late terms of heavy and severe to describe the PM 4–10 range. Hercynian tectonic movement witnessed the formation of The normal alkanes are generally more susceptible to prototype of the Chepaizi Uplift, which subsequently expe- biodegradation than the branched alkanes and isoprenoids rienced the intense uplift during the Indosinian and Yanshan (Chen et al. 2017), thus ratios of i-C /n-C , pristane/n-C movements, slow subsidence during the Himalayan tectonic 5 5 17 and phytane/n-C are widely used to assess the alteration movement. As an inherited paleo-uplift, Chepaizi Uplift was content of light to moderate biodegraded oils (Peters and characterized by the seriously lack of deposition succession Moldowan 1993). However, some case studies showed no in the structurally high parts, with the Cretaceous, Paleo- evident correlations between Ph/n-C and oil API, and gene, Neogene and Quaternary sedimentary rocks overly then the parameter “mean degradative loss” was proposed Carboniferous volcanics (Song et al. 2007). to assess the biodegradation extent by means of quantify- Due to the multi-staged transgression and uplifting, ing the depletion in volumetrically important individual oil three sets of favorable reservoir rocks were developed in the constituents, especially for the light to moderate level of Chepaizi Uplift, i.e., medium-fine sandstone and glutenite alteration (Elias et al. 2007). Additionally, the production occurred in the Neogene Shawan Formation, fine sandstones and biodegradation of oxygen-containing compounds also and siltstone occurred in the Cretaceous interval, and basalt, follow a preferential order, thereafter the acyclic (DBE 1)/ andesite, tuff and volcanic breccia in the Carboniferous cyclic (DBE 2-4) can reveal the biodegradation level, which interval. well correlated with the PM scale when it less than PM < 6 The Hongche Fault, an active fault separating the Che- (Angolini et al. 2015). paizi Uplift from the Changji Sag (Dong et al. 2017; Ni et al. Many case investigations suggest that the sequential 2019), cut through sandbodies developed in the Cretaceous, microbial degradation of oil constitutes does not occur in Neogene Shawan Formation and volcanics in the Carbonifer- a true stepwise fashion and strictly follow the schemes pre- ous, providing a favorable vertical conduit for the migration viously developed (Bennett and Larter 2008; Wang et al. of hydrocarbon generated in the Changji Sag to the Chepaizi 2013; Chang et al. 2018). All these scales are unsuitable Uplift and yielding three vertical oil-bearing intervals (Miao for heavy oil or super heavy oil, and for the mixtures of et al. 2015; Meng et al. 2016). oils (Larter et al. 2012; Zhang et al. 2014a, b). Selecting 8 compound classes (alkyl toluenes, C naphthalenes, C 0-1 2 naphthalenes, C naphthalenes, methyl dibenzothiophenes, 3 Samples and experimental C naphthalenes, C phenanthrenes and steranes) to reflect 4 0-2 the increasing resistance to biodegradation, assigning 0–4 3.1 Sample preparation scores to describe the removal extent, Larter et al. (2012) proposed a Manco scale to assess the biodegradation level Eleven DST oil samples from the Carboniferous volcanic (PM4–8), with its MN2 value positively correlates with oil interval were collected from wells in the Chepaizi Uplift viscosity, and effectively used (López 2014). for geochemical analysis. Asphaltenes were removed from Till now, published studies mainly focused on the geo- the oil samples and source rocks using n-hexane precipita- chemical behaviors of molecular biomarkers at the bio- tion, and the deasphaltened oil was divided into two aliquots. degradation level of PM8 or less, for those biodegradation The first of these underwent column chromatography using a level higher than PM8, rare studies were reported. With the routine silica gel and alumina column from which aliphatic reduction in conventional resources, extremely biodegraded and aromatic fractions were obtained using n-hexane and oils continuously discovered in the petroleum exploration dichloromethane (DCM). 1 3 382 Petroleum Science (2021) 18:380–397 1 3 Zhongguai Uplift Hongche Fault Shawan Sag Chepaizi Uplift Wulun Depression Luliang Rise Yilinheibier Mountains Thrust-fold Belt -250 Central Depression -300 -350 -400 -450 -500 -550 80° E85° E 90° E 95° E P665 080 km 04 8 km P60 P66 P666 P61 P661 P685 P663 P668 East Rise (a) 84°50' 85°10' 85°30' 45° 10' 040 km P66 P612 P70 44° 50' Su1 P702 P70 Sikeshu Sag 44° 30' (b) 15290000 15300000 15310000 15320000 15330000 (c) 44° Boundary of Structure contour of Basin boundary Faults Oil fields Wells -100 10' tectonic units carboniferous top surface 84°50' 85°10' 85°30' Fig. 1 Map showing structural elements of the Junggar Basin (a), with location of the Chepaizi Uplift (b), and a field-scale map showing the sampling wells in the Chepaizi Uplift (c). This figure was modified after (Chang et al. 2018) West Ris is i e e -150 -200 -250 -300 -350 -400 -450 -500 -550 -600 Zhayier Mountains -100 -150 -200 -50 -100 Wuxia Fault Hala’alat Mts. Zhayier Mts. -600 -750 -800 -850 -900 -950 -1000 -1050 -1100 -1150 -400 -800 -650 -700 -450 -500 -750 -150 -200 -250 -300 -350 -400 -50 -100 -150 -200 -250 -300 -350 -700 -700 -550 -50 -400 -850 -900 -650 -100 Hongche Fault Zone 44° N 46° N 48° N 4990000 5000000 5010000 Petroleum Science (2021) 18:380–397 383 As for the study of oil origins, oil family is a widely used 3.2 Gas chromatography–mass spectrometry term to distinguish oils with different genetic affinity. An oil family, a group of oils that derived from a same source Gas chromatography–mass spectrometry (GC–MS) analysis of the aliphatic and aromatic fractions was performed with rock, possibly experienced similar reservoir-forming history, belonged to a same oil system, and possessed same or simi- a Finnigan Model SSQ-710 quadrupole analytical system coupled to a DB-5 fused silica column (30 m × 0.32 mm i.d.) lar chemical compositions. Combing the ratios of C dias- terane 20S/20R (C DS 20S/20R), C diasterane/C regu- and linked to an IAIS data processing system. GC tempera- 27 27 27 ture operating conditions for the aliphatic fraction were as lar sterane (C DS/CRS), C /C triaromatic steroid (20S) 27 27 26 28 (C /C TAS(20S)), and C /C TAS(20R) with the stable follows: 100 °C (1 min) to 220 °C at 4 °C/min and, then to 26 28 27 28 300 °C (held 5 min) at 2 °C/min; for the aromatic fraction: carbon isotope distribution, previous studies concluded that the Carboniferous oils in the eastern Chepaizi Uplift were 80 °C (1 min) to 300 °C (held 15 min) at 3 °C/min. MS conditions were as follows: electron impact (EI) ionization mainly derived from the mudstone of the Middle Permian Wuerhe Formation (P w) in the Changji Sag, whereas the mode; 70-eV electron energy; 300-mA emission current; and 50–550 amu/s scan range. ones in the western Chepaizi Uplift were essentially origi- nated from the Jurassic mudstone in the Sikeshu Sag (Zhang et al. 2012; Xu et al. 2018; Mao et al. 2020). Although two oil charging episodes were defined in the Carboniferous 4 Results and discussion reservoirs based on the oil geochemistry, fluid inclusions and basin modeling (Chang et al. 2019; Shi et al. 2020), the 4.1 Oil bulk compositions and oil families later charge actually was the remigration of early reservoired oils due to the tectonic adjustment (Cao et al. 2010; Song The Carboniferous oils from the eastern Chepaizi Uplift are characterized by higher oil density (0.9285–0.9590 g/ et al. 2016; Chang et al. 2019). Although the Carbonifer- ous oils all exhibited the characteristics of lacustrine source cm ) and viscosity (154–8968 mPa·s) than those from the western Chepaizi Uplift, which can be classified as heavy facies, source-diagnostic and redox potential of depositional environment-related molecular biomarkers showed marked crude oils (Table 1). The Carboniferous oils are predomi- nantly aliphatic, as indicated by their saturate/aromatic distinction between the eastern and western parts (Xu et al. 2018), implying at least two oil groups. (ST/AR) ratio (1.92–3.56) and saturate fraction abundance (44.45%–64.38%). The oil density showed roughly positive Cluster analysis, a statistical method, is an effective tool to classify studied samples into different groups by their correlation with the viscosity and the NSO (resin + asphal- tene) fraction content, and negative correlation with the bur- similarity distances which were calculated from the different variables investigated. Allowing for the severe oil alteration, ial depth. Progressive biodegradation of crude oils may be responsible, as it decreases the content of saturate and aro- eight biomarker parameters strongly resistant to biodegrada- tion were selected to act as the variables in the clustering matic hydrocarbons and enriches the resins and asphaltenes, resulting in an increase in oil density (López et al. 2015; analysis, i.e., C /C TAS (20S), C /C TAS (20R), C tet- 26 28 27 28 24 racyclic terpane/C hopane (C Tet/C H), gammacerane/ Wenger et al. 2002). 30 24 30 Table 1 Bulk compositions of crude oils from Chepaizi Uplift. ST, saturate hydrocarbon; AR, aromatic hydrocarbon; NSO, resin + asphaltene; G/C H, gammacerane/C hopane; *, data cited from Chang et al. (2018) and Xu et al. (2018) 30 30 3 13 Area Well Depth, m Strata Density*, g/cm Viscosity*, ST*, % AR*, % NSO*, % δ C*, ‰ G/C H* mPa·s Western P70 699–713 C 0.9190 38.5 59.94 12.28 27.78 − 27.50 0.08 P702 663.7–698.69 C 0.9254 47.1 − 27.80 Eastern P66 1109.06–1123. C 0.9285 154 64.38 18.49 17.12 − 30.40 0.49 P661 1106.2–1125.0 C 0.9288 149 63.02 20.71 16.27 − 30.00 0.54 P666 922.69–1140.77 C 0.9297 359 61.40 21.58 17.02 − 29.90 0.51 P61 855.73–949.58 C 0.9398 390 56.58 17.37 26.05 − 30.10 0.63 P663 928.45–1031 C 0.9528 1182 55.59 20.68 23.73 − 30.40 0.65 P668 953.15–1069.51 C 0.9528 1880 44.45 12.50 43.05 − 30.30 0.41 P60 690–800 C 0.9389 3079 47.06 18.00 34.94 − 31.00 0.66 P665 781.5–985.85 C 0.9590 8968 45.19 23.01 31.80 − 30.50 3.93 P685 808.82–892 C 0.9524 2600 47.22 24.60 28.18 − 30.40 4.27 1 3 384 Petroleum Science (2021) 18:380–397 C hopane (G/CH), C H (22S)/C H (22S), C 18α(H)- (TAS) were essentially unchanged (Fig. 3d). Thereby, Group 30 30 35 34 29 30- norneohopane/C Hopane (C Ts/CH), C tricyclic I can be assigned to biodegradation level of PM6. 