Reservoir Characterization. Группа авторов

Figure 4.6 GC- Fingerprint of Tertiary Oil No. 3 which is severely biodegraded.

Figure 4.7 GC- Fingerprint of Cretaceous Oil No. 3 which is not biodegraded.

Table 4.2 Typical biomarker characteristics of shale and carbonate-derived hydrocarbons.

Biomarker parameter	Shales	Carbonates
Pristane/Phytane	Often ≥ 1	Often ≤ 1
Steranes/αβ-hopanes	High	Low
Diasteranes/steranes	High	Low
C24 tetracyclic/C26 tricyclic terpanes	Low-medium	Medium-high
C29/C30 hopanes	Often low	Can be high (>1)
C35 Homophone Index	Often low	Often high (>0.1)
Hexacyclic hopanes & benzohopanes	Low	High
Dibenezothiophene/phenanthrene	Low (<3)	High (>1; Often >3)
αββ steranes	Low-medium	Medium-high

       Carbonate-derived oils (Figure 4.8) often display a predominance of C29-hopane (norhopane) over C30-hopane (hopane), with C29/C30 hopane ratios above one.

       In contrast, shale-derived oils show triterpene patterns (Figure 4.9 as an example) in which C29-hopane is predominant over C30-hopane, with a C29/C30 hopane ratio being low (mostly below one).

       The Dibenzothiophene/Phananthrene ratio [10] is relatively high (mostly above 1) in carbonate oils (Figure 4.10), while those in shale-derived oils are low (mostly below 1).

Figure 4.8 Triterpane fingerprints of carbonate-derived oil.

Figure 4.9 Triterpane fingerprint of shale-derived oil.

Figure 4.10 Cross plot of dibenzothiophene/phenanthrene versus pristane/phaytane.

4.4 Summary and Conclusions

      Carbonate rocks contain major oil and gas deposits throughout the world. They commonly have been discounted as important source rock due to lower organic-carbon content, however, carbonate source rocks contain mostly sapropelic (oil-prone, Type II)) organic matter capable of generating significant quantities of oil.

      The objective of the study was to provide an overview of the geochemical characteristics of carbonate-derived oils collected from different wells and fields. A total of 6 Cretaceous carbonate core samples and 14 oils from Tertiary (5) and Cretaceous (9) reservoirs were investigated using advanced geochemical technologies.

      Almost all Tertiary oils in this study are severely biodegraded whereas, the Cretaceous oils are not.

      All the oils studied show biomarker parameters that are characteristic of oils derived from carbonate source rocks, such as:

       Pristane/phytane ratios below 1

       High C29/C30-hopane ratios

       Low steranes/hopanes ratios

       High C35-homohopane index values

       High dibenzothiophene/phenanthrene ratio

References

      1. B.P. Tissot and D.H. Welte, Petroleum Formation and Occurrence, Springer-Verlag, New York (1984).

      2. K.E. Peters, C.C. Walters, and J.M. Moldowan, The Biomarker Guide Volume 1 and 2 Biomarkers and Isotopes in Petroleum Exploration and Earth History, pp. 492, Cambridge University Press, United Kingdom (2005).

      3. GJ. Demaison and G.T. Moore, Anoxic environments and oil source bed genesis. AAPG Bulletin 64, 1179-1209 (1980).

      4. S.E. Calvert, Oceanographic controls on the accumulation of organic matter in marine sediments, in Marine Petroleum Source Rocks, J. Brooks and A.J. Fleet (Eds.), pp. 137-151, Blackwell, London (1987).

      5. K.E. Peters, Guidelines for evaluating petroleum source rocks using programed pyrolysis. AAPG Bulletin 70, 318-329 (1986).

      6. D.K. Baskin, Comparison between H/C and Rock-Eval hydrogen index as indicator of organic matter quality, in The Monterey Formation: From Rocks to Molecules, CM. Isaacs and J.Rullkoetter (Eds.), pp. 230-240, Colombia University Press, New York (2001).

      7. D.H. Welte and D. Waples, Ueber die Bevorzugung gradzahliger n-Alkane in Sedimentgesteinen. Naturwissenschaften 60, 516-517 (1973).

      8. R. Alexander, R. Kagi, and G.W. Woodhouse, Geochemical correlation of Windalia oil and extracts of Winning Group (Cretaceous) potential source rocks, Barrow Subbasin, Western Australia. AAPG Bulletin 65, 235-250 (1981).

      9. H. Ten Haven, J.W. de Leeuw, J. Rullkoetter, and J.S. Sinninghe damste, Restricted utility of the pristane/phytane ratio as paleoenvironmental indicator. Nature 330, 641-643 (1987).

      10. W.B. Hughes, A. G. Holba, and L.I.P. Dzou, The ratios of Dibenzothiophene to Phenanthrene and Pristane to Phytane as indicators of depositional environment and