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

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