ACCUMULATION ANALYSIS ON A HELIUM-ENRICHED GAS RESERVOIR IN HUAGOU AREA, THE JIYANG DEPRESSION

Helium content in natural gas reservoirs is very low,usually smaller than 0.1%.Gas helium not only has inmportant industrial values,but also is a major differential mark for the origin of natural gas especially nonhydrocarbons natural gas.In Hua-501 gas reservoir discovered in the Upper Tertiary Minghuazhen Formation(Nm)of Huagou area,the Jiyang Depression,the gas He content is up to 3.08%.The gas components of the gas reservoir is complex.It has not only nonhydrocarbons gas layers dominated by N₂and CO₂,but also gas layers dominated by CH₄.It is a special gas reservoir of multiple and composite origins.δ¹³C_CO2 is equal to-8.3‰,and ₃He/⁴He is 4.47×10^-6.These representative indexes reveal that N₂,CO₂,He and other nonhydrocarbons gases are mainly of mantle-source origins.δ¹³ Cl is equal to -48.8‰.This shows that gas CH₄ is organic genetic oil-type gas generated in the Lower Tertiary.The gas reservoir is located in the joint of the Jiyang Depression and the Luxi Uplift.It is controlled by the Gaoqing Fault which is a deep and large rift long activated since the Mesozoic,and is closely related to muti-stage magmatic activities.CH₄ gas layers and N₂,CO₂,He and other nonhydrocarbons gas layers are the products of two stages of accumulation.CH₄ accumulated early,and the accumulation of helium-enriched nonhydrocarbons gases happened within 3Ma from now.