A comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry (GC × GC/TOFMS) method has been developed for the formation and identification of unresolved complex mixtures (UCMs) in lacustrine biodegraded oils that with the same source rock, similar maturity, and increasing degradation rank from Nanxiang Basin, China. Normal alkanes, light hydrocarbons, isoprenoids, steranes, and terpanes are degraded gradually from oil B330 to oil G574. The compounds in biodegraded oil (oil G574) have fewer types, the polarity difference of compounds in different types is minor, and the relative content of individual compounds is similar. All the features make the compounds in biodegraded oil coelute in GC analysis and form the raised “baseline hump” named UCMs. By injecting standard materials and analyzing mass spectrums of target compounds, it is shown that cyclic alkanes with one to five rings are the major components of UCMs. Furthermore, UCMs were divided into six classes. Classes I and II, composed of alkyl-cyclohexanes, alkyl-naphthanes, and their isomers, are originated from the enrichment of hydrocarbons resistant to degradation in normal oils. Classes III ~ VI, composed of sesquiterpenoids, tricyclic terpanes, low molecular steranes, diasteranes, norhopanes, and their isomers, are probably from some newly formed compounds during the microbial transformation of oil.
Biodegraded oils exist widely in many oil and gas basins in the world. According to the statistics, 10% of global oil reserves have been degraded and another 10% have suffered varying degrees of biodegradation [
In this study, UCMs in five lacustrine biodegraded oils with different biodegradation ranks from Nanxiang Basin, China, were separated with comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry (GC × GC/TOFMS) and identified by standard materials validation, structured chromatograms, and mass spectrums of target compounds. The geochemical characteristics, degradation ranks of the biodegraded oils, and formation and identification of UCMs were studied in detail.
The Mesozoic to Cenozoic Biyang depression is one of the depressions in Nanxiang Basin, which is a typical lacustrine basin located in Henan Province, Eastern China. Biyang depression can be divided into three structural units, the northern slope, the middle depression, and the southern steep slope (Figure
Location, tectonic units, and sampled wells of Biyang depression.
Five biodegraded oils from the northern slope and one normal oil from the southern steep slope were analyzed (Figure
Reservoirs, depths, and group compositions of oil B330 to oil G574.
Sample | Reservoir | Depth (m) | Saturates (%) | Aromatics (%) | Resin (%) | Asphaltenes (%) |
---|---|---|---|---|---|---|
B330 |
|
1750.0 | 76.10 | 9.61 | 10.86 | 3.43 |
B276 |
|
1083.0 | 26.82 | 11.22 | 54.63 | 7.31 |
YJ3624 |
|
857.0 | 37.65 | 25.46 | 33.48 | 3.40 |
Y50714 |
|
753.0 | 25.98 | 19.22 | 49.35 | 5.45 |
L3917 |
|
282.0 | 43.10 | 16.70 | 36.80 | 3.39 |
G574 |
|
263.0 | 28.42 | 16.05 | 46.85 | 8.68 |
A suitable amount (35 mg) of oil was dissolved in 30 mL hexane and then mixed for 5 min using ultrasonic wave. The samples need to stand for 12 hours and the asphaltene was filtered. At last, the supernatant was analyzed by GC × GC/TOFMS, GC/MS, and GC, respectively. Standard materials were purchased from J&K Scientific and solvents come from Tedia High Purity Solvent.
The GC × GC system consisted of a GC (7890A, Agilent Technologies, Wilmington, DE, USA) equipped with a secondary oven and a quad-jet dual stage modulator. A detailed description of the cold-jet modulator has been made in a previous publication [
In GC × GC/TOFMS analysis, a DB-5MS column (60 m × 0.25 mm × 0.25
In GC/MS analysis, a DB-5MS column (50 m × 0.25 mm × 0.25
In GC analysis, a DB-Petro column (30 m × 0.25 mm × 0.25
The Eh3 Formation can be divided into the
Biomarker maturity indicators, such as sterane maturity index (C29
The biomarkers of the oils in the northern slope were analyzed by GC/MS. Different parameters of oils (degradation rank lower than 5) are calculated (Table
Main geochemical parameters of oils from the northern slope.
