吐哈盆地雁木西油田天然气成因分析

GENESIS OF NATURAL GAS IN YANMUXI OILFIELD,THE TURPAN-HAMI BASIN

雁木西油田所产天然气在吐哈盆地有一定的特殊性,主要表现为含有一定量的硫化氢气体,部分样品的氮气含量高、甲烷含量低、烃类碳同位素值较重,并且重烃碳同位素值发生了倒转。探讨这些特殊性有助于全面认识吐哈盆地的天然气来源、成因与分布规律等,为此在天然气地球化学特征分析的基础上,结合具体的地质条件对天然气的来源、成因进行了系统地分析并取得了如下认识:①雁木西油田的原油溶解气分布在鄯善群和白垩系油藏中,成因上为腐殖型天然气,由烃类和非烃类组成;②烃类气体由甲烷和重烃组成,不同组分的含量与干燥系数均在较宽的范围内变化,但都表现为湿气特征;③非烃类气体主要由N2、H2S、CO2等组成,其中CO2含量普遍较低;④烃类气体、部分非烃气体及其伴生的原油主要来自侏罗系烃源岩,部分高含量的氮气为大气来源,硫化氢与深部石炭系海相碳酸盐岩有关;⑤部分天然气遭受了次生变化,烃类碳同位素值偏重、重烃碳同位素值倒转均与细菌的生物降解作用有关。

Natural gas produced from Yanmuxi oilfield in the Turpan-Hami Basin shows some special characteristics,such as certain content of hydrogen sulfide,high content of nitrogen,low content of methane and relatively heavy carbon isotope of hydrocarbons in some samples,as well as inverse carbon isotope of heavy hydrocarbons.Study of these special characteristics is helpful for a thorough understanding of the source,genesis and distribution of gas in the Turpan-Hami Basin.A systematic analysis of the source and genesis of gas is performed based on the geochemical behaviors of gas and in combination with the geologic conditions and the following conclusions are obtained.① The dissolved gas in Yanmuxi oilfield is mainly distributed in the Shanshan Group and Cretaceous oil reservoirs,and belongs to humid gas composed of hydrocarbon and non-hydrocarbon gases.② The hydrocarbon gases mainly include methane and heavy hydrocarbons,which show the characteristics of wet gas,though their contents and drying coefficients change in wide ranges.③ The non-hydrocarbon gases are dominated by N2,H2S and CO2,in which the content of CO2 is generally low.④ The hydrocarbon gases,some of the non-hydrocarbon gases and the associated oil are mainly sourced from the Jurassic source rocks,some nitrogen with high content are sourced from the air,while the hydrogen sulfide is sourced from the deep Carboniferous marine carbonate.⑤ Some natural gas has experienced secondary change,and the relatively heavy carbon isotope of hydrocarbons and the inverse carbon isotope of heavy hydrocarbons are the results of biodegradation.