25th International Meeting on Organic Geochemistry IMOG 2011

O-49
Determination of effective source rocks for Quaternary biogenic
gas generation in the eastern Caidam Basin, NW China
Maowen Li 1 , Shuichang Zhang 2 , Yanhua Shuai 2 , Jenny Wong 3 , Julito Reyes 3 , Sneh
Achal 3
1 Sinopec Research Institute of Petroleum Exploration and Production, Beijing, China, 2 PetroChina Research
Institute of Petroleum Exploration and Development, Beijing, China, 3 Geological Survey of Canada, Calgary,
Canada (corresponding author:limw@pepris.com)
The Quaternary clastic sediments collected from the
eastern Qaidam Basin, NW China have been
investigated for their molecular geochemistry, stable
carbon and hydrogen isotopes, and organic and
inorganic petrology, where geochemical data of gas
samples from a number of large gas accumulations
clearly demonstrate a biogenic origin, likely formed by
anaerobic microbial breakdown of sedimentary
organic matter. The gases are dominated by
methane, with minor C2+ hydrocarbons, N2 and CO2
(all below 1%). The �13C values of methane in these
gases range from -68.51‰ to-65.00‰, typical of
biogenic gas. The��D values of methane in these
gases, ranging from-227.55‰ to -221.94‰, are much
higher than those generated from acetate
fermentation, indicating a probable origin from carbon
dioxide reduction. Quaternary strata in the study area
are characterized by inland evaporitic lacustrine
sediments, with generally low organic content (on
average only 0.3% TOC) and locally organic-rich
carbonaceous layers (up to 40% TOC). The organic
input is predominately from terrigenic herbaceous and
low shrubbery plants, relatively rich in cellulose,
hemicellulose, and carbohydrates such as starch, fat
and pectin. The average geothermal gradient in this
area is 25 oC/km, and the maximum temperature for
Quaternary sediments up to 2000 m in depth is still
below 70 oC or 0.5 %Ro. Against conventional
thinking, many previous studies on biogenic gas
generation suggest that the Quaternary deposit with a
low TOC content be the effective source rocks for the
bioigenic gases. The organic-rich carbonaceous
layers were deliberately excluded in the earlier rounds
of gas resource assessment, because mass balance
calculations in the literature claims that high TOC
content is not necessary for commercial biogenic gas
generation and a 10% conversion of sediments
containing greater than 0.2% TOC can potentially
generate a substantial amount of free gas phase.
Results of our petrographic study indicate that sulfate
occurs widely in the Quaternary sediments in eastern
Caidam Basin. Organic matter in almost all of the
organic-lean Quaternary samples (with less than 1%
TOC) are either inertinite or highly mineralized
organic detritus, co-occurring with sulfate and ferrous
oxide minerals. The trace amounts of organic extracts
in these samples also reveal strong molecular
evidence for oxidative degradation of sedimentary
organic matter. For example, the C29 sterane
transformation ratios are abnormally high, displaying
values normally expected in ―conventional peak oil
window‖. The data are clearly incompatible with the
low thermal maturity levels of the host sediments. As
factors such as contaminations from migrated
hydrocarbons, from drilling fluids, or laboratory
handling can be readily ruled out, the abnormal data
most likely indicate that these organic-lean sediments
were deposited and are still in aerobic environments
in spite of significant sedimentary burial! This is in
contrast with the organic-rich carbonaceous layers,
where molecular analysis of organic extracts indicates
highly anoxic depositional environments.
Petrographically, large amount of well-preserved plant
material co-occurs with pyrite, a reliable indicator for
anaerobic environment and sulfate-reducing bacteria.
The latter are common in anaerobic environments
where they aid in the degradation of organic
materials. The results indicate that only these organicrich
layers have been capable of providing the
competing methanogens with sufficiently available
substrates in subsequent diagenesis and biogenic
gas generation. When the sediments are buried to
some depth and reach the optimal temperatures for
methanogens to thrive, the dilution of water salinity by
groundwater infiltration would greatly facilitate the
flourish of the methanogens in shallow sediments and
hence the generation of large quantities of biogenic
methane.
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