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经济上最重要的油气藏。虽然碳酸盐生产率和台地沉积相的组成主要受海洋环境参数的控制，但台地发展（和内部台地结构）的大规模模式受海平面变化和盆地沉降或隆起之间的相互作用的影响最大。对储层潜力特别重要的是平台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积累的速度时，就会发生平台淹没。这通过影响台地表面内部和顶部的成岩密封和渗透性屏障的位置和范围，对储层潜力起到主要控制作用。这些密封和渗透屏障是由连续的成岩和埋藏蚀变循环产生的，因为台地逐渐被淹没和/或受到陆上暴露-通常它们是成岩蚀变先前阶段连续叠加的结果。由于缺乏对活跃沉降的平台边缘的详细成岩研究，解开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是特别困难的-这在很大程度上是“最近”平台边缘沉积物不可访问的功能。淹没地表开发和淹没历史的问题与CASE合作伙伴（BG集团）运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台地储层沉积。Karachaganak油田是哈萨克斯坦西北部的一个“超级巨型”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。淹没面是非常重要的，因为它们是流体流动的垂直屏障，尽管最近的开发威尔斯表明这些屏障在整个平台上并不有效。因此，了解这些淹没面的空间变化是运营商面临的一个关键地下挑战，因为它对设计用于维持储层压力支持的注气系统的设计产生了至关重要的影响。因此，缺乏适当的模拟工作淹没表面成岩作用和可能的空间范围，严重限制了运营商的能力，解释溺水的历史或模拟垂直渗透率模式。该项目的目的是利用从夏威夷和休恩湾周围连续淹没的平台中回收的独特样品收集来研究逐步淹没对平台成岩作用的影响。休恩湾层序由9个平台组成，水深239米至2,393米，年龄范围为60至450万年。在夏威夷周围，已经绘制了12个台地的地图并进行了取样。这些现象发生在约125 m至1，400 m的深度范围内，年龄跨度约为15至500 ka。学生将利用这一独特的数据集来研究与渐进式平台溺水相关的成岩和埋藏构造演化的时间变化，特别关注海洋胶结作用和埋藏蚀变（生物侵蚀，结壳，溶解）的演化和连续叠印效应。学生还将在哈萨克斯坦进行实地考察（与CASE主管一起工作），以检查与Karachaganak油田溺水表面相关的岩心，电缆测井和其他数据，以检查BG操作油田之一的平台溺水表面的岩相学和特征。将根据夏威夷和休恩湾的调查结果分析这些岩心层位的样本，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.