Abstract The Cretaceous oil reservoir located in eastern slope belt of the Oriente basin accounts for most of the discoveries in last decade in Ecuador, which makes the Oriente basin one of the most important oil exploration targets in South America. Crude oil samples (n = 31) in 31 wells together with mudstone and limestone source rocks (n = 18) from Cretaceous Napo Formation in 16 wells were analyzed to evaluate the geochemical characteristics of the oil reservoir oil. Geochemical analyses included measurements of total organic carbon (TOC) content, Rock-Eval Pyrolysis, gas chromatography (GC), and gas chromatography-mass-spectrometry (GC-MS). All the oil samples have pre-uimodal n-alkane distribution dominated by lower molecular weight components (peak at C15–C17) with a small peak at C25–C30. The carbon preference index (CPI) and odd-over-even predominance (OEP) values range between 1.0 and 1.34, and 1.0–1.21 respectively; Pr/Ph ratios range from 0.67 to 1.16; high Pr/n-C17 (0.80–3.47) and Ph/n-C18 ratios (0.82–7.86) values and dominate C27 regular steranes, all these biomarkers show that the oil derived from marine algal material. Furthermore, all oils have similar Terpanes and Steranes distribution shape, peak at C23TT, and low at C22TT (TT = tricyclic terpanes), relative low C22/C21, C24/C23 and C26/C25 tricyclic terpane ratios, higher C27R (R = regular steranes) content than C29R and C28R. These biomarkers demonstrate that all the oils are from same genetic family although they came from different zones. There are two kinds of source rocks in the Oriente basin, shale and argillaceous bioclastic limestone of Napo Formation. These rocks are good-excellent mature according to TOC and Rock-Eval assessment. The two source rocks show obvious different Terpanes and Steranes distribution shape in mz191 and mz217 maps. The Oil-Source correlation analysis demonstrates that the crude oils have similar Terpane and Steranes distribution shape and biomarker value, especially the C27R–C28R–C29R ratio, with argillaceous bioclastic limestone of Napo Formation. This new geochemical data suggests that the argillaceous bioclastic limestone is the major source rock in the Oriente basin. Thus, distribution of this stratigraphic unit should be the key point for future exploration.

中文翻译：

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南美洲厄瓜多尔 Oriente 盆地原油地球化学特征及成因

摘要 厄连特盆地东部斜坡带白垩系油藏占厄瓜多尔近10年发现的大部分，使奥连特盆地成为南美洲最重要的石油勘探目标之一。对31口井原油样品（n=31）和16口白垩系Napo组泥岩和灰岩烃源岩（n=18）进行分析，评价油藏油的地球化学特征。地球化学分析包括测量总有机碳 (TOC) 含量、Rock-Eval 热解、气相色谱 (GC) 和气相色谱-质谱 (GC-MS)。所有的油样都具有由较低分子量组分（C15-C17 处的峰）主导的前 uimodal 正构烷烃分布，C25-C30 处有一个小峰。碳偏好指数（CPI）和奇偶优势（OEP）值分别介于 1.0 和 1.34 之间，以及 1.0 到 1.21 之间；Pr/Ph 比率范围为 0.67 至 1.16；Pr/n-C17 (0.80–3.47) 和 Ph/n-C18 比值 (0.82–7.86) 值高，并在 C27 常规甾烷中占主导地位，所有这些生物标志物都表明油来自海藻材料。此外，所有油都具有相似的萜烷和甾烷分布形状，在 C23TT 处达到峰值，在 C22TT 处达到最低（TT = 三环萜烷），C22/C21、C24/C23 和 C26/C25 三环萜烷比率相对较低，C27R（R = 常规甾烷）含量高于 C29R 和 C28R。这些生物标志物表明所有的油都来自同一基因家族，尽管它们来自不同的区域。奥连特盆地有两种烃源岩，即那坡组页岩和泥质生物碎屑灰岩。根据 TOC 和 Rock-Eval 评估，这些岩石的成熟度为“优”。两种烃源岩在 mz191 和 mz217 图中显示出明显不同的萜烷和甾烷分布形状。油源相关性分析表明，原油具有相似的萜烷和甾烷分布形状和生物标志物值，尤其是 C27R-C28R-C29R 比率，与 Napo 组泥质生物碎屑灰岩。这一新的地球化学数据表明，泥质生物碎屑灰岩是 Oriente 盆地的主要烃源岩。因此，该地层单元的分布应是今后勘探的重点。油源相关分析表明，原油具有相似的萜烷和甾烷分布形状和生物标志物值，尤其是C27R-C28R-C29R比率，与Napo组泥质生物碎屑灰岩。这一新的地球化学数据表明，泥质生物碎屑灰岩是 Oriente 盆地的主要烃源岩。因此，该地层单元的分布应是今后勘探的重点。油源相关性分析表明，原油具有相似的萜烷和甾烷分布形状和生物标志物值，尤其是 C27R-C28R-C29R 比率，与 Napo 组泥质生物碎屑灰岩。这一新的地球化学数据表明，泥质生物碎屑灰岩是 Oriente 盆地的主要烃源岩。因此，该地层单元的分布应是今后勘探的重点。