Assessing the Impact of Solid Reservoir Bitumen
All phases of oil field exploitation, including exploration, development, and production are impacted by the presence of solid reservoir bitumens.
Solid reservoir bitumen (also called pyrobitumen, migrabitumen, gilsonite, tar, etc.) occurs in carbonate and siliciclastic oil and gas reservoirs in many basins throughout the world (Huc et al., 2000; Hwang et al., 1998; Littke et al., 1996; Lomando, 1992; Patterson et al., 1994; Pottorf et al., 2003; Rogers, et al, 1974; Sassen, 1986; Wavrek et al, 1999). Reservoir bitumen is different than source rock bitumen in that it is formed from petroleum in the reservoir through natural or artificial alteration processes such as thermal cracking of oil (pyrobitumen), gas deasphalting of oil (asphaltene precipitation), or by inspissation, water washing, or oxidation (tar).
Solid reservoir bitumen is a reservoir-altering cement (Lomando, 1992). Within the porosity of oil and gas reservoirs, it occurs as droplets, grain coatings, or as honeycomb or finger-like bodies. Solid reservoir bitumen may be finely dispersed throughout the porosity or may be localized in a particular interval, potentially creating a fluid barrier (e.g., a tar mat). If solid reservoir bitumens are present in the reservoir, they typically reduce permeability significantly even when found in only moderate amounts (Lomando, 1992).
Since solid bitumen may be formed by different processes, it will vary in chemistry. Some varieties contain predominately saturate-rich hydrocarbons soluble in organic solvents, while other solid bitumens may contain predominantly solvent-insoluble asphaltenes and resins (Hwang et al., 1998). The various types and chemical compositions of solid bitumen will also differentially impact the producibility of the associated fluids in the reservoir.
Solid reservoir bitumens are reported as porosity by routine log suites; consequently, their presence affects reserves calculations, recovery factors, and secondary recovery programs. Thus all phases of oil field exploitation, including exploration, development, and production are impacted by the presence of solid reservoir bitumens.
For an accurate assessment of the impact of solid reservoir bitumen on the production of fluids from the reservoir, OilTracers recommends resolution of the following properties:
Reservoir bitumen type
Volume of bitumen in the reservoir,
Distribution of solid bitumen in the reservoir.
The study of solid bitumen requires examination and analysis of conventional or sidewall core and analysis of associated hydrocarbon fluids. Geochemical analyses may include: high resolution gas chromatography, separation of compound class fractions (saturated hydrocarbons, aromatic hydrocarbons, asphaltenes, and resins), biomarker analysis, total organic carbon (TOC), Rock-Eval pyrolysis, and microscopic/reflectance analysis. In order to determine the quantity of solid bitumen, and its distribution in the reservoir, routine core analysis and thin section modal analysis are typically required.
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Huc A. Y., P. Nederlof, R. Debarre, B. Carpentier, M. Boussafir, F. Laggoun-Defarge, A. Lenail-Chouteau, and N. B. L. Floch, 2000, Pyrobitumen occurrences and formation in a Cambro-Ordovician sandstone reservoir, Fahud Salt Basin, North Oman: Chemical Geology, v. 168, p.99-112.
Hwang, R. S., S. Teerman, and R. Carlson, 1998, Geochemical comparison of reservoir solid bitumens with diverse origins: Organic Geochemistry, v.29, p.505-518.
Littke, R. F., J. Brauckmann, M. Radke, and R. G. Schaefer, 1996, Solid bitumen in Rotliegend gas reservoirs in northern Germany: Implications for their thermal and filling history: Zentralbl. Geol. Palaont., Teil I, v.11, p.1275-1292.
Lomando, A. J., 1992, The influence of solid reservoir bitumen on reservoir quality: AAPG Bulletin, v. 76, p.1137-1152.
Patterson, B. A., B. T. Robertson, and J. Dahl, 1994, Preliminary Reservoir Geochemistry Study of oil and Bitumens from the Tengiz Field, Kazakhstan, CIS: Abstract: AAPG Bulletin, v.78, p.230.
Pottorf, R. J., H. Y. Tseng, P. J. Hicks, K. L. Putney, D. J. Curry, P. J. Mankiewicz, and G. G. Gray, 2003, Timing, distribution, and origin of bitumen formation at Tengiz Field, Kazakhstan: in J. Cubbitt, W. England, S. Larter, and G. Macleod, ed., Conference Abstracts: Geochemistry of Reservoirs II: Linking Reservoir Engineering and Geochemical Models (Geological Society of London, February 3-4, 2003): Geological Society of London.
Rogers, M. A., J. D. McAlary, and N. J. L. Bailey, 1974, Significance of reservoir bitumens to thermal-maturation studies, Western Canada Bain: AAPG Bulletin, v. 5, p.1806-1824.
Sassen, R. 1986, Biodegradation of crude oil and bitumen precipitation in deep carbonate reservoirs of the Smackover Formation (Abstracts and Programs): Society of Organic Petrology, 3rd Annual Meeting, Lexington, Kentucky, Society of Organic Petrology, p.24-26.
Wavrek, D. A., D. M. Jarvie, and J. D. Burgess, 1999, Characterization of solid reservoir bitumen: Insights to formation mechanism, timing, and correlation (abstract): TSOP Abstracts and Program: v. 16, p.7-10.