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Erik Hammer, Norwegian University of Science and Technology (Norway)
Mai Britt Mørk, Norwegian University of Science and Technology (Norway)
Arve Næss, StatoilHydro (Norway)
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Large correlation difficulties are encountered in heterolithic fluvial-deltaic reservoirs, and understanding the distribution of sedimentary facies and diagenesis can help improving the correlatability of reservoir units and contribute to more optimal drainage solutions. The Upper Triassic - Lower Jurassic Åre Formation in the Heidrun Field offshore mid Norway comprises heterolithic fluvial and delta plain sediments. Sandstone diagenesis has been investigated to examine possible sedimentary facies controls on the distribution of cementation and compaction. Three phases of burial are interpreted in this area with the most rapid burial of 1?1.5 km from Late Pliocene. This study has used a combination of wireline log interpretation, core study, and petrography and SEM/microprobe analyses of samples from 3 wells at approximately 2.5 km burial depths. In spite of the considerable burial, porosity values from modal analyses in the non-cemented samples are generally high both in fluvial channel and sandy bayfill facies. The diagenetic minerals that have most significant influence on reservoir quality are kaolinite, pore-filling calcite and siderite, in particular in the finer-grained facies. Authigenic K-feldspar is rare, and pyrite nodules are locally present. The carbonate minerals show complex compositional and microstructural variation, including three appearances of siderite (Sid1-3), and in addition ankerite/Fe-dolomite and calcite. Sid1 of pure composition occurs as irregular, corroded grains and is commonly enclosed in Mg-Ca-richer siderite (Sid3) and ankerite or calcite cemented intervals. Sid2, also pure, has precipitated locally in organic rich clay bands in the bayfill facies, and in the upper part of fluvial channel facies. Rhomb shaped crystals of Sid3 typically shows internal zoning defined by outward increasing Mg/Fe contents. Sid3 appears as pore-fill and as replacive cement in all facies. The formation of Sid1 and Sid2 cement predated compaction bending of mica grains and is interpreted as eogenetic. The latest phase of carbonate cementation is defined by pore-filling calcite and ankerite which postdated authigenic growth of kaolinite and compaction bending of mica. A preliminary conclusion is that siderite cementation is favoured in the finer-grained coal-bearing sandstones and siltstones in all the different facies. The massively calcite cemented bed (1-2 m-thick) identified in several wells within the bayfill facies is thought to be related to a flooding/deepening event. We suggest that the eogenetic siderite cementation would influence differential compaction by reducing the compactability of the finer-grained sediments during later burial diagenesis. The impact of identifying flooding events and early stage cementation has significant application in correlation and de-compaction studies of fluviodeltaic successions as observed in the Åre Formation offshore mid Norway.
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