The LC-results presented in Figure 4.3 indicate that the majority of the oils investigated are rich in aromatic hydrocarbons, with their polar compounds (resins and asphaltenes) present in relatively high proportions, characteristic of the aromatic-intermediate class. This class of oils includes most crude oils from Jurassic and Cretaceous sources of the Middle East [1].
Typical gas chromatograms (GC-fingerprints) of carbonate and shale-derived oils are presented in Figures 4.4 and 4.5, respectively. Carbonate-derived oil shows a pristane/phytane ratio below (i.e. a predominance of phytane over pristane), indicative of a reducing environment, while that of oil from shale facies shows a pristane/phytane above 1 [7–9].
As mentioned above almost all Tertiary oils studied (Figure 4.6 as an example) are relatively heavy and severely biodegraded. This can be seen from the absence of all normal alkanes, while Cretaceous oils analyzed are relatively light and non-biodegraded (Figure 4.7 as an example), with all normal and branced (iso-) alkanes being present.
Saturated and aromatic fractions of the oils and source rock bitumens were further analyzed by GC-MS to investigate their aliphatic (saturated) and aromatic biomarkers. Table 4.2 shows biomarker parameters of carbonate facies oils compared to those of shale-derived hydrocarbons.
Table 4.1 Results of TOC and Rock-Evaol Pyrolysis for the rock samples analyzed.
| Well name | Depth (ft) | % TOC⋆ | RE-SI⋆ | RE-S2⋆ | RE-S3⋆ | Tmax⋆ | HI⋆ | OI⋆ | SI/TOC⋆100 | PI |
|---|---|---|---|---|---|---|---|---|---|---|
| Well A | 14144 | 11.31 | 3.76 | 15.13 | 1.09 | 454 | 134 | 10 | 33 | 0.20 |
| Well A | 14171 | 9.34 | 3.44 | 9.98 | 1.43 | 451 | 107 | 15 | 37 | 0.26 |
| Well A | 14193 | 6.25 | 2.92 | 7.9 | 1.37 | 454 | 126 | 22 | 47 | 0.27 |
| WellB | 13709 | 3.35 | 9.19 | 3.57 | 1.00 | 448 | 107 | 30 | 274 | 0.72 |
| WellB | 13718 | 9.82 | 15.67 | 12.9 | 1.00 | 454 | 131 | 10 | 160 | 0.55 |
| WellB | 13726 | 5.88 | 5.11 | 7.74 | 0.64 | 455 | 132 | 11 | 87 | 0.40 |
⋆TOC = Total organic Carbon (wt%); SI = mg HCs/g rock; S2 = mg HCs/g rock; S3 = C02/g rockmg; Tmax = deg C; HI = Hydrogen index (mg HCs/g TOC); OI = oxygen index (mg/C02/g TOC).
Figure 4.1 Type of Kerogen present in source rocks.
The following are the significant observations that could be obtained from Table 4.2 and GC-MS biomarker results:
Carbonate oils show Pristane/phytane ratios below one, whereas pristane/phytane ratios of shale facies oils are around or above one.
Carbonate oils are more enriched in αβ-hopanes than steranes, showing lower steranes/αβ-hopanes ratios, whereas in shale-derived oils steranes are more abundant than hopanes, showing high steranes/αβ- hopanes ratios [2].
Figure 4.2 It shows kerogen conversion and maturity (Tmax).
Figure 4.3 Bulk compositions of oils and bitumen studied.
Figure 4.4 GC-Fingerprint of a carbonate-derived oil.
Figure 4.5 GC-Fingerprint of a shale facies derived oil.