Abstract: In the central part of southern margin of Junggar Basin, Well Hutan1 at the Huxi anticline and Well Tianwan1 at the Dongwan anticline have obtained high-yield oil-gas flows from the Lower Cretaceous to Upper Jurassic reservoirs at a burial depth from 7 300 m to 8 100 m, which has made a major breakthrough in ultra-deep oil-gas exploration, showing a good prospect for hydrocarbon exploration in the ultra-deep lower assemblage of the thrust belt in the southern margin of Junggar Basin. The source and genesis of oil and gas in reservoirs are vital for understanding the hydrocarbon accumulation regularity and controlling factors as well as hydrocarbon resource potential in the ultra-deep lower assemblage. Based on the fine analysis and study of physicochemical and geochemical characteristics of ultra-deep condensate oil at the Huxi anticline and Dongwan anticline, the source and genesis of condensate oil was explored. The results show that the condensate oils in Well Hutan1 and Well Tianwan1 are characterized by low densities, low viscosities, low wax contents and freezing points, and high contents of alkanes, naphthenes and aromatics with low carbon numbers, as well as rich contents of n-alkanes with middle-high carbon numbers, and thus they are classified as light oils with low densities, low wax contents and low freezing points. The condensate oils have a light carbon isotope composition, the δ¹³C value of whole oil is from -30.7 ‰ to -28.6 ‰, and nC₉-nC₃₀ n-alkanes have light carbon isotope compositions, especially nC₁₅-nC₃₀ n-alkanes, and the corresponding δ¹³C value is from -29 ‰ to -33 ‰ . The condensate oils have a relatively high abundance of isoprenoids which show equalizing pristine (Pr)and phytane (Ph)contents or having a dominant Ph content, with Pr/Ph ratio from 0.69 to 1.27. In the compositions of biomarkers, C₂₇, C₂₈ and C₂₉ steranes and diasteranes are abundant, C₃₀ methylsteranes are relatively abundant, and C₂₇, C₂₈ and C₂₉ ααα-20R steranes show a V-shaped distribution. The 20S/(20S+20R)ratio and ββ/(αα＋ββ)ratio of C₂₉ sterane are greater than 0.45 and 0.60, respectively. The condensate oils have abundant tricyclic terpenes showed a near-normal distribution with C₂₁ terpane as the highest, have very abundant 18α(H)- trisnorneohopane (Ts), C₂₉Ts, C₂₉ diahopane and C₃₀ diahopane, and have a high content of gammacerane with an isomers, indicating that the condensate oil is generated by organic matters deposited in semi-saline to saline lacustrine clay-rich environment. As a whole, the condensate oils from Well Hutan1 and Well Titanwan1 in the ultra-deep reservoir of the central part of southern margin of Junggar Basin have very similar geochemical characteristics with the crude oil/condensate oil from the middle to shallow reservoirs at the Horgos anticline. The condensate oils are derived from the lacustrine source rocks of the Lower Cretaceous Qingshuihe Formation, and are formed by later gas invasion/washing in the early oil reservoirs.

Key words: Junggar Basin, southern margin, ultra-deep lower assemblage, condensate gas reservoir, condensate oil, oil source, gas invasion/gas washing

摘要： 准噶尔盆地南缘中部地区呼西背斜呼探1井和东湾背斜天湾1井在7 300~8 100 m深度段下白垩统—上侏罗统获得了高产油气流,超深层油气勘探获得重大突破,展现了准噶尔盆地南缘冲断带超深层下组合良好的油气勘探前景。这些油气藏的油气来源及成因成为认识超深层下组合油气成藏规律、控制因素和油气资源潜力的关键。基于呼西背斜和东湾背斜超深层凝析油物理化学和地球化学特征的精细分析,探讨了凝析油的来源与成因。研究结果表明,呼探1井和天湾1井的凝析油密度低、黏度小、含蜡量和凝固点低,低碳数烷烃、环烷烃和芳香烃含量较高,中—高碳数正构烷烃含量也较丰富,属于低密度、低蜡、低凝轻质油。这些凝析油具有轻的碳同位素组成,全油的δ¹³C为-30.7 ‰~-28.6 ‰,nC₉—nC₃₀正构烷烃分子的碳同位素组成轻,尤其是nC₁₅—nC₃₀分子的碳同位素组成特别轻,相应的δ¹³C为-29 ‰~-33 ‰。类异戊二烯烷烃丰度较高,显示姥鲛烷(Pr)和植烷(Ph)含量均势或Ph含量占优势,Pr/Ph为0.69~1.27。生物标志物组成中,C₂₇、C₂₈、C₂₉规则甾烷和重排甾烷含量丰富,C₃₀甲基甾烷含量较丰富,ααα-20R构型的C₂₇、C₂₈、C₂₉甾烷呈“V”型分布,C₂₉甾烷的异构化成熟度参数20S/(20S＋20R)和ββ/(αα＋ββ)分别大于0.45和0.60,三环萜烷含量丰富且呈以C₂₁三环萜烷最高的近正态分布,18α(H)-三降藿烷(Ts)、C₂₉Ts、C₂₉重排藿烷和C₃₀重排藿烷含量非常丰富,伽马蜡烷含量高且明显含有异构体,指示凝析油由半咸水—咸水湖相富黏土矿物沉积环境下的有机质所生成。总体上,准噶尔盆地南缘中部地区超深层呼探1井和天湾1井凝析油的地球化学特征与该地区霍尔果斯背斜等中—浅层油气藏的原油/凝析油非常相似,油源为下白垩统清水河组湖相烃源岩,凝析油为前期油藏遭受后期天然气气侵/气洗改造作用而形成。

关键词: 准噶尔盆地, 南缘, 超深层下组合, 凝析气藏, 凝析油, 油源, 气侵/气洗作用