Both the saturate and aromatic fractions of oils extracted from the Cretaceous oil sands have been degraded to different levels without the formation of ...
Origin of Heavy Oil in Cretaceous Petroleum Reservoirs Timothy 1School
1 Bata ,
John
1 Parnell ,
Stephen
1 Bowden
Adrian
2 Boyce
of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, United Kingdom, 2Scottish Universities Environmental Research Centre, East Kilbride, Glasgow G75 0QF, United Kingdom.
INTRODUCTION
Studies on the stratigraphic distribution of heavy oil show that much of the world’s heavy oil is in Cretaceous reservoir rocks. This is due to a combination of tectonic, climatic, geological and biological factors. Cretaceous oil sands from Neuquén Basin (Argentina), Sergipe - Alagoas Basin (Brazil), Alberta (Canada), Dahomey Basin (Nigeria), Uinta Basin (USA), Western Moray Firth Basin (United Kingdom) and the Wessex Basin (United Kingdom) were studied with the aim of understanding the origin of the heavy oils. The results indicate that the oils were generated as light oil, which later degraded into heavy oil. Some of these Cretaceous reservoir rocks occur above a prominent unconformity surface. These oils, which were generated from more deeply buried source rocks, had to travel lengthy routes along the planes of unconformity to reach the reservoir rocks. Oil degradation could have occurred during migration and/or in the reservoir rocks. Both the saturate and aromatic fractions of oils extracted from the Cretaceous oil sands have been degraded to different levels without the formation of 25-norhopane. This suggests that biodegradation occurred at shallow depths. Some of the Cretaceous oil sands also show evidence of water washing to different levels. The trisnorhopane thermal maturity indicator show that the Cretaceous oil sands have thermal maturity levels equivalent to 0.66 – 1.32% Ro, consistent with early to late oil window. These results indicate that the oils were generated as conventional light oil, which later degraded into heavy oil Pyrite associated with the Cretaceous oil sands was found to be consistently isotopically light. The isotopic fractionation between these pyrites and contemporary seawater sulphate calculated using the mean δ34S values and the established seawater composition curve exceeds the maximum known kinetic isotope fractionation of ~20‰ that is possible from non-biogenic mechanisms such as thermochemical sulphate reduction. This strongly suggests that the pyrite precipitated from an open system by means of microbial sulphate reduction.
Figure 1 Cross plot of Pristane/n-C17 versus Phytane/n-C18 for the Cretaceous oil sands showing inferred source rock depositional environment, and increasing level of biodegradation. NOTE TIC chromatograms showing UCM for the different levels of biodegradation.
Figure 2 Schematic showing non-degraded and degraded oils in reservoirs. Note biodegraded oils in very shallow reservoirs without seal cover, and water washed and/or biodegraded oils in reservoirs associated with active aquifers.
Figure 4 Trisnorhophane thermal maturity indicator showing the maturity levels of the oils in the Cretaceous oil sands plotted against maximum burial depths. All the oils were generated within the oil window.
Figure 5 SEM backscattered images of pyrite associated with some of the Cretaceous oil sands. (A) Pyrite occurring as patchy pyritic cements associated with other authigenic minerals in the Argentinian oil sand. (B) Pyrite occurring in the Nigerian oil sand. (C) Framboidal pyrite occurring as intergranular cements in the Wealden oil Sand (UK). (D) Framboidal pyrite occurring in the Captain Sand, Western Moray Firth Basin (UK).
Summary and Conclusions: Heavy oils occurring in the Cretaceous oil sands were sourced as conventional light oils from marine and mixed organic sources. Oil degradation in the Cretaceous oil sands occurred at shallow depths. Both biodegradation and water washing occurred simultaneously in most of the Cretaceous oil sands. The porous nature of the Cretaceous oil sands and the availability of metabolizable organic matter within them enhance their potential as habitats for microbial activities.
Figure 3 Cross plot of percentage asphaltene and the effect of water washing in the Cretaceous oil sands. The m/z 231 fragmentograms show the depletion of C20 + C21 triaromatic steranes relative to their C27 + C28 counterparts for different levels of water washing. Both water washing and biodegradation occurred simultaneously in most of Cretaceous oil sands.
Figure 6 Isotopic composition of pyrite sulphur in the Cretaceous oil sands compared with pyrite occurring in marine sands of other geological ages. Pyrite associated with the Cretaceous oil sands are isotopically light indicating microbially-induced isotopic fractionation, while pyrite associated with the other marine sands are isotopically heavy indicating pyrite precipitation from non-biogenic mechanisms such as thermochemical sulphate reduction or biogenically in a closed system. Shaded region indicates compositional field which could be explained by both biogenic and abiogenic sulphate reduction. Sea water composition from Machel (2001).
Key References: Bennett, B., Fustic, M., Farrimond, P., Huang, H., and Larter, S. R., 2006. 25-Norhopanes: formation during biodegradation of petroleum in the subsurface.Organic Geochemistry, 37. 787- 797. Fallick, A. E., Boyce, A. J., and McConville, P., 2012. Sulphur stable isotope systematics in diagenetic pyrite from the North Sea hydrocarbon reservoirs revealed by laser combustion analysis. Isotopes in environmental and health studies, 48, 144-165. Kuo, L. C., 1994. An experimental study of crude oil alteration in reservoir rocks by water washing. Organic Geochemistry, 21, 465- 479. López, L., 2014. Study of the biodegradation levels of oils from the Orinoco Oil Belt (Junin area) using different biodegradation scales. Organic Geochemistry, 66, 60-69. Machel, H. G., 2001, Bacterial and thermochemical sulphate reduction in diagenetic settings – old and new insights. Sedimentary. Geology. 140, 143–175. Parnell, J., Boyce, A. J., Hurst, A., Davidheiser-Kroll, B., and Ponicka, J., 2013. Long term geological record of a global deep subsurface microbial habitat in sand injection complexes. Nature Scientific Report 3. http://www.nature.com/srep/2013/130510/srep01828/pdf/srep01828.pdf Peters, K. E., Walters, C. C., and Moldowan, J. M., 2005. The biomarker guide: biomarkers and isotopes in the environment and human history Vol. 2. Cambridge University Press.
Nigeria
