25th International Meeting on Organic Geochemistry IMOG 2011

O-85
Mechanistic study of shale gas generation and gas isotope
fractionations
Yongchun Tang, Daniel Xia
PEER Institute, Covina, United States of America (corresponding author:tang@peeri.org)
Recently abundant production and geochemical data
of shale gas accumulated during the extensive
exploration and development in the U.S provide
geochemists with a close look at the details of shale
gas generation. In particular, the systematic
measurements show a surprising patterns of carbon
isotope composition (� 13 C) of C2H6 and C3H8, which
becomes more negative at deeper depth (known as
―rollover‖). [1] Similar phenomenon was also found in
the conventional gas reservoir in the Ordos Basin,
China. In addition, the hydrogen isotope of CH4 (�D1)
from the Appalachian Basin of USA also shows a
reversal trend with depth. [2] This abnormal
phenomenon may relate to some special processes of
natural gas generation under geological condition.
In order to explain the isotope reversal phenomenon,
different mechanisms have been proposed.
Fractionation caused by adsorption/desorption is
unlikely the reason, since one would not expect to see
large reversal of � 13 C2 (>10 per mil) due to small
differences of any adsorption/desorption energies of
13 C labeled and unlabeled C2H6. Diffusion might
cause strong fractionation during shale gas
production; however the effect would be stronger for
CH4 than C2H6 (due to the larger isotope effect on
molecule mass of methane).
Gas mixed from dominant amount of late gas (dry gas
generated from high mature kerogen) and minor
amount of secondary gas, with volume ratio around
99% : 1%, may fit well to the ―rollover‖ trend of � 13 C2
against wetness and the normal trend of � 13 C1; but
the �D1 values become more negative with increasing
maturity and depth remains unknown. Since �D of
water in shale formation (-50 to -80 per mil) is much
more positive than organic matter, the reversal of �D1
is unlikely due to isotope exchange. The overall of �D1
values can only be explained by the contribution of
hydrogen from oil or condensate. Since the oil or
condensate cracking will generate more wet gases
and it is impossible to explain the rollover of hydrogen
isotope by oil cracking.
One of our considerations is hydrocarbon reforming
reaction with water to form CO2 and H2 under shale
gas geological condition. To address this issue, we
performed sealed gold tube pyrolysis of model
compounds under conditions with and without water.
The model compounds included in this study are
isooctane, benzene, toluene and 1-methylnaphtalene
(1-MN). Significant amount of CO2 and H2 were
generated at vitrinite reflectance equivalent of 2%.
Based on the model compound study, the relative
rates of the hydrocarbons water reforming reactions
are: branched alkane > alkyl polyaromatics > alkly
benzene > benzene.
Hydrocarbon reforming reaction with water is very
important toward the mechanism of shale gas
generation. CO2 generated from this reaction may
have dissolved certain minerals and increased the
porosity and permeability. Based on the observation
of geothermal fluid methane formation, the reforming
product of CO2 and H2 may generate relative dry
methane with ethane and propane isotopically lighter
than methane, [3] and decrease water saturation.
References
[1] Zumberge, J. E., Ferworn, K. A., Curtis, J. B.
(2009) Gas character anomalies found in highly productive
shale gas wells. Geochim. Cosmochim. Acta 73, A1539.
[2] Burruss, R. C., Laughrey, C. D. (2010), Carbon
and hydrogen isotopic reversals in deep basin gas: Evidence
for limits to the stability of hydrocarbons. Org. Geochem. 41,
1285-1296.
[3] Sherwood Lollar, B., Westgate, T. D., Ward, J. A.,
Slater, G. F., Lacrampe-Couloume, G. (2002), Abiogenic
formation of alkanes in the Earth's crust as a minor source
for global hydrocarbon reservoirs, Nature, 416, 522-524.
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