29 29 29 24 terpane (TT)/CTT, C TT/C TT. In the clustering tree 23 22 21 graph (Fig. 2), the Carboniferous oils can be clearly divided 2. Oil group II- family II into two groups, that is, Group I for the western Chepaizi Uplift, Group II for the eastern Chepaizi Uplift. In addition, within the Groups II, three oils families can be further subdi-Family II (wells P661, P666 and P66), featured substantially vided (II, II and II ) according to their similarity distances. removed n-alkanes and iso-alkanes and prominent “UCM” 1 2 3 on the TIC chromatograms (Fig. 4a). C homohopanes 31–35 were heavily depleted. Tricyclic terpanes were far higher 4.2 Biodegradation level by PM scale than hopanes in abundance with reversed “V” distribution of C TT–C TT–C TT (Fig. 4b). Gammacerane was rela- 20 21 23 4.2.1 Qualitative evaluation by molecular compositions tively high in content as showed by the high G/C H values 1. Oil Group I (0.49–0.54). Pregnane and homopregnane were enriched and nearly equal to the regular steranes in abundance with reversed “L” distribution of ααα20R C –C –C sterane 27 28 29 Group I (wells P70 and 702), Carboniferous oils from the (Fig.  4c). Stable carbon isotope varied from −29.9‰ to western Chepaizi Uplift, was least biodegraded among −30.0  ‰ (Table 1). Naphthalenes and phenanthrenes were the investigated oils. These oils showed faintly “UCM” substantially depleted (Fig. 4d). The biodegradation can be and relatively intact n-alkanes on the TIC chromatogram classified into PM6 + to PM7. (Fig.  3a). Hopanes were slightly higher than the tricy- clic terpanes (TTs) in abundance with C hopane as the 3. Oil group II- family II 30 2 peak compound and reversed “L” -shaped distribution of C TT-C TT-C TT (Fig. 3b). Gammacerane (G) was low 20 21 23 in content as evidenced by the quietly low G/C H values Family II (wells P663, P668, P61 and P60), showed heavily 30 2 (0.08–0.09). Fully developed 25-norhopanes were detected removed hopane series with C hopane as the peak com- indicating heavy biodegradation. Pregnane, homopregnane, pound, equal tricyclic terpanes to hopanes in abundance with and diasteranes (DS) were lower than the regular steranes sequentially increased distribution of C TT–C TT–C 20 21 23 (RS) in abundance with “V”-shaped distribution of ααα20R TT (Fig. 5b), and high G/C H ratios (0.41–0.66). Pregnane C -C -C steranes (Fig. 3c). Stable carbon isotope var- and homopregnane far exceeded the regular steranes with 27 28 29 ied from −27.5 ‰ ~ −27.8  ‰ (Table 1). Naphthalenes and “L” distribution of ααα20R C –C –C sterane (Fig. 5c). 27 28 29 phenanthrenes were nearly intact and triaromatic steroids The diasteranes were prominently altered. Stable carbon Euclidean distances Euclidean distances 05 10 15 20 25 05 10 15 20 25 P663 P70-2 Group I P668 P702 P70-1 P60 P666 P661 P61 P666 Family II P66-1 P66-1 Group II P66-2 P661 P663 P66-2 P668 P665-1 Family II Group II P61 P685 P60 P665-2 P665-1 P70-1 Family II3 P685 P702 Group I P665-2 P70-2 (a) Oil groups (b) Oil families Fig. 2 Clustering tree graphs showing the different oil groups (a) and oil families (b) 1 3 Petroleum Science (2021) 18:380–397 385 Ph C30H (a) TIC (b) m/z 191 C H nC17 25-NH nC Pr Tm Ts C19 C Tet C23 C 20S (c) m/z 217 (d) m/z 231 C 20R C 20R+C 20S 26 27 C21 C2620S C 20R C P C P Retention time Fig. 3 Representative fragmentograms of oil group I showing molecular compositions (well P70). a Total ion chromatogram (TIC); Pr, pristane; Ph, phytane. b m/z 191, C -C , tricyclic terpanes with different carbon number; Tet, tetracyclic terpane; Ts, 18α(H)- trisnorneohopane; Tm, 19 26 17α(H)- trisnorhopane; 25-NH, C 25-norhopane; CH, C hopane; CH, C hopane; G, gammacerane. c m/z 217, CP, C pregnane; C P, 29 29 29 30 30 21 21 22 C homopregnane; and d m/z 231, C –C, C –C triaromatic steroids; C –C 20S and 20R,20S and 20R isomers for triaromatic steroids 22 20 21 20 21 26 28 with different carbon numbers isotope (−29.7‰ to −30.7‰) was like that of the family Although many studies reported that the microbial degrada- II (Table  1). Triaromatic steroids were still unchanged tion of oil did not appear to be strictly consistent with the (Fig. 5d). The biodegradation level reached PM8 to PM8+ . stepwise fashion in established schemes, relative variations of biomarkers in oil families II, II and II can be promi- 1 2 3 4. Oil group II- family II nently distinguished (Fig.  7). The values of C T/C TT, 3 22 21 C TT/CTT, C C Tet/C TT and (C + C )/C TT 24 23 24 24 26 20 21 26 Family II (wells P665 and P685), by contrast, exhibited ratios increased with the biodegradation extent increased the strongest biodegradation characteristics. Hopanes were from PM7 to PM8 and finally to PM9+ (Fig.  7a–d), sug- essentially removed with C 25-norhopane as the peak com- gesting the alterations of tricyclic terpanes, preferential pound, and tricyclic terpanes were prominently reduced removal of lower molecular weight homologues and more (Fig. 6b). Gammacerane was abnormally high with G/C H bioresistance of tetracyclic terpane (Huang and Li 2017). values ranging from 3.93–4.27. Pregnane and homopreg- Hopane/sterane ratio varied with biodegradation in a zig- nane were far beyond the majorly depleted regular steranes zag-shaped fashion (Fig.  7e), i.e., essentially unchanged (Fig. 6c). However, no changes of the stable carbon isotope from the level of PM6 + to PM7, sharply increased from the (−29.7‰ to −30.7‰) can be seen (Table 1). The essentially level of PM7 to PM8, and progressively decreased from the unaltered triaromatic steroids (Fig. 6d) confirmed the bio- level of PM8 to PM9+, confirming the different suscepti- degradation level of PM9+ . bility to biodegradation for hopanes and steranes at differ - ent biodegradation level (Reed 1977;). Ratios of C /C H, 29 30 G/ H and C diahopane/C hopane (C D/C H) kept 30 30 30 30 30 5. Sequential biodegradation revealed by biomarkers relatively unchanged from level PM7 to PM8, then sharply increased to PM9 + (Fig. 7f–h), indicating approximately 1 3 Relative abundance ααα(20R)C29 ααα(20R)C ααα(20R)C 28 386 Petroleum Science (2021) 18:380–397 Tricyclic terpanes (a) TIC C TT (b) m/z 191 C TT 23 Pentacyclic terpanes C23 C19TT RS C24 C H 25-NH C30H C C H 26 29 25 Tm C19 Ts (c) m/z 217 (d) m/z 231 C2620R+C2720S C2820S C P C2820R C2720R C P 22 C2620S Retention time Fig. 4 Representative fragmentograms of oil family II showing molecular compositions (well P661). a Total ion chromatogram (TIC), RS, regular sterane, b m/z 191, c m/z 217, and d m/z 231. Abbreviations are the same to Fig. 3 the same bioresistance of C hopane to C hopane, gam- (C + C )/C TAS demonstrated that the short-chained 30 29 20 21 26–28 macerane and C diahopane before PM8 level, but more counterparts were slightly destroyed above PM7 (Fig. 7p). susceptibility at level PM9 + . Gradually increased 18α(H)- trisnorneohopane/17α(H)- trisnorhopane (Ts/Tm) ratio with 4.2.2 Semi‑quantitative evaluation by parameter‑stripping biodegradation indicated the relatively more susceptible of method Tm than Ts to microbial alteration (Fig. 7i). Increasing C 25-norhopane/gammacerane (C NH/G) value with biodeg- radation confirmed the formation of 25-norhopane (Fig.  7j) Ideally, biomarkers were sequentially removed by biodegra- at level above PM6, which was consistent with the increas- dation in a stepwise fashion and can be described by the ten- ing C NH/C H and C NH/C H (Fig. 7k, 7l). However, point scales (Peters and Moldowan 1993). To further proof 28 29 29 30 its decrease from PM8 to PM9 + revealed the degradation the extent of biodegradation four biodegradation stages were of 25-norhopanes occurred under extreme biodegrada- defined following the general alteration tendency in PM tion level (Huang and Li 2017; Chang et al. 2018; Killops scheme (Fig. 8). Stage 1 covers approximately from PM1 to et al. 2019), as validated by the increasing C NH/C NH PM5, stage 2 from PM5 to PM7, stage 3 from PM7 to PM8, 29 28 (Fig. 7m). 25-norhopane ratio (∑C –C 25-norhopanes/ and stage 4 involves PM8 and higher. Biodegradation lev- 30 34 (∑C –C 25-norhopanes + ∑C –C homohopanes) els of different oil families were semi-quantitatively defined 30 34 31 35 generally exhibited a tendency to be increased with bio- by sequentially stripping the samples in the cross-plots of degradation, which was consistent with the formation of biodegradation-resistant parameters. 25-norhopanes and reduction in homohopanes (Fig.  7n). Increasing C DS/C RS with biodegradation displayed the 1. Stage 4 27 27 faster degradation of regular steranes than the diasteranes after PM7 level (Fig. 7o). Unchanged C /C TAS(20S) and Diasteranes show particular resistance to biodegradation and 26 28 C /C TAS(20R) well documented the strongest bioresist- remain where steranes and hopanes are totally removed in 27 28 ance of triaromatic steroids (Fig. 7q, 7r), yet the decreasing case of no 25-norhopanes are present (PM9) (Peters et al. 1 3 Relative abundance ααα(20R)C29 ααα(20R)C ααα(20R)C 28 Petroleum Science (2021) 18:380–397 387 (a) TIC (b) m/z 191 Pentacyclic terpanes Tricyclic terpanes 25-NH RS C23TT 25-NH C21 C21TT C26 C30H C C H 24 30 C29H C24Tet Tm C25 C26 Ts (c) m/z 217 (d) m/z 231 C 20R+C 20S 26 27 C P C 20S C 20R C P C 20R C 20S C21 C20 Fig. 5 Representative fragmentograms of oil family II showing molecular compositions (well P663). a Total ion chromatogram (TIC), b m/z 191, c m/z 217, and d m/z 23. All abbreviations are the same to Fig. 3 2005). Pregnane (P) and homopregnane have high resistance et  al. 2020); therefore, the biodegradation extent may be to biodegradation, comparable to the diasterane (PM9). Tri- responsible for the observed differences of oil compositions. cyclic terpanes can be altered at about the same level as the The tricyclic terpanes feature similarly high bioresistance to diasteranes (Lin et al. 1989). Non-hopanoid triterpanes, such the diasteranes (PM > 8; Lin et al. 1989), slightly lower than as gammacerane, diahopanes, oleanane, are highly resist- the non-hopanoid terpanes. In the extreme biodegradation ant to biodegradation (Peters et al. 2005), and even beyond cases, tricyclic terpanes with lower molecular weight would the point (PM ≥ 9) where the tricyclic terpanes have been be preferentially degraded (Huang and Li 2017; Chang et al. removed (Wenger and Isaksen 2002). Although aromatized 2018). Oil family II exhibited prominently higher C / 3 24 steroids keep unaltered in all but the most severely biode- C TT and C /C TT ratios than other oil families, implying 23 22 21 graded oils (PM10) and can be effectively used to determine a biodegradation level at least up to PM 9–PM9+ (Fig. 9c). the origin and thermal maturity for extremely biodegraded Obviously, by contrast, the biodegradation rank of family II oils (Peters and Moldowan 1993), low molecular weight tri-and II were lower than PM9. aromatic steroids (C -C ) are among the r fi st aromatic ster - 20 21 oids to be depleted during biodegradation (Wardroper et al., 2. Stage 3 1984). From the cross-plots of gammacerane/C diahopane (G/C D) vs. C DS(20S)/CDS(20R), C /C TAS(20S) Hopanes are removed before or after steranes, 25-norho- 30 27 27 26 28 vs. C /C TAS(20R), Carboniferous oils in the western and panes occur in oils where the hopanes are preferentially 27 28 eastern Chepaizi Uplift were clearly distinguished exactly removed (Reed 1977; Peters et  al. 2005), as supported corresponding to the Group I and II, respectively (Fig. 9a, by this investigation (Fig. 7e). C and C homohopanes 31 32 9b). were more susceptible to biodegradation than C hopane in the asphalts from Madagascar (Rullkotter and Wendisch Using triaromatic steroids ratios, previous studies concluded 1982), while C -C 17α-hopanes are typically biode- 28 30 that the Carboniferous oils from the eastern Chepaizi Uplift graded in the same manner and at approximately the same had a common origin and similar maturities (Xi et al. 2014; rate as the C -C extended hopanes (Williams et al. 1986). 