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Abstract

Rumaila is one of the world’s supergiant oil fields. At the end of 2009, BP entered into a Technical Service Contract as the lead contractor with the South Oil Company of Iraq (SOC) and PetroChina to develop the remaining resource. The Early Cretaceous Main Pay is one of the largest reservoirs in the field. A rich dataset has recently become available including access to nearly 1 km of core, wireline image logs, open hole logs, formation pressure tests and cased hole saturation logs. Biostratigraphic analysis of the core indicates that most samples contain both marine and terrestrial mircoflora, confirming a marginal marine gross depositional environment. Palynology has proved to be of particular value, with diverse and abundant recovery of algal and dinoflagellate cysts from fine grained deposits allowing for a high resolution bio-chronostratigraphic model to be built. A set of marginal marine genetic elements was interpreted through the integration of core sedimentology and palynofacies. Two fourth-order regressive-transgressive cycles are identified in the Main Pay and overlying Upper Shale Member with additional higher-order cycles recognised, especially in the north of the field. The cycles highlight the phased advance and retreat of a river-dominated and tidally influenced delta system. The observation of cyclic bundles of thin foreset shale laminae confirms the influence of tides. Cores from the south of the field are dominated by fine to medium grained cross-bedded sandstones that were mainly deposited in lower delta plain distributary channels. Towards the north of the field, grain sizes tend to reduce and more heterolithic mixed sand and shale deposits are preserved. In addition to distributary channels; mouth bar, shoreline and tidal flat deposits are present in the north, suggesting a more basinward setting. Non-reservoir elements in the Main Pay are dominated by prodelta “fluid” mudstones picked primarily based on their palynological assemblages. These elements typically overlie a transgressive deposit and flooding surface and mark the early phase of deltaic advance. These deposits tend to be laterally extensive and form the foundation for the stratigraphic description. The stratigraphic and depositional descriptions have been extended away from cored wells using well logs, formation pressure tests and fluid saturation data. The current description suggests the potential for targeted development of bypassed oil beneath flooding shales and in the mouth bar, shoreline and tidal flat genetic elements.

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/content/papers/10.3997/2214-4609.20131455
2013-09-15
2021-10-26
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