南海北部大陆架Y盆地天然气中N2成因及气源剖析与探讨

Y盆地天然气中普遍含有非烃N₂和CO₂,且N₂含量与无机CO₂总体呈相互消长关系,而与CH₄为主的烃类气正相关。非烃N₂依据其含量可划分为低含N₂(N₂<10％)、含N₂(N₂=10％～15％)和富(高)N₂天然气(N₂>15％)等三类,而N₂成因则根据其地质地球化学特征及伴生稀有气体同位素特点,可将其综合判识为大气成因、壳源有机成因及壳源有机―无机混合成因等三种成因类型。N₂总体分布特征多具有平面上分区分块、剖面上集中富集于400～1600m的浅层或超浅层的特点。N₂与CO₂及烃类气的共生组合关系,常常具有有机成因的富N₂天然气与低含量有机CO₂及高含量CH₄为主的烃类气伴生,而有机―无机混合成因的低含N₂、含N₂天然气则多与高含量无机CO₂及部分CH₄为主烃类气并存的组合特征。很显然,这种共生组合关系决定了N₂的成因及气源构成特点。因此,基于N₂成因类型及与CO₂及烃类气的共生组合关系和分布特征,结合地质地球化学的综合分析研究,可以综合判识N₂气源。

N_2 ORIGIN ANALYZE IN NATURAL GAS OF Y BASIN IN THE NORTH OF NANHAI, CHINA

There contains non-hydrocarbon CO2 and N2 universally in natural gas of Y basin and the N2 content appear negative-correlate with overall of inorganic CO2 and positive-correlate with hydrocarbon gas which mainly is CH4. According to N2 content .natural gas can be divided into three typesjow N2 content(N2<10%) .middle N2 content(N2 = 10%-15%),high N2 content(N2>15%). According to its geology and geochemistry behaviors and characteristics of companion rare gas isotope the origin of N2 can be comprehensively discriminated and determined to three original types:atmospheric origin,shell origin and combination of shell origin of hydrocarbon and non-hydrocarbon. The distribution traits largely possess parti-tioned-area and partitioned-block on plate and concentrated in shallow layer or overshallow of 400-1600m on cross section. The co-existed grouping relation of N2,CO2 and hydrocarbon gas usually include natural gas of high-content N2 association with low-content CO2 hydrocarbon gas of hydrocarbon origin and hydrocarbon gas of mainly high-content CH4,and N2 nuaural gas of hydrocarbon and non-hydrocarbon combination origin coexisting with high-content CO2 of non-hydrocarbon and partly hydrocarbon gas which mainly is CH4. Obviously this co-existed grouping relation determine the traits of origin and composition of N2. So based on the origin type of N2 and co-existed grouping relation and distribution characteristics of N2, CO2 and hydrocarbon gas, associated with integrated analyzing and researching of geology and geochemistry, the source of N2 can be comprehensive discriminated and determined.