31 35 Xiao et al. 2014; Xu et al. 2018; Chang et al. 2019; Mao The 25-norhopane ratio, that is ratio of the total C –C 30 34 1 3 ααα(20R)C ααα(20R)C27 ααα(20R)C 28 388 Petroleum Science (2021) 18:380–397 Pentacyclic terpanes (a) TIC (b) m/z 191 25-NH RS Tricyclic terpanes 25-NH C21P C26TT C22P C H C 29 G C24Tet Ts Tm C25 C30H C21 C19 (c) m/z 217 (d) m/z 231 C 20R+C 20S 26 27 C21P C 20S C2820R C22P C 20R C 20S C20 Retention time Fig. 6 Representative fragmentograms of oil family II showing molecular compositions (well P665). a Total ion chromatogram (TIC), b m/z 191, c m/z 217, and d m/z 23. All abbreviations are the same to Fig. 3 25-norhopanes to the sum of these compounds plus the of sterane and hopane biodegradation (Peters et al. 2005). C –C homohopanes, could be used to evaluate the alter- Regular steranes are removed after the complete removal of 31 35 nation extent among severely biodegraded oils of PM ~ 6–9 C -C isoprenoids and before or after the hopanes (Peters 15 20 (Peters et al. 1996). Oil family II , featured medium ratios et al. 2005). Comparatively, slight alteration of methyl and of C P/∑C RS and C P/∑C RS (Fig. 9d), which dimethylnapthalenes occurs during the removal of n-alkanes, 21 27–29 22 27–29 clearly distinguished from those of the family II (quite high) trimethylnaphthalenes are altered during the removal of and family II (rather low), and further verified by the vary - the isoprenoids, and tetramethylnaphthalenes persist until ing range of C 25-NH/C H and C 25-NH/C H ratios steranes are largely depleted (Fisher et al. 1998). Phenan- 29 30 30 31 (Fig. 9e). Hence, the biodegradation level of the family II threnes are generally more resistant to biodegradation than can be evaluated at PM8–PM8+ , while the family II less alkylnaphthalenes, and methylbiphenyls, dimethylbiphe- than PM8. nyls, and methyldiphenylmethanes lacks in the biodegraded oils at PM7 (Peters and Moldowan 1993). Higher-hopane 3. Stage 2 homologues, particularly the C pentahomohopanes, are preferentially bioresistant, and the alteration of C hom- Normal alkanes are preferentially removed at the biodeg- hopanes are possibly at the rank of PM ≥ 7 (Seifert et al. radation level of PM1–2; however, selective biodegrada- 1984; Moldowan et al. 1995). The G/C H and C hopane/ 30 29 tion of the isoprenoid over more bioresistant steranes or C hopane (C H/C H) varied from roughly unchanged 30 29 30 hopanes is used to determine the level of PM3–4 (Peters to rapidly increased with the increasing biodegradation et  al. 2005). Long-chained alkylated cyclopentanes and level, confirming the faster degradation of C H than C H, 30 29 cyclohexanes are about as susceptible as branched alkanes especially at level higher than PM8 (Bennett et al. 2006; and monocyclic alkanes will be depleted or in trace quanti- Chang et al. 2018); however, the substantial constancy of C ties at PM3–4 level (Peters and Moldowan 1993). C -C 18α-30-norneohopane/C hopane (C Ts/C H) implies the 14 16 29 29 29 bicyclic terpanes are less susceptible to biodegradation than similar susceptibility of C Ts and C H to biodegradation 29 29 isoprenoid and will be completely eliminated before the start (Chang et al. 2018). Oil family II was biodegraded to PM7 1 3 Relative abundance ααα(20R)C29 ααα(20R)C ααα(20R)C 28 Petroleum Science (2021) 18:380–397 389 C /C TT C /C TT C Tet/C TT (C +C )/C TT H/S C /C H 22 21 24 23 24 26 20 21 26 29 30 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 01234 0 0.2 0.4 0.6 0.8 01 24 3 01234567 0123456 P661 (a) (b) (c) (d) (e) (f) P66 P666 P61 P663 P668 P60 P665 P685 G/C H C D/C H Ts/Tm C NH/G C NH/C H C NH/C H 30 30 30 29 28 29 29 30 01 234 5 01 2 0 0.2 0.4 0.6 0.8 1.0 0123 02 1 34 02 1 3456 P661 (g) (h) (i) (j) (k) (l) P66 P666 P61 P663 P668 P60 P665 P685 25-NHs/ C NH/C NH C DS/C RS (C +C )/C TAS C /C TAS(20S) C /C TAS(20R) 29 28 27 27 20 21 26-28 26 28 27 28 ∑ ∑ ( 25NHs+ HHs) 01 23 0 0.2 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 0 0.04 0.08 0.12 0.16 0.200 0.1 0.2 0.3 0.4 0.5 0 0.2 0.4 0.6 0.8 1.0 P661 (m) (n) (o) (p) (q) (r) P66 P666 P61 P663 P668 P60 P665 P685 Fig. 7 Correlations of biomarker parameters among three oil families of Group II showing the alterations of biomarkers with different suscepti- bility to biodegradation. TT, tricyclic terpane; Tet, tetracyclic terpane; H, hopane; G, gammacerane; Ts, 18α(H)-trisnorneohopane; Tm, 17α(H)- trisnorhopane; NH, 25-norhopane; C D, C diahopane; ∑25-NHs, sum of C -C 25-norhopanes; ∑HHs, sum of C -C homohopanes; DS, 30 30 30 34 31 35 diasterane; RS, regular sterane; TAS, triaromatic steroid. All the ratio data were cited after (Chang et al. 2018) by its lower C /C hopane (22S) and HHI values (Fig. 9f, 4.3 Quantitative evaluation by refined Manco scale 35 34 g). Furthermore, the tetramethylnaphthalene ratio (TeBR = 1,3,6,7-TeMN/1,3,5,7TeMN, TeMN = tetramethylnaphtha- 4.3.1 Refined Manco scale lene; Peters et al. 2005) begins to be altered significantly at levels PM5–6 (Fisher et al. 1998). Therefore, oil family II The present biodegradation scales are essentially estab- distinctly distinguished from other families and showed bio- lished according to the presence or absence of single degradation level of PM7 as evidenced by the higher TeBR compound classes and the alteration extent within a com- values (Fig. 9h). pound class, which encountered issues for super heavy oils (Larter et al. 2012). Manco scale emerged as the require- 4. Stage 1 ment, integrating the extent of degradation of various sets of compound classes. This quantitative Manco scale pro- No biodegradation occurred. vided a higher resolution, however, only effective for the biodegradation of PM4–8. 1 3 390 Petroleum Science (2021) 18:380–397 Pristine Light Moderate Heavy Very heavy Severe References 0 1 2 3 4 5 6 7 8 9 10 Stage 4 Seifert et al., 1984; Non-hopanoids Zhang et al., 1988 Diasteranes, Diahopane Seifert & Moldowan, 1979 Tricyclic terpanes Connan, 1984; Lin et al., 1989 Stage 3 C -C steranes Peters & Moldowan, 1993 21 22 25-norhopanes Killops et al., 2019 Stage 2 C -C hopanes Williams et al., 1986 27 29 C31-C35 hopanes Rullkotter & Wendisch, 1982 Tetracyclic terpanes Schmitter et al., 1982 Seifert & Moldowan, 1979 Regular steranes C hopane (25-norhopane formed) Stage 1 Reed, 1977 C14-C16 bicyclic terpane ? Alexander et al., 1983 Isoprenoids Connan, 1984 Peters & Moldowan, 1993 Alkylcyclohexanes n-alkanes Connan, 1984 Stage 4 C -C monoaromatic steranes Wardroper et al., 1984 20 22 C -C triaromatic steroids Wardroper et al., 1984 26 28 C -C monoaromatic steranes Wardroper et al., 1984 27 29 Wardroper et al., 1984 C20-C21 triaromatic steroids Stage 3 Peters & Moldowan, 1993 Ethyl- and trimethylbiphenyls Stage 2 Peters & Moldowan, 1993 Ethyl phenanthrenes Methyl biphenyls Peters & Moldowan, 1993 Dimethyl phenanthrenes Fisher et al., 1998 Tetramethyl phenanthrenes Stage 1 Fisher et al., 1998 Fisher et al., 1998 Methyl phenanthrenes Trimethyl naphthalenes Peters & Moldowan, 1993 Fisher et al., 1998 Methyl- and dimethyl naphthalenes Monocyclic aromatic hydrocarbons Fisher et al., 1998 First altered Substantially depleted Completely eliminated Fig. 8 Schematic plot of the four-stages of the PM scale More biodegradation refractory compound class, i.e., i RMN = m 5 1 i 25-norhopane, tricyclic terpanes and triaromatic steroids, were added to refine the Manco scale to meet the situation of RMN =[n +(log RMN ⋅ S − 1 ]∕n very heavy to severe biodegraded oils from Chepaizi Uplift. 2 5 1 max Followed the academic principles of Manco scale, eight where m refers to the refined Manco score for each out of the compound classes that covered the whole range of biodegra- eight vector elements (0–4), i means the class number (0–7), dation rank (PM0–10) and possessed increasing resistance to and n is the number of compound classes. S , maximum max biodegradation were selected as vector elements. The eight for RMN , is designate to be 1000 to avoid confusion with compound classes and their GC–MS detection m/z values the currently existing scales, and to ensure enough resolu- are listed in Table 2. tion at different levels of biodegradation when using integer Five levels of refined Manco score (RMS) from 0–4 were values. assigned to the 8 compound classes, referencing after the When applied the refined Manco scale to evaluate the Manco scale (Table 3). The refined Manco number (RMN Carboniferous oils in the Chepaizi Uplift (Table 4), the and RMN ) can be calculated by the following formulas. 1 3 Aromatic hydrocarbons Saturate hydrocarbons PM6 PM7 Biodegradation increasing PM8~PM8+ Biodegradation increasing PM9~PM9+ Petroleum Science (2021) 18:380–397 391 2.00 0.85 (a) (b) Stage 4 Stage 4 0.80 Group II 0.75 1.50 0.70 0.65 1.00 0.60 Group I 0.55 Group I Group I 0.50 Group II 0.50 Family II1 Family II2 0.45 Family II3 0 0.40 0 0.10 0.20 0.30 0.40 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 G/C DH C /C TAS(20S) 30 26 28 1.00 0.40 (c) (d) Stage 4 Stage 3 0.80 PM9~PM9+ 0.30 0.60 0.20 0.40 <PM9 0.10 0.20 <PM8 0 0 0 0.50 1.00 1.50 2.00 2.50 3.00 3.50 0 0.10 0.20 0.30 0.40 0.50 C TT/C TT C (P)/∑C -C (RS) 24 23 21 27 29 6.00 0.60 (e) (f) Stage 3 Stage 2 5.00 0.50 PM9~PM9+ 4.00 0.40 3.00 0.30 2.00 0.20 PM8~PM8+ 1.00 0.10 <PM8 0 0 0 1.00 2.00 3.00 4.00 5.00 6.00 0 0.50 1.00 1.50 2.00 2.50 3.00 C (25-NH)/C (H) C H(22S)/C H(22S) 29 30 35 34 3.50 2.00 (g) (h) Stage 2 Stage 2 3.00 1.50 2.50 2.00 1.00 1.50 1.00 0.50 PM8~PM8+ 0.50 PM7 0 0 0 1.00 2.00 3.00 4.00 5.00 0 1.00 2.00 3.00 4.00 5.00 ETR DNR1 Fig. 9 Scattering plots of the Carboniferous oils by the parameter-striping method. DS, diasterane; G, gammacerane;/DH, diahopane; TAS, tri- aromatic steroid; TT, tricyclic terpane; P, pregnane; RS, regular sterane; 25-NH, 25-norhopane; H, hopane; C Ts, C 18α-30-norneohopane; 29 29 HHI, C homohopane/sum of C -C homohopanes; ETR, (C tricyclic terpane + C tricyclic terpane)/(C tricyclic terpane + C tricyclic 35 31 35 28 29 28 29 terpane +18α(H)-trisnorneohopane); TeBR, 1,3,6,7-/1,3,5,7 tetramethylnaphthalene; DNR1, (2,6- +2,7-)/1,5-dimethylnaphthalene; *, data cited from Chang et al. (2018) 1 3 PM9~PM9+ PM8~PM8+ Biodegradation increasing PM9~PM9+ PM8~PM8+ HHI C (25-NH)/C (H) C TT/C TT C DS(20S)/C DS(20R) 30 31 22 21 27 27 TeBR C Ts/C H C (P)/∑C -C (RS) C /C TAS(20R) 29 29 22 27 29 27 28 392 Petroleum Science (2021) 18:380–397 Table 2 Refined compound classes used to represent increasing bioresistance Vector element Compound class GC–MS m/z value 0 n-alkanes + C naphthalenes 85 + 156 + 170 + 184 + 198 1-4 1 Alkyl dibenzothiophenes 198 + 212 + 226 2 C phenanthrenes 178 + 192 + 206 + 220 0-3 3 C phenanthrenes 234 4 Hopanes + methylhopanes + 25-norhopanes 191 + 177 + 205 5 Regular steranes + diasterane + short-chain sterane 217 + 218 + 259 6 Tricyclic terpanes + 17-nor tricyclic terpane 191 + 177 7 Triaromatic steroid + methyl triaromatic steroid 231 + 245 RMN generally well correlated with the viscosity and 4.4 Geochemical implications NSO content (Fig. 10a, b), implying the loss of oils, espe- cially those light ends or susceptible compounds with Although the Carboniferous oils in the western and eastern increasing biodegradation extent. Summed pentacyclic Chepaizi Uplift were derived from different source kitchens, terpanes (∑PTs), summed tricyclic terpanes (∑TTs) and oils within the individual groups had common origin and diasteranes concentrations decreased with the increasing similar maturities (Zhang et al. 2014a, b; Xi et al. 2014; RMN (Fig. 10c, d, f), suggesting the slight alterations of Xiao et al. 2014; Xu et al. 2018; Mao et al. 2020). The dif- pentacyclic terpanes, tricyclic terpanes and diasteranes ferences observed in PM and refined Manco scales were from the PM7 to PM8 level, and sharp depletion from mainly attributed to the biodegradation extent. By contrast, PM8 to PM9 + . However, the variation of diahopane con- the refined Manco scale showed higher resolution than the centrations (Fig. 10e) possibly indicated its high biore- PM scale, which can differentiate the biodegradation extent sistance or other mechanism needed to further investigate. of super heavy oils with same PM