Published:January 01, 2007
The Upper Jurassic Hareelv Formation in Jameson Land, East Greenland is one of the world’s finest outcrop examples of a giant sand-injection complex. The contrast between the black, organic-rich hemipelagic mudstones and the injected light yellow sandstones is striking and allows easy recognition of geometries both in close-up and from a distance. The formation is 200–400 m (660–1310 ft) thick, and in the lower part (Katedralen Member), the sandstone/mudstone ratio is roughly 1:1, increasing to about 9:1 in the upper part (Sjsllandselv Member).
All sands in the upper Oxfordian–Volgian Katedralen Member have undergone postburial remobilization and injection into the surrounding mudstones, and virtually all primary sedimentary structures have been obliterated. It is thus not possible to provide detailed interpretations of the primary depositional processes. On the basis of sand-body geometry and comparison with the undisturbed underlying Olympen and overlying Raukelv formations, the depositional system is interpreted as comprising slope gullies and laterally extensive base-of-slope lobes. The sandstones occur as thick, virtually structureless bodies, which may be laterally extensive or form mounded or pod-shaped masses. Smaller dikes and sills are ubiquitous, and their geometries range from orthogonal or polygonal to extremely irregular, reflecting injection into mudstones with various degrees of consolidation. Mudstone slabs and fragments of all sizes occur in the sandstones and may easily be mistaken for clasts transported in concentrated gravity flows. They are formed, however, by excavation and rip-down of the mudstone during forceful injection of fluidized sand.
Vertical or lateral organizational trends of sandstone bodies are not observed, and no clear indications exist if intrusion of dikes and sills were upward, downward, or lateral with respect to the larger sandstone bodies. The mudstones above large convex- upward sandstone bodies seem, however, to be relatively undisturbed by dikes and sills. Close inspection of some thick, laterally extensive sandstone bodies show that they contain subhorizontal mudstone leaves or layers, indicating long-distance lateral injection and splitting of the injected mudstone package. The thick convex-upward sandstone bodies were, however, clearly intruded vertically upward into the mudstones. No evidence is present for sand extrusion on the sea floor, and remobilization and injection clearly were postburial, probably under a cover of tens to perhaps hundreds of meters. Several generations of injection can be demonstrated based on crosscutting relationships of dikes and sills and the presence of both straight and strongly ptygmatically folded dikes at the same levels. Similar injections are unknown from both older and younger formations in Jameson Land.
The Hareelv Formation was deposited during the climax of the most important Mesozoic rift event in East Greenland, and the pervasive remobilization of all sands in the formation is interpreted as caused mainly by cyclic loading triggered by seismic shocks. Additional factors may have included slope shear stress, buildup of pore pressure caused by sediment loading, upward movement of pore waters expelled from the compacting muds, and possibly biogenic and thermogenic gas. The well-exposed Hareelv Formation is an excellent analog for subsurface hydrocarbon reservoirs, which have been modified by remobilization and injection of sands. It provides one of the best field examples known, illustrating the degree to which a sediment can be altered and all primary features destroyed by remobilization, fluidization, and injection.
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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.