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| Titel |
Methane leakage during the evolution of petroleum systems in the Western Canada Sedimentary Basin and the Central Graben area of the North Sea |
| VerfasserIn |
L. A. Berbesi, R. di Primio, Z. Anka, B. Horsfield |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250060059
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| Zusammenfassung |
| Around 500 to 600 Tg (1 Tg = 1012 g) of methane enter the atmosphere every year, mainly as
product of microbial processes and combustion of fossil fuels and burning biomass. The
importance of another source, the geologic emissions of methane, is up to now only loosely
constrained. In this study, we addressed the potential methane emissions during the
geological evolution of the Western Canada sedimentary basin (WCSB), which holds the
largest oil sand accumulations in the world, and the Central Graben area of the North
Sea.
In the case of the WCSB, thermogenic gas generation and leakage at the sediment surface
were addressed through 3D petroleum systems modeling. In this basin, the accumulated oil
experienced intense biodegradation that resulted in large masses of biogenic methane. We
quantified this latter mass though a two-step mass balance approach. Firstly, we estimated the
rate of petroleum degradation and the magnitude of petroleum loss. After this, we calculated
the mass of biogenic methane generated using a model that assumes hexadecane (C16H34) as
representative of the saturated compounds (Zengler et al., 1999). Our 3D model suggests that
90000-150000 Tg of dry gas (mostly methane) could have leaked during the interval from
80 Ma to 60 Ma. Therefore, uniform leakage rates would have been in the order
of 10-3-10-2 Tg yr-1. Biogenic methane generation could have taken place at
rates of 10-4 to 10-2 Tg yr-1. However, the effective mass of thermogenic and
biogenic methane reaching the atmosphere might have been up to 90% lower than
calculated here due to methanotrophic consumption in soils (Etiope and Klusman,
2002).
We addressed the thermogenic gas generation and leakage in the Central Graben through
two different methods. The first is based on a previous 3D petroleum system modeling of the
region (Neumann, 2006). The second consisted of calculating the mass of generated
petroleum based on source rock extension and properties (Schmoker, 1994), and then
estimating the gas mass available for leakage based on the concept of petroleum systems and
the proportions among its constituents (Hunt, 1995). We propose that a maximum of
10-4-10-3 Tg of thermogenic gas (mostly methane) could have leaked annually
from the sediment surface. The portion of this gas that reached the atmosphere is
unconstrained, and it would depend on the extent of oxidation through the water
column.
The maximum rate of thermogenic gas generation in the WCSB is in the order
of 10-2 Tg yr-1 (10-8 Tg yr-1Km-2, when normalized to area of kitchen). In
the case of the Central Graben, the maximum would be in the order of 10-3Tg
yr-1 (10-8 Tg yr-1 Km-2). These results suggest that thermal maturation alone
would not be able to promote leakage rates as high as those reported for some single
sedimentary basins at present-day, these last reaching up to 3.5 Tg yr-1 (Judd, 2004).
Mechanisms promoting the release of previously accumulated gas masses in a short time
span are thus a basic requisite for petroleum systems to exert an impact on climate. |
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