25th International Meeting on Organic Geochemistry IMOG 2011

P-349
Fluid pressure evolution and gas preservation of the Puguang
Gasfield, Sichuan Basin, China
Xiao Xianming, Liu Dehan, Tian Hui
State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of
Sciences, Guangzhou, China (corresponding author:xmxiao@gig.ac.cn)
Previous studies have indicated that the natural gas
of Puguang Gasfield of the Sichuan Basin, China, is a
typical oil-cracking origin. However, there is no report
on the fluid pressure evolution and gas preservation
related to the oil cracking processes of this reservoir
during geological history. This study investigated the
fluid pressure variation and its influences on loss and
storage of hydrocarbons in the reservoir of the
Puguang Gasfield by means of PVTsim software
modeling based on the gas generation kinetics of oil
cracking and geothermal history of this area. The
result has shown that the fluid pressure was 25 – 50
MPa with a pressure coefficient less than 1.2 during
175 to 157 Ma in the reservoir where oil was not
cracked. The oil was significantly cracked from 157Ma,
which resulted in a fast increase of pressure in the
reservoir. At about 142 Ma, the pressure coefficient
reached 2.4 in the reservoir when about 80% oil was
cracked into gas. The seal rock was broken under this
pressure condition, the gas leaked out and the
reservoir pressure returned to the hydrostatic
pressure. It was supposed that the reservoir was resealed
after the gas leakage, and its pressure would
increase again with residual oil cracking and
subsequently the secondary cracking of C2-5
hydrocarbons. At 132 Ma, the pressure coefficient
reached 2.4, gas leaked out and the reservoir was resealed
again. It is estimated that about 23% and 33%
of petroleum in the reservoir was loosed during the
two times of break, respectively. In the interval of 132
- 96Ma for further burial of the gas reservoir, the
pressure was mainly affected by temperature increase
since the C2-5 gas was almost cracked during the
previous two stages. In the interval of 96Ma to the
present time, this area was uplifted to erode about
2800 m thick sediments, the pressure of the gas
reservoir, as affected by temperature decrease,
decreased to 56.9-61.0MPa, with a current pressure
coefficient of 1.1. It is estimated that about 46% of gas
resulted from oil cracking was preserved in the
reservoir to form the current gas pool. The
overpressure resulted from gas generation due to oil
cracking was validated by a discovery of high density
(0.34-0.35 g /cm 3 ) methane inclusions from reservoir
rocks. This type of inclusions has a trapping pressure
up to 150-160MPa, with a pressure coefficient of 2.0-
2.2. This present research provides a case study of
an oil-cracking origin gas pool and its fluid pressure
evolution, which is useful to gas exploration in areas
with similar geological and geochemical conditions in
China petroleum-bearing basins.
Keywords: Puguang Gasfield; oil-cracked gas; high
density methane inclusion
480