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1 Statoil a.s., N-4035 Stavanger, Norway
2 Statoil a.s., N-4035 Stavanger, Norway
Edward Prestholm is a technical advisor in Statoil's petroleum technology division in Stavanger, Norway, where his work is mainly focused on development and quality control of sedimentologic and structural aspects of reservoir models. He received an M.Sc. degree in petroleum geology from the University of Bergen in 1984. His main research interests are within sedimentology and structural geology, including the geology of Svalbard where he has carried out extensive field work.Olav Walderhaug is employed in Statoil's exploration technology division in Stavanger, Norway, where diagenesis and reservoir quality prediction are his main areas of responsibility. Olav holds an M.Sc. degree in petroleum geology from the University of Bergen and a D.Sc. in sandstone diagenesis from the University of Oslo. His research interests are mainly within sandstone diagenesis and related topics, including the development of quantitative models of cementation, porosity evolution, and basin subsidence.
Distributary channel sandstones of the Lower Cretaceous Helvetiafjellet Formation and underlying prodelta shales and thin-bedded sandstones of the Upper Jurassic Janusfjellet Formation exposed along the east coast of Spitsbergen are cut by syndepositional planar and listric faults forming collapse scars with depths of 35-90 m and widths up to 1.5 km. The fault zones mostly have a dip of 40-60°, a width of up to 2.3 m, and contain fault-parallel 3-5-m-long overlapping sandstone sheets with widths of 5-40 cm, and up to 2-m-thick fault-parallel sandy mudstones. The intrafault sandstones show fault-parallel banding resulting from differences in detrital clay content and grain size. The banding has been enhanced by selective, late diagenetic quartz cementation of the clay-poor bands. Thin clay laminae, now developed into fault-parallel stylolites, occur along the margins of the intrafault sandstones. The clay laminae do not emerge from clay layers in the fault blocks and are not clay smears. The laminae probably formed during faulting when fluidization within the fault zones allowed clay particles to move laterally and accumulate along the margins of the fault zones. There is no enhanced cementation or cataclastic deformation within the fault zones. The ability of the fault zones to act as capillary seals or barriers to fluid flow is therefore mostly determined by the clay laminae rimming the intrafault sandstones.
Fractures filled by quartz and calcite cement containing oil inclusions that homogenize at 58-73°C probably were not produced by the syndepositional faulting and may have formed during uplift of the area.
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