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

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.