Oil sample | Y3515 | Y50714 | YJ3624 | B276 | G412 |
---|---|---|---|---|---|
C27/C27~C29-steranes | 0.32 | 0.32 | 0.30 | 0.27 | 0.31 |
C28/C27~C29-steranes | 0.33 | 0.33 | 0.34 | 0.33 | 0.31 |
C19/C23-tricyclic terpanes | 0.05 | 0.03 | 0.03 | 0.12 | 0.07 |
C19~C22/C23~C26-tricyclic terpanes | 0.62 | 0.50 | 0.50 | 1.17 | 0.74 |
C26-tricyclic/C24-tetracyclic terpane | 2.24 | 2.32 | 2.38 | 1.72 | 2.51 |
C21~C22-pregnanes/(steranes + diasteranes) | 0.02 | 0.02 | 0.01 | 0.03 | 0.03 |
C19~C29-tricyclic terpanes/ |
0.29 | 0.25 | 0.23 | 0.32 | 0.30 |
C19~C29-tricyclic terpanes/C30-hopane | 1.30 | 1.12 | 0.97 | 1.49 | 1.33 |
C27~C29-diasteranes/steranes | 0.05 | 0.05 | 0.05 | 0.06 | 0.06 |
Ts/(Ts + Tm) | 0.15 | 0.15 | 0.15 | 0.10 | 0.14 |
C31~C35-homohopanes/C30-hopane | 0.58 | 0.61 | 0.60 | 0.61 | 0.54 |
In addition, biomarker features can reflect the oil maturity, such as
Compounds in crude oil have different kinds of capability resisting the biodegradation, especially the biomarkers [
The overall composition of the crude oil, such as the composition of n-alkanes and the relative content of steranes and terpanes, can be characterized by GC analysis. The GC chromatographs of six oils are shown in Figure
GC chromatograms of oil B330 to oil G574.
In order to determine the degradation ranks of oils Y50714 to G574, the profiles of steranes and terpanes need to be further compared. The degradation rank of oil YJ3624 is rank 3 with steranes and terpanes not being degraded. The relative content of diasteranes, low molecular weight steranes, and tricyclic terpanes in oil YJ3624 is low, and it is the same with that of normal oil B330. Similar composition of oil Y50714 and oil YJ3624 indicates that neither steranes nor terpanes in oil YJ3624 have been degraded. Steranes and terpanes in oil L3917 began to be degraded, and the relative content of diasteranes, low molecular weight steranes, and tricyclic terpanes is higher than that in oil YJ3624 and oil Y50714. Only some steranes and terpanes resistant to degradation, such as low molecular weight steranes, diasteranes, tricyclic terpanes, norhopanes, and gammacerane, were detected in oil G574, and the highest peaks of
The above analysis shows that oil B330 is normal oil and oil B276 to oil G574 are biodegraded oil with degradation ranks of 2, 3, 5, 6, and 8. Oils B276 to G574 are all low mature oils and their source rock is the same (black mudstones in
The UCMs (“baseline hump”) can be clearly observed in oil B276 to oil G574. UCMs are a typical complex system, and their compounds may be as many as 25,000 [
The total ion chromatograms (TIC) of oil B330 to oil G574 are shown in Figure
Main compounds of six oil samples in GC × GC/TOFMS TIC. 1: low molecular weight aromatic hydrocarbons; 2: light hydrocarbons; 3: n-alkanes and isoprenoids; 4: sesquiterpanes; 5 + 6: steranes and terpanes.
