准噶尔盆地东部侏罗系烃源岩和天然气地球化学特征及低熟气勘探前景

对准噶尔盆地东部侏罗系烃源岩有机地球化学特征进行了研究,结果显示准噶尔盆地东部侏罗系烃源岩有机碳含量分布范围广泛,TOC为0.13%~81.06%,烃源岩评价为差—中等生油潜力,但生气潜力较大;镜质体反射率Ro分布和与热演化程度相关的生物标志化合物参数(OEP值、CPI值、甾烷和藿烷异构化参数、甲基菲指数等)表明大多数烃源岩处于未成熟—低成熟热演化阶段;有机质类型以III型为主,部分八道湾组和西山窑组烃源岩为II2型。此外,对准噶尔盆地东部侏罗系天然气组分和碳同位素组成分析结果表明,侏罗系天然气以烃类气体为主,甲烷含量为65%~98%,C1/C1-5为0.62~0.98,以湿气为主,碳同位素组成分布范围较广,其中甲烷同位素组成δ13 C1值为-50.7‰~-28.2‰,乙烷δ13 C2为-32.5‰~-23.7‰,丙烷δ13 C3值为-30.6‰~-21.6‰,以煤型气为主,仅三台—北三台地区天然气显示为油型气-煤型气混合的特征。在对准噶尔盆地东部地区侏罗系煤系烃源岩和天然气样品分析的基础上,通过与吐哈盆地典型低熟气区烃源岩和天然气地球化学特征对比认为准噶尔盆地东部阜东和三台—北三台地区已形成一定规模的低熟气聚集,具有巨大的勘探潜力,是准噶尔盆地低熟气有望取得突破的重点地区。

Geochemical characteristic of Jurassic source rocks and natural gas in the eastern Jungar Basin and exploration potential of low-mature gas

Organic geochemical characteristics of Jurassic source rocks in the eastern Jungar Basin have been studied, showing that the Jurassic source rocks in the eastern Jungar Basin have a wide distribution of TOC ranging between 0.13% and 81.06% and was evaluated as having poor-moderate oil generation potential, but its gas generation potential is high. Distribution of vitrinite reflectance R_o and biomarker compound parameters associated with thermal evolution degree (OEP value, CPI value, sterane and hopane isomerization parameters, methylphenanthrene index, etc.) indicate that a majority of source rocks are situated in immature-low-mature stage of thermal evolution. Type-III organic matter is dominant in this area, though source rocks of Badaowan Formation and Xishanyao Formation partially have type II₂ organic matter. In addition, analysis of elemental composition and carbon isotopic composition of Jurassic natural gas in the eastern Jungar Basin demonstrate that the Jurassic natural gas is dominated by hydrocarbon gases, in which methane content is 65%-98%, C₁/C_1-5 ratio is 0.62-0.98, and wet gas is predominant; carbon isotopic composition has a wide distribution, among which δ¹³C₁ values of methane range from -50.7‰ to -28.2‰, δ¹³C₂ values of ethane range from -32.5‰ to -23.7‰, and δ¹³C₃ values of propane between -30.6‰ and -21.6‰, suggesting the dominant coaliferous gas, except that natural gas of Santai-Beisantai area is characterized by petroliferous gas-coaliferous gas mixture. Based on analyses of Jurassic coal measure source rock and natural gas samples from the eastern Jungar Basin area, the study area was compared with typical low-mature gas zone of the Turpan-Hami Basin in terms of geochemical characteristics of source rock and natural gas, and it is believed that a certain scale of low-mature gas accumulation has come into being in the eastern Jungar Basin, eastern Fukang sag and Santai-Beisantai area, which have huge exploration potential and thus are key areas of the Jungar Basin expected to make a breakthrough in low-mature gas exploration.