25th International Meeting on Organic Geochemistry IMOG 2011

P-256
Late stage gas generation in source rocks and gas shales
Nicolaj Mahlstedt, Brian Horsfield
Deutsches GeoForschungsZentrum GFZ, 14473 Potsdam, Germany (corresponding author:nick@gfzpotsdam.de)
Late dry gas generation can be expected for organic
matter with R0 > 2.0% and for geologic temperatures
well in excess of 200°C [1, 2]. This ―High Temperature
Methane‖ goes largely unnoticed when evaluation of
immature source rocks is based on routinely used
open-system pyrolysis screening-methods alone and
contributes, in addition to the cracking of unexpelled
oil, significantly to the accumulation of thermogenic
gas in low-porosity, low permeability gas shales as
well as in coal seams.
In this study we consider in detail to what extent late
stage gas generation is a function of depositional
environment and organic matter evolution using a
rapid, closed-system-pyrolysis-based screening
approach on a large world wide sample set (~100
samples) of immature source rocks as well as both
natural and artificially prepared maturity series. We
show that for almost any kind of organic matter the
late gas potential is not fixed at immature stages but
seems to increase up to ~40 mg/g TOC by the end of
catagenesis stage. Additional thermal stress during
metagenesis leads to decreasing potentials indicating
late gas generation.
The closed-system screening method basically
consists of heating crushed whole rock samples
(2°C/min) in micro-scale sealed vessels (MSSV) to 2
different end temperatures (560°C; 700°C) spanning
the range of late gas generation which occurs
subsequent to cumulative primary and secondary gas
formation. The possibility of discriminating between
source rocks with low, intermediate or high late gas
generative properties has been previously presented
[3] and can be applied to immature organic matter in
relation to its total hydrocarbon potential. High late
gas potentials are associated with heterogeneous
organic matter exhibiting a high aromaticity, mainly
terrestrial influenced type III to type II/III shales and
coals; low late gas potentials are associated with
homogeneous, paraffinic organic matter.
In the course of catagenesis late gas potentials
increase up to ~40 mg/g TOC for natural maturity
series samples (Westphalian Coals, Wealden Coals,
Barnett Shale) indicating that predicted late gas
amounts of immature equivalents are underestimates.
During natural maturation, chain shortening reactions
via β–scission related to hydrocarbon generation
might lead to a concomitant enrichment of methylaromatics
and hence late gas precursor structures
within the residual organic matter. This interpretation
is corroborated by the preparative pyrolysis of 2
immature coal samples (Åre Fm., R0 ~ 0.4%;
Westphalian Coal, R0 ~ 0.99%) under open- and
closed-system pyrolytic conditions. Starting at 250°C,
samples were heated to 400, 450, and 500°C using a
1°C/min heating rate. Despite differences in the stage
of artificially induced maturity, late gas potentials of
residues increase for both analytical set-ups. Thus,
second-order recombination reactions between firstformed
bitumen and residual organic matter seem
not, as previously assumed [1, 2], to be a prerequisite
for the generation of late gas but can be viewed as
being supportive for the retention of TOC enhancing
the amount of high temperature methane generated
from a given mature source rock unit during
metagenesis.
The decrease of late gas potentials during
metagenesis (R0 > 2.0%) demonstrates that dry gas
generation takes place under geologic conditions and
indirectly confirms previous [1, 2] and new (this study)
compositional MSSV-kinetic calculations.
References
[1] Erdmann and Horsfield (2006), Geochim. Cosmochim.
Acta 70, 3943-3956.
[2] Dieckmann et al. (2006), Mar. Pet. Geol. 23, 183-199.
[3] Mahlstedt and Horsfield (2009) Geochim. Cosmochim.
Acta 73, A817-A817
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