During the degradation of oils (oil B330 to oil G574), compounds in Zone 3 were firstly consumed by microbes, and then their relative content decreased. The relative content of compounds in Zones 1 and 2 firstly increased and then reduced. Oil B276 had the highest relative content of compounds in Zones 1 and 2. The relative content of Zones 5 and 6 increased but the kind of compounds decreased. The relative content and the kind of compounds in Zone 4 all increased. GC/MS analysis indicated that the compounds in Zone 1 were low molecular weight aromatic hydrocarbons, and they were light hydrocarbons in Zone 2, n-alkanes and isoprenoids in Zone 3, sesquiterpenes in Zone 4, and steranes and terpanes in Zones 5 and 6, respectively. Additionally, peaks in GC × GC/TOFMS TIC of oil G574 have similar heights and colors. This demonstrates that the relative content of individual compounds in severely biodegraded oil (oil G574) is similar. In general, with the increase of degradation rank, the relative content of n-alkanes and isoprenoids decreased; light hydrocarbons and low molecular weight aromatic hydrocarbons increased first and then decreased; and some sesquiterpenes, steranes, and terpanes increased gradually, while the relative content of individual compounds in severely biodegraded oils tends to be similar.
As shown in Figure
Theorthogonal separation of GC × GC/TOFMS is achieved by the association of a nonpolar column with a polar column [
Oil B330 has the “minimum” area in the second dimensional chromatographic plane because other compounds are covered by n-alkanes (Figure
In summary, compound types in biodegraded oils reduce significantly with the microbial degradation of n-alkanes, light hydrocarbons, isoprenoids, low molecular weight aromatic hydrocarbons, most steranes, and terpanes, residual compounds have minor difference in polarity, and the relative content of individual compounds tends to be similar. Thus the compounds of biodegraded oil are coeluted in the traditional GC analysis, resulting in the formation of the UCMs (“baseline hump”).
GC × GC/TOFMS, liquid chromatography associated mass spectrometry, and other techniques have been applied to the study of UCMs in biodegraded oil and bitumen [
The UCMs in oil G574 were divided into six classes (
Classification of compounds in UCMs in severely biodegraded oil G574.
In order to determine the composition of UCMs for Classes
Qualitative diagram of compounds in classes III to VI. Left: GC/MS analysis; right: GC × GC/TOFMS analysis; TT: tricyclic terpane; TeT: tetracyclic terpane; H: hopane; G: gammacerane; P: pregnane; HP: homopregnane; DS: diasterane; SeT: sesquiterpane; PD: pentamethyl-naphthane; D: drimane; HD: homodrimane.
It should be pointed out that, for UCMs, Class III has bicyclic structure (sesquiterpanes), Class V has tricyclic structure (tricyclic terpanes), Class IV has tetracyclic structure (pregnane), and Class VI has pentacyclic structure (gammacerane). Cyclohexane is the basic framework of the cyclic structure. In addition to these compounds identified, a large number of small peaks appear within the range of UCMs for Classes
Characteristic ions of UCMs Class I are
GC × GC/TOFMS TIC of standard materials and oil G574 spiked with standards. 1: naphthane (
Additionally, the zoom of different unique ions is shown in Figure
Composition of UCMs (classes I and II) identified by standard materials. 1, 2, 3, and 4 represent the same standards with Figure
Based on the above analysis, the following conclusions can be drawn. Firstly, complex UCMs can be divided into six classes (Figure
Biodegraded oils B276 to G574 are typical lacustrine oils. The source rocks of the oils are the black mudstones in the
With the degradation degree increasing (oil B330 to oil G574), n-alkanes, light hydrocarbons, isoprenoids, low molecular weight aromatic hydrocarbons, steranes, and terpanes are degraded in the relative order by microorganisms. The compound types of biodegraded oil reduce significantly, the compounds in different types have minor difference of polarity, and the relative content of individual compounds is similar. These changes result in the coelution of biodegraded oil in traditional GC analysis and form the raised “baseline hump” named as UCMs.
Standard compounds and mass spectrums analyses showed that cyclic alkanes with one to five rings (cyclohexane as basic framework) are major components of UCMs. UCMs can be divided into six classes (
The authors declare that there is no conflict of interests regarding the publication of this paper.
The study was supported by Funding Grants from Programme of Introducing Talents of Discipline to Universities (no. B14031) and the National Natural Science Foundation of China (no. 21077039 and no. 41072093).