Heterogeneity of natural gases and the episodic charging process: a case study for Dongfang 11 gas field, Yinggehai basin.
Dong, Weiliang; et al. (China
Offshore Oil Nanhai West Corp., Guangdong 524057, P. R. China).
There
is a complex distribution of the main components (CH4, CO2 and N2) of gases
in the Dongfang 11 gas field, the Yinggehai basin. The study indicates that
hydrocarbon gas (except for biogas) was generated from mature to highmature
Miocene sourcerocks; CO2 was principally formed from inorganic sources involving
the thermodecomposition of carbonate minerals and the reaction of carbonate
minerals with silicate minerals, after the Miocene calcitemudstones entered the
late period of high maturation ;N2 was derived from the maturation of organic
matters. Both lateral and vertical compositional heterogeneities and isotopic
variations of gases of the whole field suggest that those reservoir zones have
been compartmentalized by the faults, shalebreaks and even nonpermeable zones
causng in the Yinggehai basin. The data combined with fluid inclusions as
well as thermal evolution of the sourcerocks can be used to trace and identify
that the field has been charged by at least 4 episodes of gases over geologic
time. The episodic gas charging process closely relates to the actions of a
diapir caused by deep overpressure releasied by lateral lithofacies variations, and
lead to a better understanding of the charging history.
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