values but different oil Generally, RMN ranged from 546.04 to 909.63, which viscosities (Table 4). approximately distinguished the oils at the level of PM6 For the eastern Chepaizi Uplift, Carboniferous oils were to PM9 + (Table 4). Exceptionally, by contrast, the oils mostly biodegraded above PM8; however, their oil viscos- from well P60 exhibited higher oil density but lower bio- ity displayed orders of magnitude variations. Known to all, degradation extent than that from well P685. Factually, the physical properties, particularly the oil viscosity, were there is not a simple relationship between the oil viscosity critical to the choice of exploration strategies. Subtle differ - and the Manco Number (Larter et al. 2012). Processes ences revealed by the refined Manco scale could be helpful. other than biodegradation, i.e., secondary oil charge, Besides the practical application, refined Manco scale also water washing, mixing of multiple maturity oil charges, allow geochemists conducting more basic study to under- and loss of light ends from heavy oils could produce var- stand the relative differences in the extent of the biodegrada- iations in oil viscosity and API gravity (López 2014). tion process among related samples. The two episodes of oil charging, early biodegraded oils mixed with the later remigration of preexisting oils due to the structural adjustment, yet the same oil origin in the 5 Conclusions Chepaizi Uplift maybe responsible for this case (Chang et al. 2019; Shi et al., 2020). The heavy to severe biodegradation was responsible for Notably, although the RMN calculated by the refined the Carboniferous heavy oils in the Chepaizi Uplift, with Manco scale showed roughly positive correlation with oil PM6 in the western and PM7–PM9+ in the eastern part, density, some samples, especially those three biodegraded respectively. According the oil-source correlation, the at PM8, exhibited much the same RMN but with different oils in the western Chepaizi Uplift were derived from the oil densities. This indicated that biodegradation refrac- Jurassic source rock in the Sikeshu sag and those in the tory compound class added in this refined scale could not eastern Chepaizi Uplift were originated from the Permian perfectly differentiate the appearances among oils biode- source rocks in the Changji sag. Therefore, two oil groups graded at about PM8. More assemblages of compound (I and II) were distinguished. Three oil families ( II, II 1 2 classes need to be established to refine the Manco scales and II ) were subdivided with the biodegradation level of in further trials. PM7, PM8–8+ , PM9+ , respectively, based on molecular 1 3 Petroleum Science (2021) 18:380–397 393 Table 3 Refined Manco Scores used to qualitatively distinguish the biodegradation level of compound classes Vector Compound class Refined Manco Scores ele- ment 0 n-alkanes + C naphthalenes 0: intact n-alkanes 1-4 1: slightly degraded n-alkanes, C naphthalene as the peak compound of alkyl naphthalenes 2: substantially degraded n-alkane, C naphthalene as the peak compound of alkyl naphthalenes 3: essentially depleted n-alkanes, C naphthalene as the peak compound of alkyl naphthalenes 4: partially remained C naphthalene in the alkyl naphthalenes 1 Alkyl dibenzothiophenes 0: non-degraded 1: only slightly degraded 2: mid-way between the extremes 3: not quite fully degraded 4: typically absent 2 C phenanthrenes 0: complete C phenanthrenes with the C phenanthrene as peak compound 0-3 0-3 0 1: slightly degraded phenanthrenes with the C phenanthrene as peak compounds 1-2 2: dimethyl and ethyl phenanthrene begun to be altered with the C phenanthrene as peak compound 3: substantially degraded dimethyl and ethyl phenanthrene, C phenanthrene begun to be altered with the C phenanthrene as peak compound 4: wholly removed dimethyl and ethyl phenanthrene, only trace C phenanthrene remained 3 C phenanthrenes 0: non-degraded 1: only slightly degraded 2: mid-way between the extremes 3: not quite fully degraded 4: typically absent 4 Hopanes +methylhopanes + 25-norhopanes 0: intact hopanes with C hopane as the peak compound 1: hopanes begun to be degraded still with C hopane as the peak compound, and 25-norhopane was present 2: hopanes were substantially degraded with C or C hopane as the peak com- 30 29 pound, 25-norhopane was prominently enriched in abundance 3: C -25norhopane was the peak compound in m/z 191 chromatograms, 25,28-binorhopane begun to come into existing. 4: completely depleted hopanes with C 25-norhopane as the peak compound, 25,28-binorhopane increased prominently in abundance. 5 Regular steranes + diasterane + short-chain sterane 0: completed regular steranes with higher contents compared to diasteranes 1: slightly degraded regular steranes, slightly lower contents than diasteranes 2: substantially degraded regular steranes, which were evidently lower than the slightly degraded diasteranes 3: essentially depleted regular steranes, substantially degraded diasteranes 6 Tricyclic terpane + 17-nor tricyclic terpane 0: intact TTs 1: slightly degraded TTs with the presence of 17-nor tricyclic terpane 2: substantially degraded TTs with relatively increased 17-nortricyclic terpane 3: essentially degraded TTs, with far higher 1contents of 17-nortricyclic terpane than TTs 7 Triaromatic steroid + methyl triaromatic steroid 0: intact triaromatic steroid and methyl triaromatic steroid 1: slightly degraded C -C triaromatic steroids, and unaltered C -C triaromatic 19 20 26 28 steroids and methyl triaromatic steroids 2: substantially removed low-carbon-number triaromatic steroids and methyl triaro- matic steroids, high-carbon-number triaromatic steroids still unchanged 1 3 394 Petroleum Science (2021) 18:380–397 Table 4 Refined Manco numbers for Carboniferous oils of Chepaizi Uplift calculated by the refined Manco scores. P, pregnane; DH, diahopane Well Vector element RMN PM DS*, μg/g G*, μg/g ∑PTs*, μg/g ∑TTs*, μg/g P*, μg/g DH*, μg/g 0 1 2 3 4 5 6 7 P70 4 4 4 3 1 0 0 0 546.04 6 – – – – – – P702 4 4 4 3 2 0 0 0 580.34 6 – – – – – – P66 4 4 4 3 3 1 0 0 669.22 7 44.96 175.41 1478.11 2596.9 83.42 55.75 P661 4 4 4 4 3 1 0 0 670.97 7 115.02 139.3 1110.92 2825.58 62.29 20.05 P666 4 4 4 4 3 1 0 0 670.97 7 120.37 141.77 1313.7 2465.01 61.97 29.49 P61 4 4 4 4 4 2 1 0 786.71 8 37.05 144.57 1218.28 2140.41 68.76 45.95 P663 4 4 4 4 4 2 1 0 786.71 8 35.14 161.56 1353.03 1525.6 73.84 56.51 P668 4 4 4 4 3 2 1 0 784.45 8 44.04 129.12 1285.58 1452.22 64.23 47.65 P60 4 4 4 4 3 2 1 0 784.35 8+ – – – – – – P665 4 4 4 4 4 3 2 1 909.63 9+ – – – – – – P685 4 4 4 4 4 3 2 0 830.14 9+ 27.76 220.18 945.75 780.71 103.28 69.37 compositions and parameter-stripping method of strongly scale to quantify the biodegradation extent of Carbonif- bioresistant parameters. Three biodegradation refractory erous oils from Chepaizi Uplift. The evaluation results compound class (25-norhopane, tricyclic terpanes and tri- clearly differentiate the biodegradation extent of heavy oils aromatic steroids) were added to establish a refined Manco with same PM values but different oil viscosities, indicat- ing a high resolution and potential prospect. 1 3 Petroleum Science (2021) 18:380–397 395 10 45 (a) (b) R = 0.8958 10 0 400 500 600 700 800 900 1000 400 500 600 700 800 900 1000 RMN RMN 2 2 1600 3000 (c) (d) 800 1500 0 0 400 450 500 550 600 650 700 750 800 850 900 400 450 500 550 600 650 700 750 800 850 900 RMN RMN 2 2 80 140 (e) (f) 0 0 400 450 500 550 600 650 700 750 800 850 900 400 450 500 550 600 650 700 750 800 850 900 RMN RMN 2 2 Fig. 10 Correlations of the refined Manco number (MN2) with oil viscosity (a), NSO content (b), summed pentacyclic terpane (c) and tricyclic terpane concentrations (d), and absolute concentrations of diahopane (e) and diasteranes (f). All the concentration data were cited after (Chang et al. 2018) Acknowledgement This work was funded by the National Natural provide a link to the Creative Commons licence, and indicate if changes Science Foundation of China (Grant No. 42072172, 41772120), the were made. The images or other third party material in this article are Shandong Province Natural Science Fund for Distinguished Young included in the article’s Creative Commons licence, unless indicated Scholars (Grant No. JQ201311), and the SDUST Research Fund (Grant otherwise in a credit line to the material. If material is not included in No. 2015TDJH101). Associated editor Jie Hao and seven anonymous the article’s Creative Commons licence and your intended use is not reviewers were deeply acknowledged for their critical comments and permitted by statutory regulation or exceeds the permitted use, you will helpful suggestions, which greatly improved the early version of this need to obtain permission directly from the copyright holder. To view a manuscript. copy of this licence, visit http://creativ ecommons .or g/licenses/b y/4.0/. Open Access This article is licensed under a Creative Commons Attri- bution 4.0 International License, which permits use, sharing, adapta- tion, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, 1 3 ΣPTs, µg/g Viscosity, mPa·s Diahopane, µg/g ΣTTs, µg/g Diasterane, µg/g NSO, % 396 Petroleum Science (2021) 18:380–397 Killops SD, Nytoft HP, di Primio R. 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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

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Springer Journals
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Copyright © The Author(s) 2021
ISSN
1672-5107
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1995-8226
DOI
10.1007/s12182-020-00542-x
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Abstract

The Carboniferous volcanic reservoir in the Chepaizi Uplift became an exploration hot target in recent years for its sub- stantial 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 (II , II and II ), 1 2 3 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 (RMN ) positively correlated with the oil density, NSO contents, and absolute concentrations of diasteranes and gammacerane, negatively correlated with the absolute concentrations of diaho- pane, 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. Keywords Super heavy oil · Biodegradation · Parameter-striping method · Refined Manco scale · Junggar Basin 1 Introduction degradation of the conventional oils over geological time- scales (Larter et al. 2012). With the proceeding of biodegra- Heavy oil and oil sand bitumen dominate the world’s oil dation, the oil will exhibit decreases in mass and net volume, inventory (Marcano et al. 2013), even exceed the normal increases in oil density, oil viscosity, oil acidity and sulfur oil in quantity (Wang et al. 2016; Li and Huang 2020). Bio- content, enrichments in nitrogen, sulfur and oxygen-contain- degradation may be responsible for these unconventional ing organic compounds, and trace mental elements, which resources, which primarily formed from the microbial heavily affect the oil’s chemical compositions and physical properties (Larter et al. 2006; Zhou et al. 2008; Forsythe et al. 2019). Even within a single petroleum reservoir, bio- Edited by Jie Hao and Chun-Yan Tang degradation may create orders of magnitude variation in oil viscosity and fluid property laterally and vertically (Adams * Xiang-Chun Chang et  al. 2012). Allowing for their great impact on the eco- xcchang@sina.com nomic value and predictability, knowledge of the petroleum College of Earth Science and Engineering, Shandong biodegradation is critical in selecting exploitation strategies University of Science and Technology, Qingdao 266590, (Marcano et al. 2013; Yin et al. 2013), evaluating the alter- China ing extent (Bautista et al. 2015) and forecasting the oil com- Laboratory for Marine Mineral Resources, Pilot National positional characteristics and viscosity (Larter et al. 2012; Laboratory for Marine Science and Technology, Zhang et al. 2014a, b). Qingdao 266071, China Experimental examinations and case studies indicate that Research Institute of Petroleum Exploration relative sensitivity to microbial attack exist among differ - and Development, Shengli Oil Company, ent compound class, even among the individual compound Sinopec, Dongying 257001, China Vol:.