25th International Meeting on Organic Geochemistry IMOG 2011

P-258
Worldwide distribution and significance of secondary microbial
methane formed during petroleum biodegradation in
conventional reservoirs
Alexei Milkov
BP Russia, Moscow, Russian Federation (corresponding author:alexei.milkov@bp.com)
Around half of world‘s endowment of in-place oil and
bitumen experienced biodegradation, which is now
believed to be largely an anaerobic methanogenic
process. However, the distribution and scale of
methanogenic biodegradation in world‘s petroleum
accumulations and the significance of its terminal
product, secondary microbial methane, in global gas
endowment and carbon cycle are largely unknown.
Here, I present geological and geochemical criteria to
recognize secondary microbial methane in
conventional petroleum reservoirs. These include the
presence of biodegraded oil (as pools, legs or shows)
in the reservoir or down-dip, the relatively dry
(methane-dominated) gas containing methane with
δ 13 C values between -55‰ and -35‰ and, most
importantly, CO2 with δ 13 C >+2‰ (Fig. 1). Based on
these criteria, the presence of secondary microbial
methane is apparent in 22 basins, probable in 12
basins and possible in six basins worldwide (Fig. 2).
Reservoirs apparently containing secondary microbial
methane are mostly Cenozoic and clastic and occur
at depths 37–1834 m below surface/mudline and
temperatures 12–71 ºC. Using the current global
endowment of in-place oil and bitumen and
reasonable assumptions about conversion of oil into
methane during biodegradation, I estimated that
~65,500 tcf of secondary microbial methane could
have been generated in existing worldwide
accumulations of oil and bitumen through their
geological history. From 1461-2760 tcf in-place (845-
1644 tcf recoverable) of secondary microbial methane
may be accumulated as free and oil-dissolved gas in
petroleum reservoirs. I also updated the inventory of
primary microbial methane and estimated that the
global primary microbial gas endowment (free and oildissolved)
is from 676-797 tcf in-place (407-589 tcf
recoverable). Secondary microbial methane may
account for ~5–11% of the global conventional
recoverable gas endowment and appears more
abundant than primary microbial gas (~3-4% of the
global gas endowment). Most of the generated
secondary microbial methane probably is aerobically
and anaerobically oxidized to CO2 in the overburden
above petroleum reservoirs. However, some
secondary microbial methane may escape from
shallow reservoirs into the atmosphere and affect
present and past global climate.
Fig. 1. δ 13 C of methane vs δ 13 C of CO2 in gases
within global biodegradation zone with genetic fields
defined after [1] and the field of significant secondary
microbial gas defined in this study. Thick brown line
shows the pathway of secondary microbial gas
generation consistent with the model in [2].
Fig 2. Worldwide distribution of secondary microbial
methane.
[1] Whiticar, M.J., Chem. Geol., 161, 291–314.
[2] Jones, D.M. et al., Nature, 451, 176–180.
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