25th International Meeting on Organic Geochemistry IMOG 2011

P-260
Geochemical characteristics of biogenic gases in China
Yunyan Ni, Jinxing Dai, Caineng Zou
Research Institute of Petroleum Exploration and Development, PetroChina, Beijing, China (corresponding
author:niyy@petrochina.com.cn)
Alkane gases are formed largely by the digestion of
organic compounds by microorganisms (biogenic gas)
and the thermal decomposition of organic matter
(thermogenic gas). Bacterial process is a widespread
phenomenon in nature and over 20% of the world
natural gas accumulations are of biogenic origin. In
China biogenic gases account for 7% of the total
proved gas reserves. Recent discovery of the largest
terrestrial biogenic gas field with proved gas reserves
of 3000×10 8 m 3 in the Qaidam Basin has
demonstrated the huge resource potential. Due to its
wide distribution, shallow burial depth and low
exploiting cost, biogenic gases play more and more
important role in the gas explorations. Here we
demonstrate the main geochemical characteristics of
biogenic gases in China and their possible formation
mechanisms according to around 200 samples from
various biogenic gas fields, wells or gas seeps.
To date, biogenic gases are mainly distributed in the
strata of Quaternary, Paleogene, Neogene and
Cretaceous with shallow burial depth. The deepest
one is from south Qaidam basin, where burial depth
can be up to 1900 m. The shallowest one is from
Zhejiang, where biogenic gas reservoirs are only
several meters deep. Burial depth of most commercial
biogenic gas reservoirs ranges from 400 to 1800
meters.
Biogenic gases in China are dominated by methane,
with little amount of ethane and propane (normally
less than 1%). � 13 C1 values vary from -89‰~-
55‰ and �D1 values range from -287‰~-108‰.
Previous studies proposed that variations in �D in
biogenic methane are predominantly due to
environmental changes. A differentiation of ‗terrestrial‘
(�D1-190‰
or - 200‰) biogenic gases has been suggested
(Schoell, 1980; Shen et al., 1991). As shown in Figure
1, biogenic gases in China are mainly of terrestrial
origin and a result of bacterial carbonate reduction.
Samples from Yingqiong Basin and Jiangsu province
are relatively more enriched in D, which are closely
related to the marine environments or seawater
intrusion in these areas. Several samples from
Baoshan Basin show a mixture feature of bacterial
carbonate reduction and bacterial methy-type
fermentation. This might be related to its relatively
high geothermal temperature (7-8 ℃ ) which is
favorable for the bacterial methyl-type fermentation.
Fig. 1 Stable carbon and hydrogen isotopes of
biogenic gases in China
References:
[1] Schoell, M. The hydrogen and carbon isotopic
composition of methane from natural gases of various
origins. Geochimica et Cosmochimica Acta, 1980, 44,
649-661.
[2] Shen, P., Xu, Y.C., Wang, X.B., et al. Geochemical
characteristics of gas source rocks and natural gas
genesis. Gansu Science and Technology Press,
LanZhou, 1991.
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