塔里木盆地含氮天然气地球化学特征及氮气来源

Geochemical characteristics and N_2 source of nitrogen riched natural gas in Tarim Basin

为了研究塔里木盆地不同构造带氮气含量和同位素组成的成因和来源,通过对塔里木盆地重点油气田含氮天然气的地球化学特征进行系统研究发现,塔里木盆地天然气中氮气含量与天然气的类型和成熟度有关。研究表明,油型气中氮气含量多大于5%,主要位于中央隆起和塔北隆起的古生代地层中;煤型气中氮气含量一般小于5%,主要位于库车坳陷、塔北隆起和塔西南坳陷的中生代—新生代地层中;天然气中氮气含量在成熟阶段最高,且随天然气成熟度增高氮气含量逐渐降低;天然气中氮同位素与天然气类型关系密切,含轻氮同位素的天然气分布在油型气藏中,含重氮同位素的天然气则分布在煤型气藏中。进一步对塔里木盆地主要油气田天然气中的氮气来源进行研究后发现,虽然塔里木盆地内大多数气田天然气中的氮气与相同储层中的烃类同源,为烃源岩在成熟、高成熟、过成熟阶段产生的氮气,但阿克莫木气田天然气中却存在较为明显的幔源氮气混入。

This study investigates the content of N2 and the origin and source of nitrogen isotope composition in different tectonic zones of Tarim Basin by examining the geochemical characteristics of nitrogen mched natural gas in the major oil-gas fields in Tarim Basin. Results show that the N2 content is determined by the type and maturity of natural gas. Oil-type gas generally contains more than 5% N2 and is mainly distributed in Paleozoic strata in the central uplift and Tabei uplift. Coal-type gas generally contains less than 5 % N2 and is mainly found in Mesozoic-Cenozoic strata in Kuqa depression, Tabei uplift, and Taxinan depression. The content of N~ in natural gas is highest in the stage of maturity and gradually decreases with increasing maturity. The nitrogen isotope fractionation of natural gas is closely related to the type of natural gas, and light-and heavy-nitrogen isotope fractions are respectively found in nat- ural gas in oil-and coal-type gas reservoirs. The N2 origin of natural gas in the major oil-gas fields in Tarim Basin is further analyzed. Results show that the N2 composition of natural gas in most of the oil-gas fields in Tarim Basin have identical origin with hydrocar- bons gases in the same reservoirs, i. e. , the products of source rocks in the stages of maturity and high to post maturity. An excep- tion is natural gas in Akemomu gas field, which is associated with obvious mixing of mantle-derived N2.