Mounded Structures of Injected Sandstones in Deep-marine Paleocene Reservoirs, Cecilie Field, Denmark
Published:January 01, 2007
L. Hamberg, A.-M. Jepsen, N. Ter Borch, G. Dam, M.K. Engkilde, J.B. Svendsen, 2007. "Mounded Structures of Injected Sandstones in Deep-marine Paleocene Reservoirs, Cecilie Field, Denmark", Sand Injectites: Implications for Hydrocarbon Exploration and Production, Andrew Hurst, Joseph Cartwright
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The Cecilie field is located at the mouth of the Paleocene Siri Canyon near the Danish Central Graben. The field comprises a stratified succession of lower to upper Paleocene deep-marine sandstones and hemipelagic mudstones. Part of the field is occupied by a remarkable mounded structure, circular in planform and approximately 600 m (1968 ft) in diameter. Continuous coring through the mound and in the surrounding undisturbed sections provides significant data for the interpretation of this mounded feature. The Cecilie Mound gained its present shape by differential compaction over a dome-shaped injection of sand. In cores, the domeshaped injection is seen to have eroded and removed 25–30 m (82–98 ft) of the original stratigraphy preserved only outside the mound. Excavation of this attic chamber resembles magmatic stoping and occurred primarily by sand injection, splitting the overlying, semiconsolidated mudstones, so that clasts were spalled off. This resulted in the formation of common mudstone-clast conglomerates, which, along with flow structures formed by density segregation of the glauconitic sands, are important for the differentiation of injected and in-situ depositional sandstones. The close relationship of the Cecilie Mound to deep-seated faults in the Chalk Group indicates that initial fluidization occurred in response to the reactivation of the graben faults, breaching the overburden and releasing the overpressured pore waters of the Paleocene sands. Mounded injections like the Cecilie Mound have the potential to form hydrocarbon traps, and during production, the crosscutting nature of the mounded injection has ensured good vertical and lateral hydraulic communication in the otherwise depositionally stratified reservoirs.
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Sand Injectites: Implications for Hydrocarbon Exploration and Production
Sand injectites are described in scientific literature as an increasingly common occurrence in hydrocarbon reservoirs, in particular in deep-water clastic systems, where they are known to influence reserves distribution and recovery. Seismically-detectable injected sand bodies constitute targets for exploration and development wells and, subseismic sand bodies provide excellent intra-reservoir flow units that create field-wide vertical communication through depositionally extensive, low-permeability units. As sand injectites form permeable conduits in otherwise low-permeability units they facilitate the expulsion of basinal fluids; hence they act both as a seal risk and mitigate timing and rate of hydrocarbon migration. Injected sand bodies form intrusive traps, which are distinct from structural or stratigraphic traps. Included in this publication are 10 chapters on subsurface examination of sand injectites, 1 chapter on theoretical considerations, and 13 outcrop analogs in reservoirs across the world. Captured in this volume is at least a taste of the global and stratigraphic distribution of sand injectites, and an attempt to introduce readers to sand injectites and their significance in the context of hydrocarbon exploration and production. The book is not intended as a complete review of the field-based literature, but emphasizes high quality case studies from the surface and subsurface. The geographic scope of the book is large, and illustrates the diversity of geological settings in which these fascinating and economically significant features are found.