Stratigraphic organization of carbonate ramps and organic-rich intrashelf basins: Natih Formation (middle Cretaceous) of northern Oman
Stratigraphic organization of carbonate ramps and organic-rich intrashelf basins: Natih Formation (middle Cretaceous) of northern Oman
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碳酸盐坡道和富含有机物的陆架盆地的地层组织：阿曼北部的 Natih 组（白垩纪中期）
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发表时间：
2002
期刊：
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通讯作者：
J. Philip
中科院分区：
文献类型：
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作者：
F. Buchem;P. Razin;P. Homewood;W. Oterdoom;J. Philip
中东白垩纪中期碳酸盐矿床是世界上产油量最高的地层层段之一，包含许多巨型油田，例如阿拉伯联合酋长国（Mauddud 和 Mishrif 地层）、伊朗（Sarvak 地层）和阿曼（Natih 地层）。造成这种碳氢化合物集中的主要原因之一是地质因素：由于富含有机质的陆架盆地的反复形成，在同一沉积序列中储集相和烃源岩共存。阿曼 Natih 组的高分辨率层序地层研究证实了这一点，该研究显示了储层、烃源岩和密封相的分布和几何形状的独特且可预测的模式。本文提出的层序地层模型可为中东地区的时间当量矿床提供参考。沉积学分析表明，Natih组是由两种沉积体系交替形成的：（1）以底栖有孔虫为主的平层碳酸盐-粘土混合坡道；（2）与陆架内盆地接壤的以碳酸盐为主的坡道，中坡道环境中存在丰富的铁质沉积物和富含有机质的盆地相。区分了三个完全发育的三级层序，显示了沉积体系的相似演化，底部是碳酸盐-粘土混合坡道系统，上部是碳酸盐主导的坡道系统。然而，这种模式会发生变化，具体取决于海平面稳定、环境因素和构造作用的相对影响。晚阿尔叠纪近塞诺曼阶序列 I 显示了从混合平坦斜坡到碳酸盐为主的斜坡和富含有机物的陆架盆地（第 22 页开始）的演化，沉积主要受海平面上升控制。在中塞诺曼阶II层序中，也观察到从混合缓坡向碳酸盐缓坡的演化，但研究区未发育陆架内盆地地形。这可能是由于粘土的大量涌入影响了该序列中的环境，抑制了碳酸盐的产生，可能与缺乏足够的住宿空间有关。层序 III 的塞诺曼晚期近土伦阶海侵部分显示出与层序 I 中观察到的类似的演化，并发育了富含有机质的内陆盆地。然而，在高位期间，观察到构造控制的沉积模式，伴随着强迫海退楔的发展（由于前陆盆地的弯曲隆起）。 Natih 组三级沉积层序海侵部分曾两次发生陆架内盆地形成。我们的研究表明，这主要是不同沉积速率的结果，即碳酸盐沉积系统本身响应相对海平面（快速）上升的动态，可能是海平面上升的原因。构造作用只是盆地地形形成过程中的一个次要因素，可能是通过形成小的初始地貌来实现的。有机物的积累不仅是由于形成足够深的水柱以保证其保存的无氧条件的结果。阿尔布阶晚期和土伦阶晚期塞诺曼阶也是全球范围内普遍有利于高有机质生产力的时期。这里提出的三级层序的时间线和地层结构在阿拉伯板块的尺度上具有应用潜力。我们的模型预测了一般的沉积模式，但可能会因不同的当地条件而发生修改。
The middle Cretaceous carbonate deposits in the Middle East are among the most productive oil-bearing stratigraphic intervals in the world, containing numerous giant fields in, for instance, the United Arab Emirates (Mauddud and Mishrif formations), Iran (Sarvak Formation), and Oman (Natih Formation). One of the main reasons for this concentration of hydrocarbons is a geological factor: the coexistence of both reservoir facies and source rocks in the same depositional sequences due to the repeated creation of organic-rich intrashelf basins. This is demonstrated in a high-resolution sequence stratigraphic study of the Natih Formation in Oman, which shows distinct and predictive patterns in the distribution and geometries of reservoir, source rock, and seal facies. The sequence stratigraphic model presented here may serve as a reference for time-equivalent deposits in the Middle East. The sedimentological analysis showed that the Natih Formation was formed by the alternation of two types of depositional systems: (1) a flat-bedded, mixed carbonate-clay ramp, dominated by benthic foraminifera, and (2) a carbonate-dominated ramp bordering an intrashelf basin, with abundant rudists in the mid-ramp environment and organic-rich basinal facies. Three fully developed third-order sequences are distinguished, showing a similar evolution of the depositional system, with a mixed carbonate-clay ramp system at the base, followed by a carbonate-dominated ramp system in the upper part. Variations occur on this pattern, however, depending on the relative influence of eustasy, environmental factors, and tectonism. The late Albiannearly Cenomanian sequence I shows an evolution from a mixed, flat ramp to a carbonate-dominated ramp and organic-rich intrashelf (Begin page 22) basin, and sedimentation is predominantly controlled by eustatic sea level. In the middle Cenomanian sequence II, the evolution from a mixed ramp to a carbonate ramp is also observed, but no intrashelf basin topography was developed in the studied area. This may be due to the high influx of clay that influenced the environment in this sequence, inhibiting the carbonate production, probably in combination with the lack of sufficient creation of accommodation space. The late Cenomaniannearly Turonian transgressive part of sequence III shows a similar evolution to that observed in sequence I, with the development of an organic-rich intrashelf basin. During highstand, however, a tectonically controlled sedimentation pattern is observed, with the development of forced regressive wedges (due to the flexural bulge of the foreland basin). Intrashelf basin formation occurred twice in the transgressive part of the third-order depositional sequences of the Natih Formation. Our study shows that this is mainly the result of differential sedimentation rates, that is, the dynamics of the carbonate sedimentary system itself in response to (rapid) rises in relative sea level, probably of eustatic origin. Tectonism was only a minor factor in the creation of the basin topography, possibly through the creation of small initial relief. The accumulation of the organic matter is not only a result of the creation of a sufficiently deep-water column to guarantee dysaerobic conditions for its preservation. The late Albian and late Cenomaniannearly Turonian were also periods of generally favorable conditions worldwide for high organic matter productivity. The time lines and stratigraphic architecture of the third-order sequences presented here have an application potential at the scale of the Arabian plate. The general sedimentation pattern is predicted by our model, but modifications due to different local conditions are likely to occur.