25th International Meeting on Organic Geochemistry IMOG 2011

P-089
Addressing thermogenic and biogenic gas emissions during
the formation of the oil sands deposits of the Western Canada
Basin
Luiyin Berbesi, Rolando di Primio, Zahie Anka, Brian Horsfield, Heinz Wilkes
GFZ German Research Centre for Geosciences, Potsdam, Germany (corresponding author:berbesi@gfzpotsdam.de)
The thermal maturation of organic matter in
sedimentary basins implies, in general lines, the
generation, migration, and accumulation of
hydrocarbons, and in many cases, its subsequent
alteration in reservoir. During the different stages of
this process, a portion of the generated hydrocarbons
escapes from the sedimentary basin and may enter
the hydro- and atmosphere, with methane (CH4) being
one of the main expelled gases.
The strong greenhouse gas potential of methane is
well known [1] , and it could suggest the existence of a
link between gaseous hydrocarbon leakage and
climate history. In this context, an evaluation of the
amount of methane released during the evolution of
sedimentary basins worldwide as well as the timing of
these processes is required.
A very good target for this approach is the Western
Canada Basin, given its immense quantities of heavy
oil deposits, estimated over 250 x10 9 m 3 [2] . The
geological setting is that of a foreland basin, where
the generation and accumulation of oil was followed
by uplift during the Laramide orogeny and subsequent
biodegradation of original oil.
In this project, the hydrocarbon generation and
accumulation, as well as thermogenic gas emission
were assessed using a 3D basin model that covers
most of Alberta and Saskatchewan, and a part of the
Rocky Mountains in British Columbia, Canada. The
stratigraphic sequences were reproduced by 22
isopach maps covering middle Devonian to Tertiary
strata. The lithologies correspond to those described
in the Atlas of the Western Canada Sedimentary
Basin [3] . The kinetic parameters applied to source
rocks were based on the relationship between the
organic sulfur content of kerogen and the kinetic
parameters determined by hydrous pyrolysis [4] . We
used a constant heat flow map containing values
between 45 mW/m 2 and 70 mW/m 2 for the study
area [3] . The magnitude of the Laramide erosion
ranges from 180 to 2400 meters [5] . A hybrid
calculation method combining Darcy flow and flowpath
(ray tracing) techniques was used for
hydrocarbon migration simulation. The model
suggests that a peak of gas leakage at the upper
boundary of the model occurred between 80 to 60
Ma, concordant with the major phase of generation
and migration. The amount of thermogenic gas
released during this period of time was approximated
to be in the order of 10 16 to 10 17 grams.
Additionally, the amount of generated, and possibly
released secondary biogenic methane was addressed
by estimating the rate of petroleum degradation and
the magnitude of petroleum loss. We calculated the
amount of generated biogenic methane following a
model [6] that assumes hexadecane (C16H34) as a
representative compound for the saturated
compounds. Based on this methodology, the total
amount of generated methane during 60 million years
of biodegradation is in the order of 10 17 g , of which
50 % would probably have been generated during the
first 20 million years after the main charge phase.
Our estimates for combined thermogenic and
biogenic methane emitted during the evolution of the
Western Canada Basin, amount to 8 x 10 17 g. The
estimated amount of secondary methane, generated
during the first million years of biodegradation,
corresponds to around 50 times the total present-day
annual flux of methane into the atmosphere.
However, the possible influence of the estimated
leaked gas on past climate evolution will depend not
only on the rate at which this methane leaks from the
basin, but also on the rate of its further oxidation.
These processes remain to be constrained.
References
[1] United Nations Framework Convention on Climate Change
(UNFCCC) 1995.
[2] National Energy Board Canada, 2000,NEB Report, p.5.
[3] Mossop, G., and I. Shetson, eds., 1994, Geological atlas of the
Western Canada sedimentary basin.
[4] Higley, D., Lewan, D., Roberts, N., and Mitchell, H., 2009, AAPG
Bulletin, v. 93, no 2, p. 203-230.
[5] Nurkowski, J.,1984, AAPG Bulletin, v. 68, no 3, p. 285-295.
[6] Zengler, K., Richnow, H., Rosselló, Mo., Michaelis, W, and F.
Widdel, 1999, Nature, v. 401, 266-269.
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