Reservoir fluids in the Ledong gas field in Yinggehai basin have obvious heterogeneity.The natural gas is mainly composed of methane

CO

2

and nitrogen.Thermogenic hydrocarbon gases were sourced from the mudstones in Miocene Meishan-Sanya Fms.Most of nitrogen were generated from the organic materials of the source rocks during mature and post-mature stages.Inorganic CO

2

might be the results of thermal decomposition of carbonate minerals and reactions between carbonate and clay minerals in deeply buried sedimentary strata.In addition

some of the CO

2

might be generated by thermal decomposition of the pre-Tertiary carbonate rocks and a little amount of the CO

2

might be added from the mantle.According to the chemical composition and carbon isotope data

it is preliminarily determined that there are at least three stages of gas charging

i.e.successively biogas

thermogenic hydrocarbon-rich gas and inorganic CO

2

.The time of gas reservoiring is about 1.2 to 0.1 Ma.The diapiric faults that link up source rocks would have acted as good pathways for upward migration of natural gas.Episodic discharging of high-pressure fluids have induced the periodic opening and closing of diapiric faults

resulting in the mixing of gases from multiple sources and episodic accumulation of gases in the diapiric zone of Yinggehai basin.