A case study of crude oil alteration in a clastic reservoir by waterflooding

Zhao

et al. 2018

Energy Fuels

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Biodegraded oils have been widely discovered throughout the world, whereas the alteration of the molecular composition of oils at extreme levels (>PM8) has been insufficiently documented. A suite of crude oils from Carboniferous volcanic reservoirs in the eastern Chepaizi Uplift, Junggar Basin, experienced severe to extreme biodegradation (from PM6+ to PM9+), which provided an ideal case for the present study. This investigation showed that the variations in molecular composition were not strictly consistent with their stepwise fashion in established schemes. The idea that 25-norhopanes are derived from hopanes was confirmed by the sharp decreases in the C29 hopane/gammacerane (C29H/G) and C30 hopane/gammacerane (C30H/G) values at the level of extreme biodegradation, which were associated with the increases in their counterparts of C28 25-norhopane/gammacerane (C28 25-NH/G) and C29 25-norhopane/gammacerane (C29 25-NH/G). 25-Norhopanes were also biodegraded at an extreme level, with C29 25-NH being more susceptible than C28 25-NH. The preferential biodegradation of individual homohopanes by carbon number occurred at an extreme level, whereas C29H featured more bioresistance than C30H and shared a similar susceptibility to biodegradation as 18α-30-norneohopane (C29Ts). The formation of 22R isomers for 25-norhopanes seemed to be favored over that of 22S isomers, although the 22S isomer was degraded faster that the 22R epimer for the C31, C32, and C33 homohopanes. However, the constant values of 22S/(22S + 22R) for the C34 homohopane implied no preferential biodegradation of 22S or 22R isomers for this extended hopane. Lower molecular weight tricyclic terpanes (TTs) were preferentially removed at extreme biodegradation levels, and the late eluting stereoisomers were degraded faster than the early eluting stereoisomers for C26TT, C28TT, and C29TT. C24 tetracyclic terpane (C24Tet) is much more resistant to biodegradation than TTs. Pregnanes have a similar susceptibility to biodegradation as gammacerane, but they are more resistant than C23TT. The biodegradation of regular steranes was characterized by their faster depletion than diasteranes and the preferential depletion of C27 regular sterane to the C29 homologue and the 20R isomers to the 20S isomers. At the extreme level, even C20 and C21 triaromatic steroids (TAS) were distinctively reduced, coexisting with the relatively highly degraded steranes and terpanes, although water washing can also be responsible for the decreases in (C20 + C21)-TAS/C26–28-TAS values.

Section: Resultsmentioning

confidence: 99%

Improved Understanding of the Alteration of Molecular Compositions by Severe to Extreme Biodegradation: A Case Study from the Carboniferous Oils in the Eastern Chepaizi Uplift, Junggar Basin, Northwest China

Zhao

et al. 2018

Energy Fuels

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“…Terpanes are always significant biomarkers in oil, as they can provide some information about the organic matter (Cao et al., 2008; Chang et al., 2016; Hanson et al., 2000; Peters et al., 2005). The distribution of terpanes using GC-MS by monitoring the m/z 191 ions is shown in Figure 7(c) and 7(d).…”

Section: Discussionmentioning

confidence: 99%

“…Terpane and sterane biomarkers. Terpanes are always significant biomarkers in oil, as they can provide some information about the organic matter (Cao et al, 2008;Chang et al, 2016;Figure 8. Plot of biomarkers of the samples, showing a mixed organic matter under marine environment.…”

Section: Paleoenvironmentmentioning

confidence: 99%

Geochemical characterization and possible hydrocarbon contribution of the Carboniferous interval natively developed in the Chepaizi Uplift of Junggar Basin, northwestern China

Mao

Energy Exploration & Exploitation

et al. 2019

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Previous studies on Chepaizi Uplift mainly focused on its reservoirs, and the potential source rocks natively occurred was ignored. During the exploration process, dark mudstones and tuffaceous mudstones were found in the Carboniferous interval. These possible source rocks have caused great concern about whether they have hydrocarbon generation potential and can contribute to the reservoirs of the Chepaizi Uplift. In this paper, the potential source rocks are not only evaluated by the organic richness, type, maturity, and depositional environment, but also divided into different kinetics groups. The Carboniferous mudstones dominated by Type III kerogen were evolved into the stage of mature. Biomarkers indicate that the source rocks were deposited in a marine environment under weakly reducing conditions and received mixed aquatic and terrigenous organic matter, with the latter being predominant. The effective source rocks are characterized by the total organic carbon values >0.5 wt.% and the buried depth >1500 m.

“…The Carboniferous oil from well P70 in the western Chepaizi Uplift, by contrast, exhibited a lighter oil density (0.919 g/cm 3 ) and viscosity (38.5 mPaÁs), but similar saturate (59.94%) and NSO fractional abundances (27.78%). Progressive biodegradation of crude oils may be responsible, as it decreases the content of saturate and aromatic hydrocarbons and enriches the resins and asphaltenes, resulting in an increase of oil density (Chang et al, 2016(Chang et al, , 2017Larter et al, 2003;L opez et al, 2015;Wenger et al, 2002).…”

Section: Oil Bulk Compositionsmentioning

confidence: 99%

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

Energy Exploration & Exploitation

Shi

et al. 2018

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A substantial amount of petroleum was recently discovered in the Carboniferous volcanic reservoir of the Chepaizi Uplift in the western Junggar Basin, yet the source is still indefinitive. Geochemical investigation indicates that the Carboniferous oils from the eastern and western Chepaizi Uplift are characterized by different source facies, although they are all typically of lacustrine origin. The eastern oils exhibit a restricted, clastic starved, highly reducing hypersaline condition during source rock deposition, which is distinct from the western oils. The Carboniferous oils were subjected to biodegradation ranging from rank 6 to rank 9, as indicated by the presence of 25-norhopane, evident depletion of hopanes and regular steranes, and even selective reduction of tricyclic terpanes. The maturities for the Carboniferous oils correspond to the onset of oil generation. The eastern oils contain lower (C 19 þ C 20)TT/ (C 23 þ C 24)TT and C 19 TT/C 21 TT, and lighter stable carbon isotopes than the western ones, correlating well with the Middle Permian Wuerhe (P 2 w) source rocks and the Jurassic source rocks, respectively. The good correlation of tricyclic terpanes source-related parameters further implies less contribution to the eastern oils by the Carboniferous source rocks.