柴达木盆地东坪地区一类新的原油及其地球化学特征

各类生物标志物的组成特征表明:柴达木盆地北缘的冷湖油田原油具有姥植比高（Pr/Ph>2.0），重排甾烷（C₂₇重排甾烷/规则甾烷=0.5~0.7）、重排藿烷（diaC₃₀H/C₃₀H=0.2~0.4）和新藿烷（C₂₉Ts/C₂₉H=0.4~0.7）含量中等，伽马蜡烷含量低（伽马蜡烷指数<0.05）的特征，它们源于该地区发育的下侏罗统淡水湖沼相烃源岩；而柴西北区咸水湖相原油的姥植比低（Pr/Ph<0.8），重排甾烷（C₂₇重排甾烷/规则甾烷<0.1）、重排藿烷（diaC₃₀H/C₃₀H<0.05）和新藿烷（C₂₉Ts/C₂₉H=0.2~0.4）含量低和伽马蜡烷含量高（伽马蜡烷指数=0.4~0.8），这一系列特征与柴西地区发育的古近系-新近系咸水湖相烃源岩一致。但是，东坪地区原油呈现完全不同的生物标志物组合，主要表现为丰富的伽马蜡烷（伽马蜡烷指数=1.2~3.0）与高含量的新藿烷（C₂₉Ts/C₂₉H>1.5）、重排藿烷（diaC₃₀H/C₃₀H>0.7）和重排甾烷（C₂₇重排甾烷/规则甾烷>0.4）并存，这与地质样品中伽马蜡烷和重排类标志物的分布特征相矛盾，因为依据现有的认识无法解释这一特殊地球化学现象。生物标志物组成特征表明东坪地区原油所具有的特殊生物标志物组合并非源于不同类型原油之间的混合，而是一种客观存在，尽管其确切的地球化学意义和形成条件目前并不清楚。由此可见，东坪地区原油为柴达木盆地一个新的原油类型，推测其烃源岩可能形成于偏酸性的咸水环境。

A New Kind of Crude Oils and the Geochemical Characteristics in the Dongping Area,Qaidam Basin

Based on compositions of various biomarkers, the crude oils from northern Qaidam Basin have relatively higher Pr/Ph ratios (>2.0), moderate contents of rearranged steranes (diaC₂₇/reC₂₇=0.5-0.7), rearranged hopane (diaC₃₀H/C₃₀H=0.2-0.4) and neohopane (C₂₉Ts/C₂₉H=0.4-0.7), and lower gammacerane (gammacerane index<0.05), consistenting with depositional environments of Lower Jurassic source rocks. However, the crude oils from the north oilfields in western Qaidam Basin have the characteristics of lower Pr/Ph ratios (<0.8), lower contents of rearranged steranes (diaC₂₇/reC₂₇<0.1), rearranged hopane (diaC₃₀H/C₃₀H<0.05) and neohopane (C₂₉Ts/C₂₉H=0.2-0.4), and abundant gammacerane (gammacerane index=0.4-0.8), which may relate to saline source rocks. The crude oils from the Dongping area have a completely different biomarker assemblage from the two kinds of crude oils mentioned above in the study area, that is, abundant gammacerane (gammacerane index=1.2-3.0) coexists with high rearranged steranes (diaC₂₇/reC₂₇>0.4), rearranged hopane (diaC₃₀H/C₃₀H>0.7) and neohopane (C₂₉Ts/C₂₉H>1.5). It is very difficult to be explained by the distributions of gammacerane, rearranged steranes, rearranged hopanes and neohopanes in geological samples, because occurrence and formation of abundant gammacerane, rearranged biomarkers and neohopanes require completely different geological conditions. Furthermore, the relative compositions of various biomarkers in different crude oils from the study area suggest that this unusual biomarker assemblage in crude oils from the Dongping area is not from mixing of different crude oils, but an objective geological phenomenon, although its generation condition and geochemical significance remain unknown at present. Therefore, the crude oils having unusual biomarker assemblage are a new kind of crude oils in the Qaidam Basin, and their source rocks are probably deposited in an acid and saline environment.