Biodegradation and origin of oil sands in the Western Canada Sedimentary Basin

The oil sands deposits in the Western Canada Sedimentary Basin (WCSB) comprise of at least 85% of the total immobile bitumen in place in the world and are so concentrated as to be virtually the only such deposits that are economically recoverable for conversion to oil. The major deposits are in three geographic and geologic regions of Alberta: Athabasca, Cold Lake and Peace River. The bitumen reserves have oil gravities ranging from 8 to 12° API, and are hosted in the reservoirs of varying age, ranging from Devonian (Grosmont Formation) to Early Cretaceous (Mannville Group).They were derived from light oils in the southern Alberta and migrated to the north and east for over 100 km during the Laramide Orogeny, which was responsible for the uplift of the Rocky Mountains. Biodegradation is the only process that transforms light oil into bitumen in such a dramatic way that overshadowed other alterations with minor contributions. The levels of biodegradation in the basin increasing from west (non-biodegraded) to east (extremely biodegraded) can be attributed to decreasing reservoir temperature, which played the primary role in controlling the biodegradation regime. Once the reservoir was heated to approximately 80 °C, it was pasteurized and no biodegradation would further occur. However, reservoir temperature could not alone predict the variations of the oil composition and physical properties. Compositional gradients and a wide range of biodegradation degree at single reservoir column indicate that the water-leg size or the volume ratio of oil to water is one of the critical local controls for the vertical variations of biodegradation degree and oil physical properties. Late charging and mixing of the fresh and degraded oils ultimately dictate the final distribution of compositions and physical properties found in the heavy oil and oil sand fields. Oil geochemistry can reveal precisely the processes and levels that control these variations in a given field, which opens the possibility of model-driven prediction of oil properties and sweet spots in reservoirs.

[1]  R. Macdonald,et al.  Origin and Alteration of Lower Cretaceous Mannville Group Oils from the Provost Oil Field, East Central Alberta, Canada , 1999 .

[2]  C. Mendoza,et al.  Coupled fluid flow, heat and mass transport, and erosion in the Alberta basin: implications for the origin of the Athabasca oil sands , 2004 .

[3]  W. Ayrton.  Canadian Society of Petroleum Geologists: GEOLOGIC NOTES , 1977 .

[4]  P. Brooks,et al.  Biological marker and conventional organic geochemistry of oil sands/heavy oils, Western Canada basin , 1988 .

[5]  S. Bachu,et al.  Regional-scale hydrogeology of the Upper Devonian-Lower Cretaceous sedimentary succession, south-central Alberta basin, Canada , 2001 .

[6]  Jennifer D. Adams,et al.  Geological Controls on the Origin of Heavy Oil and Oil Sands and Their Impacts on In Situ Recovery , 2006 .

[7]  M. Fustic,et al.  25-Norhopanes: Formation during biodegradation of petroleum in the subsurface , 2006 .

[8]  Steve Larter,et al.  Thermal controls on biodegradation around the Peace River tar sands: Paleo-pasteurization to the west , 2006 .

[9]  J. Connan.  Biodegradation of Crude Oils in Reservoirs , 1984 .

[10]  W. K. Muwais,et al.  Application of Outcrop Analogues and Detailed Reservoir Characterization to the AOSTRA Underground Test Facility, McMurray Formation, North Eastern Alberta. , 1997 .

[11]  Ian D. Gates,et al.  The Origin, Prediction and Impact of Oil Viscosity Heterogeneity on the Production Characteristics of Tar Sand and Heavy Oil Reservoirs , 2006 .

[12]  S. Larter,et al.  Reservoir geochemistry: methods, applications and opportunities , 1995, Geological Society, London, Special Publications.

[13]  M. Fowler,et al.  Devonian Hydrocarbon Source Rocks and Their Derived Oils in the Western Canada Sedimentary Basin , 2001 .

[14]  Ian M. Head,et al.  The controls on the composition of biodegraded oils in the deep subsurface: Part II—Geological controls on subsurface biodegradation fluxes and constraints on reservoir-fluid property prediction , 2006 .

[15]  Steve Larter,et al.  A dynamic biodegradation model suggested by petroleum compositional gradients within reservoir columns from the Liaohe basin, NE China , 2004 .

[16]  P. Farrimond,et al.  Biodegradation of oil in uplifted basins prevented by deep-burial sterilization , 2001, Nature.

[17]  Lloyd M. Wenger,et al.  Multiple Controls on Petroleum Biodegradation and Impact on Oil Quality , 2002 .

[18]  K. Peters,et al.  The Biomarker Guide: Interpreting Molecular Fossils in Petroleum and Ancient Sediments , 1992 .

[19]  Grant D. Mossop,et al.  Geological Atlas of the Western Canada Sedimentary Basin , 1988 .

[20]  Ian M. Head,et al.  Biological activity in the deep subsurface and the origin of heavy oil , 2003, Nature.

[21]  D. Welte,et al.  Advances in petroleum geochemistry , 1984 .

[22]  K. Osadetz,et al.  Variable alteration in heavy crude oils of west-central Saskatchewan, Canada , 2004 .

[23]  S. Larter,et al.  Biodegradation and mixing of crude oils in Eocene Es3 reservoirs of the Liaohe basin, northeastern China , 2002 .

[24]  T. Austad,et al.  The geochemistry of reservoirs , 1996 .

[25]  S. Larter,et al.  Influence of biodegradation on carbazole and benzocarbazole distributions in oil columns from the Liaohe basin, NE China , 2003 .

[26]  A. K. Rowan,et al.  Crude-oil biodegradation via methanogenesis in subsurface petroleum reservoirs , 2008, Nature.

[27]  Ian M. Head,et al.  The controls on the composition of biodegraded oils in the deep subsurface - part 1: biodegradation rates in petroleum reservoirs , 2003 .

[28]  E. Howard,et al.  Regional Geology and Sedimentology of the Basal Cretaceous Peace River Oil Sands Deposit, North-Central Alberta , 1999 .

[29]  L. Ellis,et al.  Evidence for biodegradation and evaporative fractionation in West Sak, Kuparuk and Prudhoe Bay field areas, North Slope, Alaska , 2001 .