The sedimentary model of the LU Sub-member of Cretaceous Napo Formation in Oriente Basin, Ecuador
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摘要:
厄瓜多尔 Oriente 盆地北部白垩系Napo组LU亚段地层油气资源极其丰富，具有较大的油气勘探潜力，由于目前对LU亚段地层沉积模式认识不清晰，极大程度上限制了研究区进一步勘探开发。依据LU亚段岩芯观察、测井、成熟度分析以及镜下薄片等资料，通过岩芯岩性、结构等岩石学特征，沉积构造、生物遗迹化石、沉积韵律等沉积学特征，以及测井相标志认为，研究区LU亚段地层发育于潮汐作用为主的海陆过渡环境，并在前人研究基础上提出潮控河口湾与陆棚之间存在过渡带沉积，形成了潮控河口湾-过渡带-陆棚沉积体系，识别出潮汐水道、潮汐砂坝、砂坪、海绿石砂席、混合坪、陆棚泥和灰质滩共7种沉积微相，其中，优势储层相带为潮汐水道和潮汐砂坝，砂坪次之。建立研究区潮控河口湾-过渡带-陆棚沉积模式，以期对Oriente盆地白垩系Napo组油气勘探提供有利的科学依据。
Abstract:The LU Sub-member (LUSM) of the Cretaceous Napo Formation in the northern part of Oriente Basin, Ecuador, is extremely rich in hydrocarbon resources and has a large potential for hydrocarbon exploration. However, the depositional pattern of the LUSM remains poorly understood, which had largely limited the further exploration and development of the area. Based on the core observation, logging, maturity analysis, and thin-section study, the petrological and sedimentary features were described; the biological relics and fossils, sedimentary structure and rhythm, and the logging phase signatures were analyzed. Results show that the LUSM strata were developed in a transitional environment between sea and land, in which tidal action was dominated. Referring to previous studies, we proposed that there was a transitional zone between tide-dominated estuary and a continental shelf, forming a tide-dominated estuary - transitional zone - continental shelf sedimentary system, showing seven sedimentary microfacies: tidal channel, tidal sand bar, sand flat, glauconite sand sheet, mixed flat, shelf mud, and calcareous beach. Among them, the tidal channel and tidal sand bar are sweet reservoir facies, followed by sand flat. By analyzing the depositional system, a tidal-dominated estuarine - transition zone - shelf depositional model was established for the study area. This study provided a favorable scientific basis for oil and gas exploration in the Cretaceous Napo Formation in the Oriente Basin.
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Key words:
- sedimentary model /
- sedimentary microfacies /
- tidal-dominated estuary /
- Oriente Basin
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表 1 Oriente盆地Napo组LU亚段岩相类型及成因解释
Table 1. Genetic interpretation in lithofacies types of the LU Sub-member of the Napo Formation in the Oriente Basin
岩相类型 沉积构造 生物特征 成因解释 大型交错层理中—粗砂岩（图5a—d） 以大型（板状、槽状、羽状）交错层理和泥披层为主，其中，羽状交错层理和泥披层是典型潮汐标志，碳质泥披层表现为韵律发育特征，指示具有一定陆源沉积注入 生物扰动指数0～1 由高能单向潮汐流和次要的河流逆流
产生含泥披层交错层理中—细砂岩（图5e） 高角度板状交错层理和泥披层较为常见 生物扰动指数0～1 砂质纹层为涨退潮期牵引流导致的沙丘迁移所形成，而泥披层则是在停潮期由水体中悬浮物缓慢沉降产生 脉状层理中—细砂岩（图5f） 脉状层理发育 生物扰动指数1～2 水动力条件强弱交替下形成 平行层理中—细砂岩（图5g） 以平行层理为主，泥披层不甚发育，通常与等岩相互层 生物扰动指数<1 高流态砂质沉积物迁移形成 波状层理砂泥岩互层（图5h） 发育波状层理，含有菱铁矿泥质层及黄铁矿结核，可见收缩裂缝 生物扰动指数1～2，发育Planolites、Lockeia、Thalassoides遗迹相 过渡带环境沉积介质的波浪振荡造成 块状泥质细—粉砂岩（图5i） 块状构造及波状交错层理，可见部分砂质纹层受生物破坏进入泥质纹层形成收缩裂缝等软沉积变形构造 生物扰动指数2～5，发育Planolites、Chondrites等多种遗迹相 形成于正常浪基面之下、物源供给不足的陆棚环境 波纹层理细砂岩
（图5j）为波纹层理，含有泥质薄层 生物扰动指数<1，可见Planolites、Thalassoides遗迹相 事件沉积 块状海绿石细砂岩
（图5k）块状构造及正向递变层理，具有底部冲
刷面生物扰动指数为3，含瓣鳃类钙质介壳、Planolites遗迹相 风暴浪基面与正常浪基面之间的弱氧化过渡带环境中 生物碎屑泥质细砂岩（图5l） 生物扰动指数<1，生物碎屑主要为破碎的软体动物瓣膜，可见Planolites、Thalassoides遗迹相 陆棚环境 块状含粉砂泥岩/页岩（图5m） 水平层理为主 可见钙质生物介壳，如瓣鳃类 悬浮状态的泥质颗粒缓慢沉积形成 块状泥质石灰岩
（图5n）生物扰动指数为2，含有丰富的化石遗迹，生物碎屑、软体动物瓣膜、植物碎屑、棘皮动物、有孔虫等 正常盐度的开阔海洋陆棚环境 
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