Criteria for interpreting carbonate platform drowning histories and resultant diagenetic fabrics : insights from actively subsiding platform margins

碳酸盐岩台地形成了一些最具经济价值的油气藏。虽然碳酸盐产量和台地沉积相的组成主要受海洋环境参数的控制，但更大规模的台地发展模式（和台地内部结构）最强烈地受到海平面上升和盆地沉降或隆升之间的相互作用的影响。对储层潜力特别重要的是平台所经历的沉降状态和淹没历史。当相对海平面上升的速度超过垂直沉积物堆积的速度时，就会发生平台溺水。这对储层潜力起着重要的控制作用，因为它会影响到成岩封印的位置和范围，以及平台内部和顶部的渗透率屏障。这些封闭性和渗透性屏障是由连续的成岩和地层蚀变循环产生的，因为平台逐渐被淹没和/或受到地面暴露-通常它们是前阶段成岩蚀变的连续复印的结果。由于缺乏对积极沉降的平台边缘进行详细的成岩研究，阐明平台淹没表面的成岩历史并利用这些信息来解释岩石记录中的平台淹没事件尤其困难——这在很大程度上是由于“最近”的平台边缘沉积物难以接近。淹没地表开发和淹没历史问题与CASE合作伙伴（BG Group）经营的几个碳酸盐岩储层利益直接相关，特别是目前在哈萨克斯坦的Karachaganak油田，以及许多其他碳酸盐岩平台储层。Karachaganak气田是哈萨克斯坦西北部的一个“超大型”凝析气田，估计储量为123亿桶石油和57万亿立方英尺天然气。溺水面至关重要，因为它们是流体流动的垂直屏障，尽管最近的开发井表明这些屏障在整个平台上并不有效。因此，了解这些淹没表面的空间变化对作业者来说是一个关键的地下挑战，因为它对旨在保持油藏压力支撑的注气系统的设计至关重要。因此，由于缺乏对溺水地表成岩作用的适当模拟工作，以及可能的空间范围，严重限制了作业者解释溺水历史或模拟垂直渗透率模式的能力。该项目的目的是利用从夏威夷和休恩湾附近连续淹没平台中收集的独特样本，来研究逐渐淹没对平台成岩作用的影响。Huon Gulf层序包括9个平台，保存在水深239米至2393米之间，年龄范围为~60至~450 ka。在夏威夷周围，已经绘制了12个平台并进行了采样。这些地层分布深度为~125米至1400米，跨度为~15至500 ka。该学生将利用这一独特的数据集来研究与渐进式平台淹没相关的成岩和埋藏结构演变的时间变化，特别关注海洋胶结和埋藏变化（生物侵蚀、结壳、溶解）的演变和连续的过度印刷效应。该学生还将在哈萨克斯坦进行现场工作（与CASE主管合作），检查Karachaganak油田的岩心、电缆测井和其他与淹水面相关的数据，以检查BG运营的一个油田的平台淹水面的岩石学和特征。将在夏威夷和休恩湾的发现的背景下分析这些核心层的样本，以便根据先前制定的标准改进对溺水历史的解释。

Carbonate platforms form some of the most economically important hydrocarbon reservoirs. Whilst carbonate production rates and the composition of platform sedimentary facies are primary controlled by marine environmental parameters, larger-scale patterns of platform development (and internal platform architecture) are most strongly influenced by the interplay between eustatic sea-level change and basinal subsidence or uplift. Particularly important to reservoir potential is the subsidence regime and drowning history experienced by a platform. Platform drowning occurs when rates of relative sea-level rise exceed rates of vertical sediment accumulation. This acts as a major control on reservoir potential by influencing the location and extent of diagenetic seals and permeability barriers within and on top of platform surfaces. These seals and permeability barriers are produced by successive cycles of diagenetic and taphonomic alteration as platforms are progressively drowned and/or subject to subaerial exposure- typically they are the result of sequential over-printing of previous phases of diagenetic alteration. Unravelling the diagenetic history of platform drowning surfaces and using this information to interpret platform drowning events in the rock record is particularly difficult due to the paucity of detailed diagenetic studies from actively subsiding platform margins - this is largely a function of the inaccessibility of 'recent' platform margin deposits. The issue of drowning surface development and drowning histories has direct relevance to several carbonate reservoir interests operated by the CASE partner (BG Group), most especially at the present time within the Karachaganak Field in Kazakhstan, as well as to many other carbonate platform reservoir deposits. The Karachaganak Field is a 'supergiant' gas condensate field in NW Kazakhstan with estimated reserves of 12.3 billion barrels of oil and 57 trillion cubic feet of gas. Drowning surfaces are critically important because they act as vertical barriers to fluid flow, although recent development wells have indicated that these barriers are not effective over the entire platform. Understanding spatial variations in these drowning surfaces is thus a key subsurface challenge for the operators, since it crucially impacts the design of a gas injection system designed to maintain reservoir pressure support. The paucity of appropriate analogue work on drowning surface diagenesis and likely spatial extent thus severely restricts the operator's ability to interpret drowning histories or to model vertical permeability patterns. The aim of this project is to utilise the unique sample collections recovered from successive drowned platforms around Hawaii and the Huon Gulf to examine the effects of progressive drowning on platform diagenesis. The Huon Gulf sequence comprises nine platforms, preserved across water depths of 239 m to 2,393 m and spans an age range from ~60 to ~450 ka. Around Hawaii twelve platforms have been mapped and sampled. These occur across depths from ~125 m to 1,400 m and span an age of ~15 to 500 ka. The student will utilise this unique dataset to examine temporal variations in the evolution of diagenetic and taphonomic fabrics associated with progressive platform drowning, focusing specifically on the evolution and successive over-printing effects of marine cementation and taphonomic alteration (bioerosion, encrustation, dissolution). The student will also undertake fieldwork in Kazakhstan (working with the CASE supervisor) to examine core, wireline log and other data related to the drowning surfaces in the Karachaganak Field in order to examine the petrography and character of platform drowning surfaces in one of the BG operated fields. Samples from these core horizons will be analysed in the context of the findings from Hawaii and the Huon Gulf in order to improve interpretation of the drowning history based on the previously developed criteria.