Changes In The Compositions And The Secondary Hydrocarbon Generation Behavior Of Crude Oils Under The Influences Of Superimposed Secondary Alterations

Due to multi-stage tectonic movements,many oil reservoirs in typical superimposed basins of China have undergone more than one secondary alteration.Typically,these oil reservoirs experienced biodegradation at an early stage followed by thermal alteration at a later stage.The extent of each secondary alteration can be variable.Here,the succession of biodegradation and subsequent thermal alteration is called superimposed secondary alterations.Such superimposed secondary alterations can lead to significant variations in the characteristics of both oil composition and biomarker distribution.Hydrocarbon gas generation potential and thermal stability of crude oil may also be altered during biodegradation.However,there have not been systematic studies on this subject so far.In this study,molecular compositions of two immature crude oils were thoroughly investigated by the combination of gas chromatography–mass spectrometry(GC-MS)and Fourier transform ion cyclotron resonance mass spectrometry(FT-ICR MS)under both positive-and negative-ion electrospray ionization(ESI)modes to test their capability.The result showed that these two immature crude oils were consisted of varying polar organic sulfur compounds(OSCs),with the dominant presence of five-or six-membered sulfur rings in their molecular structures.These polar OSCs were likely produced by intramolecular sulfurization during early diagenesis.A simulated aerobic biodegradation experiment was later performed on one of the two immature crude oils,from which a series of biodegraded oils from slight to moderate degrees of biodegradation were obtained.n-Alkanes were almost depleted during the biodegradation.A gradual decrease of organic sulfur compounds(OSCs)with alkyl chains during biodegradation,especially in those with few sulfur-containing or naphthenic/aromatic rings,was observed.Terminal oxidation,rather than sulfur-specific degradation,is the more likely biodegradation pathway of these OSCs,as in the case of n-alkanes in this study.Sulfur-containing carboxylic acids may be the biodegradation products of these OSCs.The simulated aerobic biodegradation suggested a preferential removal of the hydrogen-rich oil components such as OSCs and n-alkanes,which should influence the gas generation behavior of residual oil under thermal maturation to some extent.A series of naturally biodegraded oils were artificially matured to simulate the superimposed secondary alterations in oil reservoirs.The results show that hydrocarbon gas generation potential was significantly reduced at the low to moderate biodegradation stage because of the selective removal of normal and branched alkanes.For Easy%R_o of 1.6–2.5,pyro-bitumen yields from biodegraded oils were about 2–4times that of non-biodegraded oil.Therefore,hydrocarbon gas resource potential based on pyro-bitumen content must be very carefully considered in cases of superimposed secondary alterations.In addition,kinetic modeling suggests that moderately and severely biodegraded oils are thermally less stable than the non-biodegraded and slightly biodegraded oils.Moreover,the variations of biomarkers and polar heteroatom-containing compounds under the influences of superimposed secondary alterations were investigated by GC-MS and ESI FT-ICR MS,respectively.Commonly used parameters for oil-source correlations such as the distributions of C_27–29 regular steranes were significantly altered at the moderate biodegradation stage,while the ratios of TT₂₃/H₃₀and H₂₉/H₃₀were significantly altered at the severe biodegradation stage.In addition,these parameters would also be modified by advanced thermal maturation(Easy%R_o?0.91).By comparison,TT₂₃/(TT₂₃+TT₂₄)ratio remained constant after severe biodegradation and thermal maturation,making it reliable for oil-source correlations in superimposed oil reservoirs.Besides,prior biodegradation could alter some commonly used parameters for maturity assessments.Moderate biodegradation would cause an increase in???20S/(20S+20R)and???/(???+???)ratios for C₂₉ steranes while a decrease in MPI-R ratio.Severe biodegradation would modify the 22S/(22S+22R)ratio for C₃₂ homohopanes.Such differences caused by biodegradation could be partly offset by subsequent maturation.For example,the MPI-R ratio in severely biodegraded oils became close with that in non-biodegraded oils in the Easy%R_o range of 0.91–2.11.Oils that have suffered varying extents of superimposed secondary alterations were found to be different mainly in the distributions of neutral nitrogen compounds and carboxylic acids.The neutral nitrogen compounds were preferentially removed during biodegradation while relatively enriched during maturation,with carbazoles being preferentially removed under both biodegradation and maturation.Besides,the acyclic acids were more susceptible to biodegradation than naphthenic acids,while the acyclic acids were thermally more stable than naphthenic acids.PCA performed on the relative distribution of acids with varying ring numbers in a group of samples could provide a fast way to characterize their relative maturities and biodegradation severities.