(1234567890) 1 3 Petroleum Science (2021) 18:380–397 381 in each class. Therefore, biodegradation is a quasi-stepwise attracted much more attention. Using semi-quantitative process with consume of biomarkers in a preferential order parameter-stripping method and refined Manco scale, this (Peters et  al. 2005). Based on the alteration/removal of paper aims to deeply understand and quantitatively deter- saturated biomarkers, i.e., normal alkanes, acyclic isopre- mine the biodegradation extent of heavy oils in the Chepaizi noids, terpanes and steranes, and selected aromatic com- Uplift, Junggar Basin, NW China. pounds (mainly aromatic steroids), Peters and Moldowan (1993) developed a classical scale to assess the extent of oil biodegradation, which is typically abbreviated as PM 2 Geological setting scale. Five terms, i.e., light (PM1–3), moderate (PM4–5), heavy (PM6–7), very heavy (PM8–9) and severe (PM10) The Chepaizi Uplift, covering an area of 10,500 km , is a are assigned to ranges of the PM scale. Another scheme was petroliferous target of the northwestern Junggar Basin (Zhao established by Wenger et al. (2002) fundamentally based on et al. 2019), which is surrounded by two hydrocarbon gen- the alteration extent with a compound class and the presence erative source kitchens, i.e., the Changji Sag to the east and or depletion of single key compound class, only with the Sikeshu sag to the south (Fig.  1). The early stage of late terms of heavy and severe to describe the PM 4–10 range. Hercynian tectonic movement witnessed the formation of The normal alkanes are generally more susceptible to prototype of the Chepaizi Uplift, which subsequently expe- biodegradation than the branched alkanes and isoprenoids rienced the intense uplift during the Indosinian and Yanshan (Chen et al. 2017), thus ratios of i-C /n-C , pristane/n-C movements, slow subsidence during the Himalayan tectonic 5 5 17 and phytane/n-C are widely used to assess the alteration movement. As an inherited paleo-uplift, Chepaizi Uplift was content of light to moderate biodegraded oils (Peters and characterized by the seriously lack of deposition succession Moldowan 1993). However, some case studies showed no in the structurally high parts, with the Cretaceous, Paleo- evident correlations between Ph/n-C and oil API, and gene, Neogene and Quaternary sedimentary rocks overly then the parameter “mean degradative loss” was proposed Carboniferous volcanics (Song et al. 2007). to assess the biodegradation extent by means of quantify- Due to the multi-staged transgression and uplifting, ing the depletion in volumetrically important individual oil three sets of favorable reservoir rocks were developed in the constituents, especially for the light to moderate level of Chepaizi Uplift, i.e., medium-fine sandstone and glutenite alteration (Elias et al. 2007). Additionally, the production occurred in the Neogene Shawan Formation, fine sandstones and biodegradation of oxygen-containing compounds also and siltstone occurred in the Cretaceous interval, and basalt, follow a preferential order, thereafter the acyclic (DBE 1)/ andesite, tuff and volcanic breccia in the Carboniferous cyclic (DBE 2-4) can reveal the biodegradation level, which interval. well correlated with the PM scale when it less than PM < 6 The Hongche Fault, an active fault separating the Che- (Angolini et al. 2015). paizi Uplift from the Changji Sag (Dong et al. 2017; Ni et al. Many case investigations suggest that the sequential 2019), cut through sandbodies developed in the Cretaceous, microbial degradation of oil constitutes does not occur in Neogene Shawan Formation and volcanics in the Carbonifer- a true stepwise fashion and strictly follow the schemes pre- ous, providing a favorable vertical conduit for the migration viously developed (Bennett and Larter 2008; Wang et al. of hydrocarbon generated in the Changji Sag to the Chepaizi 2013; Chang et al. 2018). All these scales are unsuitable Uplift and yielding three vertical oil-bearing intervals (Miao for heavy oil or super heavy oil, and for the mixtures of et al. 2015; Meng et al. 2016). oils (Larter et al. 2012; Zhang et al. 2014a, b). Selecting 8 compound classes (alkyl toluenes, C naphthalenes, C 0-1 2 naphthalenes, C naphthalenes, methyl dibenzothiophenes, 3 Samples and experimental C naphthalenes, C phenanthrenes and steranes) to reflect 4 0-2 the increasing resistance to biodegradation, assigning 0–4 3.1 Sample preparation scores to describe the removal extent, Larter et al. (2012) proposed a Manco scale to assess the biodegradation level Eleven DST oil samples from the Carboniferous volcanic (PM4–8), with its MN2 value positively correlates with oil interval were collected from wells in the Chepaizi Uplift viscosity, and effectively used (López 2014). for geochemical analysis. Asphaltenes were removed from Till now, published studies mainly focused on the geo- the oil samples and source rocks using n-hexane precipita- chemical behaviors of molecular biomarkers at the bio- tion, and the deasphaltened oil was divided into two aliquots. degradation level of PM8 or less, for those biodegradation The first of these underwent column chromatography using a level higher than PM8, rare studies were reported. With the routine silica gel and alumina column from which aliphatic reduction in conventional resources, extremely biodegraded and aromatic fractions were obtained using n-hexane and oils continuously discovered in the petroleum exploration dichloromethane (DCM). 1 3 382 Petroleum Science (2021) 18:380–397 1 3 Zhongguai Uplift Hongche Fault Shawan Sag Chepaizi Uplift Wulun Depression Luliang Rise Yilinheibier Mountains Thrust-fold Belt -250 Central Depression -300 -350 -400 -450 -500 -550 80° E85° E 90° E 95° E P665 080 km 04 8 km P60 P66 P666 P61 P661 P685 P663 P668 East Rise (a) 84°50' 85°10' 85°30' 45° 10' 040 km P66 P612 P70 44° 50' Su1 P702 P70 Sikeshu Sag 44° 30' (b) 15290000 15300000 15310000 15320000 15330000 (c) 44° Boundary of Structure contour of Basin boundary Faults Oil fields Wells -100 10' tectonic units carboniferous top surface 84°50' 85°10' 85°30' Fig. 1 Map showing structural elements of the Junggar Basin (a), with location of the Chepaizi Uplift (b), and a field-scale map showing the sampling wells in the Chepaizi Uplift (c). This figure was modified after (Chang et al. 2018) West Ris is i e e -150 -200 -250 -300 -350 -400 -450 -500 -550 -600 Zhayier Mountains -100 -150 -200 -50 -100 Wuxia Fault Hala’alat Mts. Zhayier Mts. -600 -750 -800 -850 -900 -950 -1000 -1050 -1100 -1150 -400 -800 -650 -700 -450 -500 -750 -150 -200 -250 -300 -350 -400 -50 -100 -150 -200 -250 -300 -350 -700 -700 -550 -50 -400 -850 -900 -650 -100 Hongche Fault Zone 44° N 46° N 48° N 4990000 5000000 5010000 Petroleum Science (2021) 18:380–397 383 As for the study of oil origins, oil family is a widely used 3.2 Gas chromatography–mass spectrometry term to distinguish oils with different genetic affinity. An oil family, a group of oils that derived from a same source Gas chromatography–mass spectrometry (GC–MS) analysis of the aliphatic and aromatic fractions was performed with rock, possibly experienced similar reservoir-forming history, belonged to a same oil system, and possessed same or simi- a Finnigan Model SSQ-710 quadrupole analytical system coupled to a DB-5 fused silica column (30 m × 0.32 mm i.d.) lar chemical compositions. Combing the ratios of C dias- terane 20S/20R (C DS 20S/20R), C diasterane/C regu- and linked to an IAIS data processing system. GC tempera- 27 27 27 ture operating conditions for the aliphatic fraction were as lar sterane (C DS/CRS), C /C triaromatic steroid (20S) 27 27 26 28 (C /C TAS(20S)), and C /C TAS(20R) with the stable follows: 100 °C (1 min) to 220 °C at 4 °C/min and, then to 26 28 27 28 300 °C (held 5 min) at 2 °C/min; for the aromatic fraction: carbon isotope distribution, previous studies concluded that the Carboniferous oils in the eastern Chepaizi Uplift were 80 °C (1 min) to 300 °C (held 15 min) at 3 °C/min. MS conditions were as follows: electron impact (EI) ionization mainly derived from the mudstone of the Middle Permian Wuerhe Formation (P w) in the Changji Sag, whereas the mode; 70-eV electron energy; 300-mA emission current; and 50–550 amu/s scan range. ones in the western Chepaizi Uplift were essentially origi- nated from the Jurassic mudstone in the Sikeshu Sag (Zhang et al. 2012; Xu et al. 2018; Mao et al. 2020). Although two oil charging episodes were defined in the Carboniferous 4 Results and discussion reservoirs based on the oil geochemistry, fluid inclusions and basin modeling (Chang et al. 2019; Shi et al. 2020), the 4.1 Oil bulk compositions and oil families later charge actually was the remigration of early reservoired oils due to the tectonic adjustment (Cao et al. 2010; Song The Carboniferous oils from the eastern Chepaizi Uplift are characterized by higher oil density (0.9285–0.9590 g/ et al. 2016; Chang et al. 2019). Although the Carbonifer- ous oils all exhibited the characteristics of lacustrine source cm ) and viscosity (154–8968 mPa·s) than those from the western Chepaizi Uplift, which can be classified as heavy facies, source-diagnostic and redox potential of depositional environment-related molecular biomarkers showed marked crude oils (Table 1). The Carboniferous oils are predomi- nantly aliphatic, as indicated by their saturate/aromatic distinction between the eastern and western parts (Xu et al. 2018), implying at least two oil groups. (ST/AR) ratio (1.92–3.56) and saturate fraction abundance (44.45%–64.38%). The oil density showed roughly positive Cluster analysis, a statistical method, is an effective tool to classify studied samples into different groups by their correlation with the viscosity and the NSO (resin + asphal- tene) fraction content, and negative correlation with the bur- similarity distances which were calculated from the different variables investigated. Allowing for the severe oil alteration, ial depth. Progressive biodegradation of crude oils may be responsible, as it decreases the content of saturate and aro- eight biomarker parameters strongly resistant to biodegrada- tion were selected to act as the variables in the clustering matic hydrocarbons and enriches the resins and asphaltenes, resulting in an increase in oil density (López et al. 2015; analysis, i.e., C /C TAS (20S), C /C TAS (20R), C tet- 26 28 27 28 24 racyclic terpane/C hopane (C Tet/C H), gammacerane/ Wenger et al. 2002). 