吐哈盆地台北凹陷天然气碳氢同位素组成特征

Stable carbon and hydrogen isotopic characteristics of natural gas from Taibei sag, Turpan-Hami Basin, NW China

吐哈盆地为中国重要的富油气盆地,目前该盆地的天然气勘探主要集中在台北凹陷,有关台北凹陷的天然气成因和来源一直存在诸多争议。通过分析台北凹陷巴喀、丘陵、鄯善和温米等4个油气田23个天然气样品的组分和碳氢同位素组成,结合前期红台和丘东气田天然气地球化学资料以及前人研究成果和区域地质背景,开展天然气成因和来源研究。结果表明:台北凹陷巴喀、丘陵、鄯善和温米等地区天然气以烃类气体为主。甲烷含量为65.84%～97.94%,重烃(C_(2—5))含量高达34.98%,非烃气体(CO_2、N_2)含量非常低,为湿气。天然气δ^(13)C_1值为–44.9‰～–40.4‰,δ^(13)C2值为–28.2‰～–24.9‰,δ^(13)C_3值为–27.1‰～–18.0‰,δ^(13)C_4值为–26.7‰～–22.1‰;天然气δD1值变化不大,为–272‰～–252‰,δD_2为–236‰～–200‰,δD_3为–222‰～–174‰。甲烷及其同系物(C_(2—5))基本上为碳氢同位素组成正序排列(δ^(13)C1<δ^(13)C_2<δ^(13)C_3<δ^(13)C_4<δ^(13)C_5、δD_1<δD_2<δD_3),与典型的有机成因烷烃气碳氢同位素组成特征一致。研究区天然气为成熟度较低的煤成气(Ro均值为0.7%),主要来自中下侏罗统煤系源岩。天然气氢同位素组成受到烃源岩热演化程度和形成环境水介质的影响,数据表明研究区天然气烃源岩为陆相淡水湖沼沉积。巴喀油田巴23井和柯19井天然气后期发生次生改造,为生物改造气。

Turpan-Hami Basin is a major petroliferous basin in China.To date the natural gas exploration is concentrated in the Taibei sag.The origin and source of the natural gas in the Taibei sag has long been controversial.To further investigate the origin and source of the natural gas in the Taibei sag,combined with previous studies and the local geological backgrounds,this study collected 23 gas samples from the Baka,Qiuling,Shanshan and Wenmi oil/gas fields in the Taibei sag and analyzed the sample composition,stable carbon and hydrogen isotopes of all the gas samples.The results show that,the gases from the four oil/gas fields in the Taibei sag are dominated by hydrocarbon gas and belong to wet gas.Methane accounts for 65.84%to 97.94%,the content of heavy hydrocarbon(C2-5)can be up to 34.98%,while the content of nonhydrocarbon(CO2,N2)is trace.Theδ^13C1 value is–44.9‰to–40.4‰,δ^13C2 is–28.2‰to–24.9‰,δ^13C3 is–27.1‰to–18.0‰andδ^13C4 is–26.7‰to–22.1;while the variation ofδD1 is not significant from–272‰to–252‰,δD2 is–236‰to–200‰andδD3 is–222‰to–174‰.Methane and its homologues(C2-5)are characterized by normal stable carbon and hydrogen isotopic distribution pattern,i.e.,with the increase of carbon number,methane and its homologues become more and more enriched in 13C or D(δ^13C1<δ^13C2<δ^13C3<δ^13C4<δ^13C5,δD1<δD2<δD3),which is consistent with the carbon and hydrogen isotopic features of typical organic origin alkane gas.All these results show that the natural gases in the four oil/gas fields are coal-derived gas with low maturity(Ro averaged at 0.7%),and are sourced from the Middle-Lower Jurassic coal measure.The hydrogen isotopic data of natural gas are affected by both thermal maturity and depositional environment of the source rocks.The hydrogen isotopic data indicate that the source rocks were deposited in terrestrial limnetic facies with freshwater.Natural gases from Well Ba23 and Well Ke19 experienced biodegradation in the late stage.