Reaction kinetics of gas generation in selected source rocks of the West Siberian Basin: implications for the mass balance of early-thermogenic methane
Rg. Schaefer et al., Reaction kinetics of gas generation in selected source rocks of the West Siberian Basin: implications for the mass balance of early-thermogenic methane, CHEM GEOL, 156(1-4), 1999, pp. 41-65
About one third of the actual global recoverable natural gas reserves is ac
cumulated in the West Siberian Basin in several super-giant and mega-giant
as well as numerous smaller gas fields. The elucidation of the nature of th
is gas, its mechanisms of formation and accumulation are still a matter of
discussion. For instance, it is still not yet clear whether the enormous dr
y-gas accumulations in Cenomanian reservoirs are of microbial, 'early-therm
ogenic' or catagenetic/metagenetic origin, or if they represent a mixture o
f these possible formation mechanisms. The present contribution provides ne
w results on the thermal gas generation properties of selected source rocks
of the Cretaceous Pokur Formation in the West Siberian Basin. Eight select
ed source rock core samples of the Pokur Formation and a recent peat sample
of the same area were submitted to open-system programmed-temperature pyro
lysis, in order to quantify the formation of individual gas compounds, i.e.
, methane as well as C-2 and C-3 hydrocarbons. The rock samples were select
ed according to geological age and maturity, i.e., from the Aptian, Albian,
and Cenomanian, with the most immature samples from the latter. Organic ca
rbon contents comprise the whole range from 3 to more than 60%, the elevate
d values representing typical lignites. A reaction kinetic model system was
applied to the 'generation curves' of the gases resulting in activation en
ergy distributions and the corresponding pre-exponential 'Arrhenius factors
'. The kinetic data were combined with the temperature history of two selec
ted wells of the Urengoy area obtained from basin modelling in order to cal
culate the gas generation on the geological time-scale. It was found that,
even for the low temperatures occurring in the Pokur Formation, 'early' gas
generation could have taken place to a certain, yet small, degree. Under t
he assumption of a respective catchment area and the accumulation of this g
as in the reservoirs, however, methane quantities are in the order of magni
tude of the Urengoy gas accumulation. Thus, 'early-thermogenic' methane cou
ld, at least partially, contribute to the gas accumulated in the reservoirs
. (C) 1999 Elsevier Science B.V. All rights reserved.