30 24 30 Table 1 Bulk compositions of crude oils from Chepaizi Uplift. ST, saturate hydrocarbon; AR, aromatic hydrocarbon; NSO, resin + asphaltene; G/C H, gammacerane/C hopane; *, data cited from Chang et al. (2018) and Xu et al. (2018) 30 30 3 13 Area Well Depth, m Strata Density*, g/cm Viscosity*, ST*, % AR*, % NSO*, % δ C*, ‰ G/C H* mPa·s Western P70 699–713 C 0.9190 38.5 59.94 12.28 27.78 − 27.50 0.08 P702 663.7–698.69 C 0.9254 47.1 − 27.80 Eastern P66 1109.06–1123. C 0.9285 154 64.38 18.49 17.12 − 30.40 0.49 P661 1106.2–1125.0 C 0.9288 149 63.02 20.71 16.27 − 30.00 0.54 P666 922.69–1140.77 C 0.9297 359 61.40 21.58 17.02 − 29.90 0.51 P61 855.73–949.58 C 0.9398 390 56.58 17.37 26.05 − 30.10 0.63 P663 928.45–1031 C 0.9528 1182 55.59 20.68 23.73 − 30.40 0.65 P668 953.15–1069.51 C 0.9528 1880 44.45 12.50 43.05 − 30.30 0.41 P60 690–800 C 0.9389 3079 47.06 18.00 34.94 − 31.00 0.66 P665 781.5–985.85 C 0.9590 8968 45.19 23.01 31.80 − 30.50 3.93 P685 808.82–892 C 0.9524 2600 47.22 24.60 28.18 − 30.40 4.27 1 3 384 Petroleum Science (2021) 18:380–397 C hopane (G/CH), C H (22S)/C H (22S), C 18α(H)- (TAS) were essentially unchanged (Fig. 3d). Thereby, Group 30 30 35 34 29 30- norneohopane/C Hopane (C Ts/CH), C tricyclic I can be assigned to biodegradation level of PM6. 29 29 29 24 terpane (TT)/CTT, C TT/C TT. In the clustering tree 23 22 21 graph (Fig. 2), the Carboniferous oils can be clearly divided 2. Oil group II- family II into two groups, that is, Group I for the western Chepaizi Uplift, Group II for the eastern Chepaizi Uplift. In addition, within the Groups II, three oils families can be further subdi-Family II (wells P661, P666 and P66), featured substantially vided (II, II and II ) according to their similarity distances. removed n-alkanes and iso-alkanes and prominent “UCM” 1 2 3 on the TIC chromatograms (Fig. 4a). C homohopanes 31–35 were heavily depleted. Tricyclic terpanes were far higher 4.2 Biodegradation level by PM scale than hopanes in abundance with reversed “V” distribution of C TT–C TT–C TT (Fig. 4b). Gammacerane was rela- 20 21 23 4.2.1 Qualitative evaluation by molecular compositions tively high in content as showed by the high G/C H values 1. Oil Group I (0.49–0.54). Pregnane and homopregnane were enriched and nearly equal to the regular steranes in abundance with reversed “L” distribution of ααα20R C –C –C sterane 27 28 29 Group I (wells P70 and 702), Carboniferous oils from the (Fig.  4c). Stable carbon isotope varied from −29.9‰ to western Chepaizi Uplift, was least biodegraded among −30.0  ‰ (Table 1). Naphthalenes and phenanthrenes were the investigated oils. These oils showed faintly “UCM” substantially depleted (Fig. 4d). The biodegradation can be and relatively intact n-alkanes on the TIC chromatogram classified into PM6 + to PM7. (Fig.  3a). Hopanes were slightly higher than the tricy- clic terpanes (TTs) in abundance with C hopane as the 3. Oil group II- family II 30 2 peak compound and reversed “L” -shaped distribution of C TT-C TT-C TT (Fig. 3b). Gammacerane (G) was low 20 21 23 in content as evidenced by the quietly low G/C H values Family II (wells P663, P668, P61 and P60), showed heavily 30 2 (0.08–0.09). Fully developed 25-norhopanes were detected removed hopane series with C hopane as the peak com- indicating heavy biodegradation. Pregnane, homopregnane, pound, equal tricyclic terpanes to hopanes in abundance with and diasteranes (DS) were lower than the regular steranes sequentially increased distribution of C TT–C TT–C 20 21 23 (RS) in abundance with “V”-shaped distribution of ααα20R TT (Fig. 5b), and high G/C H ratios (0.41–0.66). Pregnane C -C -C steranes (Fig. 3c). Stable carbon isotope var- and homopregnane far exceeded the regular steranes with 27 28 29 ied from −27.5 ‰ ~ −27.8  ‰ (Table 1). Naphthalenes and “L” distribution of ααα20R C –C –C sterane (Fig. 5c). 27 28 29 phenanthrenes were nearly intact and triaromatic steroids The diasteranes were prominently altered. Stable carbon Euclidean distances Euclidean distances 05 10 15 20 25 05 10 15 20 25 P663 P70-2 Group I P668 P702 P70-1 P60 P666 P661 P61 P666 Family II P66-1 P66-1 Group II P66-2 P661 P663 P66-2 P668 P665-1 Family II Group II P61 P685 P60 P665-2 P665-1 P70-1 Family II3 P685 P702 Group I P665-2 P70-2 (a) Oil groups (b) Oil families Fig. 2 Clustering tree graphs showing the different oil groups (a) and oil families (b) 1 3 Petroleum Science (2021) 18:380–397 385 Ph C30H (a) TIC (b) m/z 191 C H nC17 25-NH nC Pr Tm Ts C19 C Tet C23 C 20S (c) m/z 217 (d) m/z 231 C 20R C 20R+C 20S 26 27 C21 C2620S C 20R C P C P Retention time Fig. 3 Representative fragmentograms of oil group I showing molecular compositions (well P70). a Total ion chromatogram (TIC); Pr, pristane; Ph, phytane. b m/z 191, C -C , tricyclic terpanes with different carbon number; Tet, tetracyclic terpane; Ts, 18α(H)- trisnorneohopane; Tm, 19 26 17α(H)- trisnorhopane; 25-NH, C 25-norhopane; CH, C hopane; CH, C hopane; G, gammacerane. c m/z 217, CP, C pregnane; C P, 29 29 29 30 30 21 21 22 C homopregnane; and d m/z 231, C –C, C –C triaromatic steroids; C –C 20S and 20R,20S and 20R isomers for triaromatic steroids 22 20 21 20 21 26 28 with different carbon numbers isotope (−29.7‰ to −30.7‰) was like that of the family Although many studies reported that the microbial degrada- II (Table  1). Triaromatic steroids were still unchanged tion of oil did not appear to be strictly consistent with the (Fig. 5d). The biodegradation level reached PM8 to PM8+ . stepwise fashion in established schemes, relative variations of biomarkers in oil families II, II and II can be promi- 1 2 3 4. Oil group II- family II nently distinguished (Fig.  7). The values of C T/C TT, 3 22 21 C TT/CTT, C C Tet/C TT and (C + C )/C TT 24 23 24 24 26 20 21 26 Family II (wells P665 and P685), by contrast, exhibited ratios increased with the biodegradation extent increased the strongest biodegradation characteristics. Hopanes were from PM7 to PM8 and finally to PM9+ (Fig.  7a–d), sug- essentially removed with C 25-norhopane as the peak com- gesting the alterations of tricyclic terpanes, preferential pound, and tricyclic terpanes were prominently reduced removal of lower molecular weight homologues and more (Fig. 6b). Gammacerane was abnormally high with G/C H bioresistance of tetracyclic terpane (Huang and Li 2017). values ranging from 3.93–4.27. Pregnane and homopreg- Hopane/sterane ratio varied with biodegradation in a zig- nane were far beyond the majorly depleted regular steranes zag-shaped fashion (Fig.  7e), i.e., essentially unchanged (Fig. 6c). However, no changes of the stable carbon isotope from the level of PM6 + to PM7, sharply increased from the (−29.7‰ to −30.7‰) can be seen (Table 1). The essentially level of PM7 to PM8, and progressively decreased from the unaltered triaromatic steroids (Fig. 6d) confirmed the bio- level of PM8 to PM9+, confirming the different suscepti- degradation level of PM9+ . bility to biodegradation for hopanes and steranes at differ - ent biodegradation level (Reed 1977;). Ratios of C /C H, 29 30 G/ H and C diahopane/C hopane (C D/C H) kept 30 30 30 30 30 5. Sequential biodegradation revealed by biomarkers relatively unchanged from level PM7 to PM8, then sharply increased to PM9 + (Fig. 7f–h), indicating approximately 1 3 Relative abundance ααα(20R)C29 ααα(20R)C ααα(20R)C 28 386 Petroleum Science (2021) 18:380–397 Tricyclic terpanes (a) TIC C TT (b) m/z 191 C TT 23 Pentacyclic terpanes C23 C19TT RS C24 C H 25-NH C30H C C H 26 29 25 Tm C19 Ts (c) m/z 217 (d) m/z 231 C2620R+C2720S C2820S C P C2820R C2720R C P 22 C2620S Retention time Fig. 4 Representative fragmentograms of oil family II showing molecular compositions (well P661). a Total ion chromatogram (TIC), RS, regular sterane, b m/z 191, c m/z 217, and d m/z 231. Abbreviations are the same to Fig. 3 the same bioresistance of C hopane to C hopane, gam- (C + C )/C TAS demonstrated that the short-chained 30 29 20 21 26–28 macerane and C diahopane before PM8 level, but more counterparts were slightly destroyed above PM7 (Fig. 7p). susceptibility at level PM9 + . Gradually increased 18α(H)- trisnorneohopane/17α(H)- trisnorhopane (Ts/Tm) ratio with 4.2.2 Semi‑quantitative evaluation by parameter‑stripping biodegradation indicated the relatively more susceptible of method Tm than Ts to microbial alteration (Fig. 7i). Increasing C 25-norhopane/gammacerane (C NH/G) value with biodeg- radation confirmed the formation of 25-norhopane (Fig.  7j) Ideally, biomarkers were sequentially removed by biodegra- at level above PM6, which was consistent with the increas- dation in a stepwise fashion and can be described by the ten- ing C NH/C H and C NH/C H (Fig. 7k, 7l). However, point scales (Peters and Moldowan 1993). To further proof 28 29 29 30 its decrease from PM8 to PM9 + revealed the degradation the extent of biodegradation four biodegradation stages were of 25-norhopanes occurred under extreme biodegrada- defined following the general alteration tendency in PM tion level (Huang and Li 2017; Chang et al. 2018; Killops scheme (Fig. 8). Stage 1 covers approximately from PM1 to et al. 2019), as validated by the increasing C NH/C NH PM5, stage 2 from PM5 to PM7, stage 3 from PM7 to PM8, 29 28 (Fig. 7m). 25-norhopane ratio (∑C –C 25-norhopanes/ and stage 4 involves PM8 and higher. Biodegradation lev- 30 34 (∑C –C 25-norhopanes + ∑C –C homohopanes) els of different oil families were semi-quantitatively defined 30 34 31 35 generally exhibited a tendency to be increased with bio- by sequentially stripping the samples in the cross-plots of degradation, which was consistent with the formation of biodegradation-resistant parameters. 25-norhopanes and reduction in homohopanes (Fig.  7n). Increasing C DS/C RS with biodegradation displayed the 1. Stage 4 27 27 faster degradation of regular steranes than the diasteranes after PM7 level (Fig. 7o). Unchanged C /C TAS(20S) and Diasteranes show particular resistance to biodegradation and 26 28 C /C TAS(20R) well documented the strongest bioresist- remain where steranes and hopanes are totally removed in 27 28 ance of triaromatic steroids (Fig. 7q, 7r), yet the decreasing case of no 25-norhopanes are present (PM9) (Peters et al. 1 3 Relative abundance ααα(20R)C29 ααα(20R)C ααα(20R)C 28 Petroleum Science (2021) 18:380–397 387 (a) TIC (b) m/z 191 Pentacyclic terpanes Tricyclic terpanes 25-NH RS C23TT 25-NH C21 C21TT C26 C30H C C H 24 30 C29H C24Tet Tm C25 C26 Ts (c) m/z 217 (d) m/z 231 C 20R+C 20S 26 27 C P C 20S C 20R C P C 20R C 20S C21 C20 Fig. 5 Representative fragmentograms of oil family II showing molecular compositions (well P663). a Total ion chromatogram (TIC), b m/z 191, c m/z 217, and d m/z 23. All abbreviations are the same to Fig. 3 2005). Pregnane (P) and homopregnane have high resistance et  al. 2020); therefore, the biodegradation extent may be to biodegradation, comparable to the diasterane (PM9). Tri- responsible for the observed differences of oil compositions. cyclic terpanes can be altered at about the same level as the The tricyclic terpanes feature similarly high bioresistance to diasteranes (Lin et al. 1989). Non-hopanoid triterpanes, such the diasteranes (PM > 8; Lin et al. 1989), slightly lower than as gammacerane, diahopanes, oleanane, are highly resist- the non-hopanoid terpanes. In the extreme biodegradation ant to biodegradation (Peters et al. 2005), and even beyond cases, tricyclic terpanes with lower molecular weight would the point (PM ≥ 9) where the tricyclic terpanes have been be preferentially degraded (Huang and Li 2017; Chang et al. removed (Wenger and Isaksen 2002). Although aromatized 2018). Oil family II exhibited prominently higher C / 3 24 steroids keep unaltered in all but the most severely biode- C TT and C /C TT ratios than other oil families, implying 23 22 21 graded oils (PM10) and can be effectively used to determine a biodegradation level at least up to PM 9–PM9+ (Fig. 9c). the origin and thermal maturity for extremely biodegraded Obviously, by contrast, the biodegradation rank of family II oils (Peters and Moldowan 1993), low molecular weight tri-and II were lower than PM9. aromatic steroids (C -C ) are among the r fi st aromatic ster - 20 21 oids to be depleted during biodegradation (Wardroper et al., 2. Stage 3 1984). From the cross-plots of gammacerane/C diahopane (G/C D) vs. C DS(20S)/CDS(20R), C /C TAS(20S) Hopanes are removed before or after steranes, 25-norho- 30 27 27 26 28 vs. C /C TAS(20R), Carboniferous oils in the western and panes occur in oils where the hopanes are preferentially 27 28 eastern Chepaizi Uplift were clearly distinguished exactly removed (Reed 1977; Peters et  al. 2005), as supported corresponding to the Group I and II, respectively (Fig. 9a, by this investigation (Fig. 7e). C and C homohopanes 31 32 9b). were more susceptible to biodegradation than C hopane in the asphalts from Madagascar (Rullkotter and Wendisch Using triaromatic steroids ratios, previous studies concluded 1982), while C -C 17α-hopanes are typically biode- 28 30 that the Carboniferous oils from the eastern Chepaizi Uplift graded in the same manner and at approximately the same had a common origin and similar maturities (Xi et al. 2014; rate as the C -C extended hopanes (Williams et al. 1986). 