柴达木盆地原油地球化学特征及其源岩时代判识

在系统分析柴达木盆地北缘和西部各油田60余个原油样品轻烃、饱和烃和芳烃组成的基础上,全面揭示了两地区原油的标志性地球化学特征;并结合源岩分析资料,应用断代生物标志物建立了识别原油源岩时代的标志。研究结果表明,北缘各油田原油Mango轻烃参数K_1值波动在1.1上下,富含甲基环已烷和甲苯;正烷烃呈奇偶优势分布,姥鲛烷优势显著;反映侏罗系淡水湖沼相沉积有机质特征。西部原油K_1值大多在1.2以上,轻烃中富含异构支链化合物;正烷烃系列呈奇碳优势(C_(11)～C(17))和偶碳优势(C_(18)～C_(28))双重分布模式,强植烷优势;C_(28)甾烷相对含量高(>30％);脱羟基维生素E系列化合物丰富,5,7,8-三甲基-/8-甲基-MTTC比值大都低于10;表征古近系—新近系咸水湖相有机质性质。奥利烷和C_(26)降胆甾烷是区分侏罗系和古近系—新近系油源油的有效断代生物标志物。侏罗系原油无奥利烷,24-/(24-+27-)降胆甾烷比值小于0.25;古近系—新近系原油含有奥利烷,24-/(24-+27-)降胆甾烷比值高于0.6。

Geochemical Characteristics and Source Rock Age Identification of Crude Oils in the Qaidam Basin, Northwest China

Based on the systematic analyses of light hydrocarbon, saturate and aromatic fractions of over 60 oil samples collected from oil fields in the northern and western Qaidam basin, the geochemical characteristics of the oils in the two regions was studied. Based on the calibrating of analytical data of source rocks, identification parameters on age-specific biomarkers for oil source rock age were put forward. The research results show that the oils in northern margin have K1 (Mango's light hydrocarbon parameter) values of around 1. 1, abundant methylcyclohexane and toluene, odd over even w-alkane preference, high Pr/Ph ratios, reflecting the feature of organic matter deposited in Jurassic freshwater lacustrine facies. Whereas, the oils in the western region are characterized by K1 values of almost over 1.2, high abundance of isoalkanes in light hydrocarbons, bimodal n-alkane distribution with odd-to-even (C11~C17) and even-to-odd (C18~C28) predominance, low Pr/Ph, high relative concentration (>30%) of C28sterane, abundant alkyl-chromans (MTTC) with 5, 7, 8-trimethyl-/8-methyl-MTTC ratios<10, indicating the signature of Tertiary saline lacustrine organic matter. Oleanane and norcholestane are believed to be useful age-specific biomarkers for differentiating the oils derived from Jurassic and Tertiary source rocks. Tertiary oils are confirmed by the appearance of oleanane and high 24-/24-+27-norcholestane ratios>0. 6. Jurassic oils lack oleanane and have low 24-/24-+27- norcholestane ratios (<0. 25).