鄂尔多斯盆地西南部长8段砂岩沥青地球化学特征及其来源分析

Geochemical Characteristics and Oil-source Correlation of Bitumen in Sandstone of Chang-8 Member in Southwestern Ordos Basin

系统采集了鄂尔多斯盆地西南部地区延长组8段沥青砂岩,对砂岩沥青的地球化学特征进行了研究,探讨了沥青的母质特征、形成环境和成熟度。研究结果表明:(1)长8段砂岩沥青中高碳数的正构烷烃优势明显,规则甾烷中C_(29)含量相对较高,萜烷系列中四环萜烷含量较高,C_(24)四环萜烷/(C_(24)四环萜烷+C_(26)三环萜烷)的值为0.26~0.52,C_(23)三环萜烷/(C_(23)三环萜烷+C_(30)藿烷)的值分布范围为0.06~0.41,这些都指示研究区长8段砂岩沥青母质来源为混合型,并且陆源有机质贡献较大;(2)砂岩沥青中Pr/Ph为0.67~1.07,同时,沥青中检测出低丰度的β胡萝卜烷,说明沥青形成环境为弱氧化—弱还原环境,较低丰度的伽马蜡烷与C_(30)+藿烷指示沥青的母质形成于淡水或微咸水湖泊环境;(3)长8段砂岩沥青的C31藿烷22S/(22S+22R)、C_(29)甾烷20S/(20S+20R)、C_(29)甾烷ββ/(αα+ββ)和C_(29)重排甾烷20S/(20S+20R)均接近或已经达到平衡值,反映沥青为成熟沥青。将研究结果与已报道潜在烃源岩的生物标志化合物特征进行对比,显示出研究区长8段砂岩沥青主要来源于延长组7段烃源岩,表明长7段烃源岩是研究区最主要的烃源岩之一。

21 bituminous sandstone samples of Chang-8 member in southwestern Orders Basin were collected, and the biomarkers of bitumen were detected by means of GC-MS. The compositional and distributional characteristics of biomarker compounds of bitumen samples were studied and the origin of bitumen is discussed. The result shows that： （1） The relative content of tetracyclic terpane to hopanes and C29 regular steranes to C27 are high, C24 tetracyclic terpane/（C24 tetracyclic terpane ＋C26 tricyclic terpane） ranges from 0.26 to 0.52, C23 tricyclic terpane/（C23 tricyclic terpane＋Caj hopanes） ranges from 0.06 to 0.41, these data indicate that their organic parent materials are mixed ones and the higher land plants make a great contribution to the oil generation. （2） The low-content of β -carotane, gammacerane and C30 ＋hopanes, and that the ratio of Pr/Ph is 0.67- 1.07 , indicate that the oil was formed under the weak oxidizing to weak reducing freshwater or brackish-water lacustrine environment.⑶ Several ratios, including C31 hopanes 22S/（22S＋22R）,Ca sterane 20S/（20S＋20R）,C29 sterane ββ/（αα＋ββ） and C29 diasterane 20S/（20S＋20R）, completely or almost completely reached such equilibrium value that it indicates the studied bitumens are mature. The results of oil -source rock correlation show that the geochemical characteristics of bitumen in sandstone of Chang -8 Member from southwest area of Ordos Basin are similar to those of the upper Triassic Chang-7 subsection, so it is proposed that the bitumen originated mainly from the Chang-7 source rock.