31 35 Xiao et al. 2014; Xu et al. 2018; Chang et al. 2019; Mao The 25-norhopane ratio, that is ratio of the total C –C 30 34 1 3 ααα(20R)C ααα(20R)C27 ααα(20R)C 28 388 Petroleum Science (2021) 18:380–397 Pentacyclic terpanes (a) TIC (b) m/z 191 25-NH RS Tricyclic terpanes 25-NH C21P C26TT C22P C H C 29 G C24Tet Ts Tm C25 C30H C21 C19 (c) m/z 217 (d) m/z 231 C 20R+C 20S 26 27 C21P C 20S C2820R C22P C 20R C 20S C20 Retention time Fig. 6 Representative fragmentograms of oil family II showing molecular compositions (well P665). a Total ion chromatogram (TIC), b m/z 191, c m/z 217, and d m/z 23. All abbreviations are the same to Fig. 3 25-norhopanes to the sum of these compounds plus the of sterane and hopane biodegradation (Peters et al. 2005). C –C homohopanes, could be used to evaluate the alter- Regular steranes are removed after the complete removal of 31 35 nation extent among severely biodegraded oils of PM ~ 6–9 C -C isoprenoids and before or after the hopanes (Peters 15 20 (Peters et al. 1996). Oil family II , featured medium ratios et al. 2005). Comparatively, slight alteration of methyl and of C P/∑C RS and C P/∑C RS (Fig. 9d), which dimethylnapthalenes occurs during the removal of n-alkanes, 21 27–29 22 27–29 clearly distinguished from those of the family II (quite high) trimethylnaphthalenes are altered during the removal of and family II (rather low), and further verified by the vary - the isoprenoids, and tetramethylnaphthalenes persist until ing range of C 25-NH/C H and C 25-NH/C H ratios steranes are largely depleted (Fisher et al. 1998). Phenan- 29 30 30 31 (Fig. 9e). Hence, the biodegradation level of the family II threnes are generally more resistant to biodegradation than can be evaluated at PM8–PM8+ , while the family II less alkylnaphthalenes, and methylbiphenyls, dimethylbiphe- than PM8. nyls, and methyldiphenylmethanes lacks in the biodegraded oils at PM7 (Peters and Moldowan 1993). Higher-hopane 3. Stage 2 homologues, particularly the C pentahomohopanes, are preferentially bioresistant, and the alteration of C hom- Normal alkanes are preferentially removed at the biodeg- hopanes are possibly at the rank of PM ≥ 7 (Seifert et al. radation level of PM1–2; however, selective biodegrada- 1984; Moldowan et al. 1995). The G/C H and C hopane/ 30 29 tion of the isoprenoid over more bioresistant steranes or C hopane (C H/C H) varied from roughly unchanged 30 29 30 hopanes is used to determine the level of PM3–4 (Peters to rapidly increased with the increasing biodegradation et  al. 2005). Long-chained alkylated cyclopentanes and level, confirming the faster degradation of C H than C H, 30 29 cyclohexanes are about as susceptible as branched alkanes especially at level higher than PM8 (Bennett et al. 2006; and monocyclic alkanes will be depleted or in trace quanti- Chang et al. 2018); however, the substantial constancy of C ties at PM3–4 level (Peters and Moldowan 1993). C -C 18α-30-norneohopane/C hopane (C Ts/C H) implies the 14 16 29 29 29 bicyclic terpanes are less susceptible to biodegradation than similar susceptibility of C Ts and C H to biodegradation 29 29 isoprenoid and will be completely eliminated before the start (Chang et al. 2018). Oil family II was biodegraded to PM7 1 3 Relative abundance ααα(20R)C29 ααα(20R)C ααα(20R)C 28 Petroleum Science (2021) 18:380–397 389 C /C TT C /C TT C Tet/C TT (C +C )/C TT H/S C /C H 22 21 24 23 24 26 20 21 26 29 30 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 01234 0 0.2 0.4 0.6 0.8 01 24 3 01234567 0123456 P661 (a) (b) (c) (d) (e) (f) P66 P666 P61 P663 P668 P60 P665 P685 G/C H C D/C H Ts/Tm C NH/G C NH/C H C NH/C H 30 30 30 29 28 29 29 30 01 234 5 01 2 0 0.2 0.4 0.6 0.8 1.0 0123 02 1 34 02 1 3456 P661 (g) (h) (i) (j) (k) (l) P66 P666 P61 P663 P668 P60 P665 P685 25-NHs/ C NH/C NH C DS/C RS (C +C )/C TAS C /C TAS(20S) C /C TAS(20R) 29 28 27 27 20 21 26-28 26 28 27 28 ∑ ∑ ( 25NHs+ HHs) 01 23 0 0.2 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 0 0.04 0.08 0.12 0.16 0.200 0.1 0.2 0.3 0.4 0.5 0 0.2 0.4 0.6 0.8 1.0 P661 (m) (n) (o) (p) (q) (r) P66 P666 P61 P663 P668 P60 P665 P685 Fig. 7 Correlations of biomarker parameters among three oil families of Group II showing the alterations of biomarkers with different suscepti- bility to biodegradation. TT, tricyclic terpane; Tet, tetracyclic terpane; H, hopane; G, gammacerane; Ts, 18α(H)-trisnorneohopane; Tm, 17α(H)- trisnorhopane; NH, 25-norhopane; C D, C diahopane; ∑25-NHs, sum of C -C 25-norhopanes; ∑HHs, sum of C -C homohopanes; DS, 30 30 30 34 31 35 diasterane; RS, regular sterane; TAS, triaromatic steroid. All the ratio data were cited after (Chang et al. 2018) by its lower C /C hopane (22S) and HHI values (Fig. 9f, 4.3 Quantitative evaluation by refined Manco scale 35 34 g). Furthermore, the tetramethylnaphthalene ratio (TeBR = 1,3,6,7-TeMN/1,3,5,7TeMN, TeMN = tetramethylnaphtha- 4.3.1 Refined Manco scale lene; Peters et al. 2005) begins to be altered significantly at levels PM5–6 (Fisher et al. 1998). Therefore, oil family II The present biodegradation scales are essentially estab- distinctly distinguished from other families and showed bio- lished according to the presence or absence of single degradation level of PM7 as evidenced by the higher TeBR compound classes and the alteration extent within a com- values (Fig. 9h). pound class, which encountered issues for super heavy oils (Larter et al. 2012). Manco scale emerged as the require- 4. Stage 1 ment, integrating the extent of degradation of various sets of compound classes. This quantitative Manco scale pro- No biodegradation occurred. vided a higher resolution, however, only effective for the biodegradation of PM4–8. 1 3 390 Petroleum Science (2021) 18:380–397 Pristine Light Moderate Heavy Very heavy Severe References 0 1 2 3 4 5 6 7 8 9 10 Stage 4 Seifert et al., 1984; Non-hopanoids Zhang et al., 1988 Diasteranes, Diahopane Seifert & Moldowan, 1979 Tricyclic terpanes Connan, 1984; Lin et al., 1989 Stage 3 C -C steranes Peters & Moldowan, 1993 21 22 25-norhopanes Killops et al., 2019 Stage 2 C -C hopanes Williams et al., 1986 27 29 C31-C35 hopanes Rullkotter & Wendisch, 1982 Tetracyclic terpanes Schmitter et al., 1982 Seifert & Moldowan, 1979 Regular steranes C hopane (25-norhopane formed) Stage 1 Reed, 1977 C14-C16 bicyclic terpane ? Alexander et al., 1983 Isoprenoids Connan, 1984 Peters & Moldowan, 1993 Alkylcyclohexanes n-alkanes Connan, 1984 Stage 4 C -C monoaromatic steranes Wardroper et al., 1984 20 22 C -C triaromatic steroids Wardroper et al., 1984 26 28 C -C monoaromatic steranes Wardroper et al., 1984 27 29 Wardroper et al., 1984 C20-C21 triaromatic steroids Stage 3 Peters & Moldowan, 1993 Ethyl- and trimethylbiphenyls Stage 2 Peters & Moldowan, 1993 Ethyl phenanthrenes Methyl biphenyls Peters & Moldowan, 1993 Dimethyl phenanthrenes Fisher et al., 1998 Tetramethyl phenanthrenes Stage 1 Fisher et al., 1998 Fisher et al., 1998 Methyl phenanthrenes Trimethyl naphthalenes Peters & Moldowan, 1993 Fisher et al., 1998 Methyl- and dimethyl naphthalenes Monocyclic aromatic hydrocarbons Fisher et al., 1998 First altered Substantially depleted Completely eliminated Fig. 8 Schematic plot of the four-stages of the PM scale More biodegradation refractory compound class, i.e., i RMN = m 5 1 i 25-norhopane, tricyclic terpanes and triaromatic steroids, were added to refine the Manco scale to meet the situation of RMN =[n +(log RMN ⋅ S − 1 ]∕n very heavy to severe biodegraded oils from Chepaizi Uplift. 2 5 1 max Followed the academic principles of Manco scale, eight where m refers to the refined Manco score for each out of the compound classes that covered the whole range of biodegra- eight vector elements (0–4), i means the class number (0–7), dation rank (PM0–10) and possessed increasing resistance to and n is the number of compound classes. S , maximum max biodegradation were selected as vector elements. The eight for RMN , is designate to be 1000 to avoid confusion with compound classes and their GC–MS detection m/z values the currently existing scales, and to ensure enough resolu- are listed in Table 2. tion at different levels of biodegradation when using integer Five levels of refined Manco score (RMS) from 0–4 were values. assigned to the 8 compound classes, referencing after the When applied the refined Manco scale to evaluate the Manco scale (Table 3). The refined Manco number (RMN Carboniferous oils in the Chepaizi Uplift (Table 4), the and RMN ) can be calculated by the following formulas. 