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.