Uncertainties

Emission factors for microseepage and macroseeps and related ranges are well known. Uncertainties in global emission estimates are mainly due to a poor knowledge of the actual area of shallow submarine macroseeps and, even more importantly, the dryland area of invisible microseepage. It is evident that all seeps and microseepage zones occur within hydrocarbon provinces, in particular within TPSs (Etiope and Klusman, 2010), but actual microseepage areas are not known. Accordingly, three main levels of spatial disaggregation can be defined, with increasing estimates for uncertainty: an area including (encompassing) sites of verified microseepage flux; an area encompassing macroseeps (where microseepage very likely occurs); and an area encompassing oil-gas fields (where microseepage likely occurs).

The classification can be used for upscaling procedures but, presently, there are no detailed maps of verified microseepage. The definition of the area used for emission calculations depends on the recognition of homogeneous wetlands wetlands

0 25 50 75 100 125 150 175 200

Tg CH4 yr1

Figure 4.5 Global methane sources revisited

0 25 50 75 100 125 150 175 200

Tg CH4 yr1

Figure 4.5 Global methane sources revisited

Source: Natural and anthropogenic sources are from IPCC AR4 (Denman et al, 2007; average of values in Table 7.6); geological source is from Etiope et al (2008).

identifiable areas and the spatial variability of the flux measured. Currently, estimations are performed based on the distribution of oil fields, assuming that approximately 50 per cent of oil field areas have positive fluxes of CH4 from soils, as suggested by preliminary data sets (Etiope and Klusman, 2010). The area identified in oil field maps is then transformed into polygons that are later used in calculations. The polygons drawn are used as a rough method for estimating the emitting area. In addition, the use of polygons most likely results in an over and/or under estimation of emitting areas. Somehow in the overall scenario emission values may be closer in reality than one may think since errors in area estimations are balanced.

Qualitatively, it is known that microseepage is higher in winter and lower in summer, due to differences in methanotrophic activity between the two seasons, removing CH4 before it can reach the atmosphere. Other short-term or seasonal variability is due to meteorology and soil conditions. Longer-time variability (decades, centuries and millennia) can be induced by endogenic factors (changes of pressure gradients in rocks, tectonic stress, etc.).

For macroseepage, the main source of uncertainty is temporal variations in emissions. The largest fraction of emissions occurs during 'individual' events/eruptions that are difficult to simulate. The resultant emissions are also not easily quantifiable. Thus, calculations are normally done using assumptions of continuous gas release from counted vents. Also, the census of vents is an additional source of uncertainty. Either on land or at sea, the majority of large macroseeps have been identified and studied, but most of the small ones have not been determined, surveyed or characterized.

0 0

Post a comment