1 3 Aromatic hydrocarbons Saturate hydrocarbons PM6 PM7 Biodegradation increasing PM8~PM8+ Biodegradation increasing PM9~PM9+ Petroleum Science (2021) 18:380–397 391 2.00 0.85 (a) (b) Stage 4 Stage 4 0.80 Group II 0.75 1.50 0.70 0.65 1.00 0.60 Group I 0.55 Group I Group I 0.50 Group II 0.50 Family II1 Family II2 0.45 Family II3 0 0.40 0 0.10 0.20 0.30 0.40 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 G/C DH C /C TAS(20S) 30 26 28 1.00 0.40 (c) (d) Stage 4 Stage 3 0.80 PM9~PM9+ 0.30 0.60 0.20 0.40 <PM9 0.10 0.20 <PM8 0 0 0 0.50 1.00 1.50 2.00 2.50 3.00 3.50 0 0.10 0.20 0.30 0.40 0.50 C TT/C TT C (P)/∑C -C (RS) 24 23 21 27 29 6.00 0.60 (e) (f) Stage 3 Stage 2 5.00 0.50 PM9~PM9+ 4.00 0.40 3.00 0.30 2.00 0.20 PM8~PM8+ 1.00 0.10 <PM8 0 0 0 1.00 2.00 3.00 4.00 5.00 6.00 0 0.50 1.00 1.50 2.00 2.50 3.00 C (25-NH)/C (H) C H(22S)/C H(22S) 29 30 35 34 3.50 2.00 (g) (h) Stage 2 Stage 2 3.00 1.50 2.50 2.00 1.00 1.50 1.00 0.50 PM8~PM8+ 0.50 PM7 0 0 0 1.00 2.00 3.00 4.00 5.00 0 1.00 2.00 3.00 4.00 5.00 ETR DNR1 Fig. 9 Scattering plots of the Carboniferous oils by the parameter-striping method. DS, diasterane; G, gammacerane;/DH, diahopane; TAS, tri- aromatic steroid; TT, tricyclic terpane; P, pregnane; RS, regular sterane; 25-NH, 25-norhopane; H, hopane; C Ts, C 18α-30-norneohopane; 29 29 HHI, C homohopane/sum of C -C homohopanes; ETR, (C tricyclic terpane + C tricyclic terpane)/(C tricyclic terpane + C tricyclic 35 31 35 28 29 28 29 terpane +18α(H)-trisnorneohopane); TeBR, 1,3,6,7-/1,3,5,7 tetramethylnaphthalene; DNR1, (2,6- +2,7-)/1,5-dimethylnaphthalene; *, data cited from Chang et al. (2018) 1 3 PM9~PM9+ PM8~PM8+ Biodegradation increasing PM9~PM9+ PM8~PM8+ HHI C (25-NH)/C (H) C TT/C TT C DS(20S)/C DS(20R) 30 31 22 21 27 27 TeBR C Ts/C H C (P)/∑C -C (RS) C /C TAS(20R) 29 29 22 27 29 27 28 392 Petroleum Science (2021) 18:380–397 Table 2 Refined compound classes used to represent increasing bioresistance Vector element Compound class GC–MS m/z value 0 n-alkanes + C naphthalenes 85 + 156 + 170 + 184 + 198 1-4 1 Alkyl dibenzothiophenes 198 + 212 + 226 2 C phenanthrenes 178 + 192 + 206 + 220 0-3 3 C phenanthrenes 234 4 Hopanes + methylhopanes + 25-norhopanes 191 + 177 + 205 5 Regular steranes + diasterane + short-chain sterane 217 + 218 + 259 6 Tricyclic terpanes + 17-nor tricyclic terpane 191 + 177 7 Triaromatic steroid + methyl triaromatic steroid 231 + 245 RMN generally well correlated with the viscosity and 4.4 Geochemical implications NSO content (Fig. 10a, b), implying the loss of oils, espe- cially those light ends or susceptible compounds with Although the Carboniferous oils in the western and eastern increasing biodegradation extent. Summed pentacyclic Chepaizi Uplift were derived from different source kitchens, terpanes (∑PTs), summed tricyclic terpanes (∑TTs) and oils within the individual groups had common origin and diasteranes concentrations decreased with the increasing similar maturities (Zhang et al. 2014a, b; Xi et al. 2014; RMN (Fig. 10c, d, f), suggesting the slight alterations of Xiao et al. 2014; Xu et al. 2018; Mao et al. 2020). The dif- pentacyclic terpanes, tricyclic terpanes and diasteranes ferences observed in PM and refined Manco scales were from the PM7 to PM8 level, and sharp depletion from mainly attributed to the biodegradation extent. By contrast, PM8 to PM9 + . However, the variation of diahopane con- the refined Manco scale showed higher resolution than the centrations (Fig. 10e) possibly indicated its high biore- PM scale, which can differentiate the biodegradation extent sistance or other mechanism needed to further investigate. of super heavy oils with same PM values but different oil Generally, RMN ranged from 546.04 to 909.63, which viscosities (Table 4). approximately distinguished the oils at the level of PM6 For the eastern Chepaizi Uplift, Carboniferous oils were to PM9 + (Table 4). Exceptionally, by contrast, the oils mostly biodegraded above PM8; however, their oil viscos- from well P60 exhibited higher oil density but lower bio- ity displayed orders of magnitude variations. Known to all, degradation extent than that from well P685. Factually, the physical properties, particularly the oil viscosity, were there is not a simple relationship between the oil viscosity critical to the choice of exploration strategies. Subtle differ - and the Manco Number (Larter et al. 2012). Processes ences revealed by the refined Manco scale could be helpful. other than biodegradation, i.e., secondary oil charge, Besides the practical application, refined Manco scale also water washing, mixing of multiple maturity oil charges, allow geochemists conducting more basic study to under- and loss of light ends from heavy oils could produce var- stand the relative differences in the extent of the biodegrada- iations in oil viscosity and API gravity (López 2014). tion process among related samples. The two episodes of oil charging, early biodegraded oils mixed with the later remigration of preexisting oils due to the structural adjustment, yet the same oil origin in the 5 Conclusions Chepaizi Uplift maybe responsible for this case (Chang et al. 2019; Shi et al., 2020). The heavy to severe biodegradation was responsible for Notably, although the RMN calculated by the refined the Carboniferous heavy oils in the Chepaizi Uplift, with Manco scale showed roughly positive correlation with oil PM6 in the western and PM7–PM9+ in the eastern part, density, some samples, especially those three biodegraded respectively. According the oil-source correlation, the at PM8, exhibited much the same RMN but with different oils in the western Chepaizi Uplift were derived from the oil densities. This indicated that biodegradation refrac- Jurassic source rock in the Sikeshu sag and those in the tory compound class added in this refined scale could not eastern Chepaizi Uplift were originated from the Permian perfectly differentiate the appearances among oils biode- source rocks in the Changji sag. Therefore, two oil groups graded at about PM8. More assemblages of compound (I and II) were distinguished. Three oil families ( II, II 1 2 classes need to be established to refine the Manco scales and II ) were subdivided with the biodegradation level of in further trials. PM7, PM8–8+ , PM9+ , respectively, based on molecular 1 3 Petroleum Science (2021) 18:380–397 393 Table 3 Refined Manco Scores used to qualitatively distinguish the biodegradation level of compound classes Vector Compound class Refined Manco Scores ele- ment 0 n-alkanes + C naphthalenes 0: intact n-alkanes 1-4 1: slightly degraded n-alkanes, C naphthalene as the peak compound of alkyl naphthalenes 2: substantially degraded n-alkane, C naphthalene as the peak compound of alkyl naphthalenes 3: essentially depleted n-alkanes, C naphthalene as the peak compound of alkyl naphthalenes 4: partially remained C naphthalene in the alkyl naphthalenes 1 Alkyl dibenzothiophenes 0: non-degraded 1: only slightly degraded 2: mid-way between the extremes 3: not quite fully degraded 4: typically absent 2 C phenanthrenes 0: complete C phenanthrenes with the C phenanthrene as peak compound 0-3 0-3 0 1: slightly degraded phenanthrenes with the C phenanthrene as peak compounds 1-2 2: dimethyl and ethyl phenanthrene begun to be altered with the C phenanthrene as peak compound 3: substantially degraded dimethyl and ethyl phenanthrene, C phenanthrene begun to be altered with the C phenanthrene as peak compound 4: wholly removed dimethyl and ethyl phenanthrene, only trace C phenanthrene remained 3 C phenanthrenes 0: non-degraded 1: only slightly degraded 2: mid-way between the extremes 3: not quite fully degraded 4: typically absent 4 Hopanes +methylhopanes + 25-norhopanes 0: intact hopanes with C hopane as the peak compound 1: hopanes begun to be degraded still with C hopane as the peak compound, and 25-norhopane was present 2: hopanes were substantially degraded with C or C hopane as the peak com- 30 29 pound, 25-norhopane was prominently enriched in abundance 3: C -25norhopane was the peak compound in m/z 191 chromatograms, 25,28-binorhopane begun to come into existing. 4: completely depleted hopanes with C 25-norhopane as the peak compound, 25,28-binorhopane increased prominently in abundance. 5 Regular steranes + diasterane + short-chain sterane 0: completed regular steranes with higher contents compared to diasteranes 1: slightly degraded regular steranes, slightly lower contents than diasteranes 2: substantially degraded regular steranes, which were evidently lower than the slightly degraded diasteranes 3: essentially depleted regular steranes, substantially degraded diasteranes 6 Tricyclic terpane + 17-nor tricyclic terpane 0: intact TTs 1: slightly degraded TTs with the presence of 17-nor tricyclic terpane 2: substantially degraded TTs with relatively increased 17-nortricyclic terpane 3: essentially degraded TTs, with far higher 1contents of 17-nortricyclic terpane than TTs 7 Triaromatic steroid + methyl triaromatic steroid 0: intact triaromatic steroid and methyl triaromatic steroid 1: slightly degraded C -C triaromatic steroids, and unaltered C -C triaromatic 19 20 26 28 steroids and methyl triaromatic steroids 2: substantially removed low-carbon-number triaromatic steroids and methyl triaro- matic steroids, high-carbon-number triaromatic steroids still unchanged 1 3 394 Petroleum Science (2021) 18:380–397 Table 4 Refined Manco numbers for Carboniferous oils of Chepaizi Uplift calculated by the refined Manco scores. P, pregnane; DH, diahopane Well Vector element RMN PM DS*, μg/g G*, μg/g ∑PTs*, μg/g ∑TTs*, μg/g P*, μg/g DH*, μg/g 0 1 2 3 4 5 6 7 P70 4 4 4 3 1 0 0 0 546.04 6 – – – – – – P702 4 4 4 3 2 0 0 0 580.34 6 – – – – – – P66 4 4 4 3 3 1 0 0 669.22 7 44.96 175.41 1478.11 2596.9 83.42 55.75 P661 4 4 4 4 3 1 0 0 670.97 7 115.02 139.3 1110.92 2825.58 62.29 20.05 P666 4 4 4 4 3 1 0 0 670.97 7 120.37 141.77 1313.7 2465.01 61.97 29.49 P61 4 4 4 4 4 2 1 0 786.71 8 37.05 144.57 1218.28 2140.41 68.76 45.95 P663 4 4 4 4 4 2 1 0 786.71 8 35.14 161.56 1353.03 1525.6 73.84 56.51 P668 4 4 4 4 3 2 1 0 784.45 8 44.04 129.12 1285.58 1452.22 64.23 47.65 P60 4 4 4 4 3 2 1 0 784.35 8+ – – – – – – P665 4 4 4 4 4 3 2 1 909.63 9+ – – – – – – P685 4 4 4 4 4 3 2 0 830.14 9+ 27.76 220.18 945.75 780.71 103.28 69.37 compositions and parameter-stripping method of strongly scale to quantify the biodegradation extent of Carbonif- bioresistant parameters. Three biodegradation refractory erous oils from Chepaizi Uplift. The evaluation results compound class (25-norhopane, tricyclic terpanes and tri- clearly differentiate the biodegradation extent of heavy oils aromatic steroids) were added to establish a refined Manco with same PM values but different oil viscosities, indicat- ing a high resolution and potential prospect. 1 3 Petroleum Science (2021) 18:380–397 395 10 45 (a) (b) R = 0.8958 10 0 400 500 600 700 800 900 1000 400 500 600 700 800 900 1000 RMN RMN 2 2 1600 3000 (c) (d) 800 1500 0 0 400 450 500 550 600 650 700 750 800 850 900 400 450 500 550 600 650 700 750 800 850 900 RMN RMN 2 2 80 140 (e) (f) 0 0 400 450 500 550 600 650 700 750 800 850 900 400 450 500 550 600 650 700 750 800 850 900 RMN RMN 2 2 Fig. 10 Correlations of the refined Manco number (MN2) with oil viscosity (a), NSO content (b), summed pentacyclic terpane (c) and tricyclic terpane concentrations (d), and absolute concentrations of diahopane (e) and diasteranes (f). All the concentration data were cited after (Chang et al. 2018) Acknowledgement This work was funded by the National Natural provide a link to the Creative Commons licence, and indicate if changes Science Foundation of China (Grant No. 42072172, 41772120), the were made. The images or other third party material in this article are Shandong Province Natural Science Fund for Distinguished Young included in the article’s Creative Commons licence, unless indicated Scholars (Grant No. JQ201311), and the SDUST Research Fund (Grant otherwise in a credit line to the material. If material is not included in No. 2015TDJH101). Associated editor Jie Hao and seven anonymous the article’s Creative Commons licence and your intended use is not reviewers were deeply acknowledged for their critical comments and permitted by statutory regulation or exceeds the permitted use, you will helpful suggestions, which greatly improved the early version of this need to obtain permission directly from the copyright holder. To view a manuscript. copy of this licence, visit http://creativ ecommons .or g/licenses/b y/4.0/. Open Access This article is licensed under a Creative Commons Attri- bution 4.0 International License, which permits use, sharing, adapta- tion, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, 1 3 ΣPTs, µg/g Viscosity, mPa·s Diahopane, µg/g ΣTTs, µg/g Diasterane, µg/g NSO, % 396 Petroleum Science (2021) 18:380–397 Killops SD, Nytoft HP, di Primio R. 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