Sandstone in Lower Cretaceous Helvetiafjellet Formation, Svalbard: Bearing on Reservoir Potential of Barents Shelf

Abstract

The Helvetiafjellet Formation consists of 50 to 100 m of largely fluvial sandstones and shales deposited throughout Svalbard during a major regression. Sandstones consist chiefly of monocrystalline and polycrystalline quartz, feldspar, chert, and volcanic (basaltic) rock fragments in (1) quartz and quartzose arenites, (2) quartzose lithic arenites, and (3) volcanic arenites. Quartz and quartzose arenites dominate in western Spitsbergen and were derived from a mixed sedimentary-metamorphic terrane in the west. Volcanic arenites are present in Kong Karls Land in the east, and are intercalated with basaltic lava flows which are considered to be the major source for the sandstones. Quartzose lithic arenites in southeast Spitsbergen are a mixture of quartz sands derived from the west and reworked volcanic sands transported from the east. Cementing minerals are variably developed in these sandstones. Quartz overgrowths are dominant in quartz and quartzose arenites. Kaolinite is present in many feldspar-bearing quartz and quartzose arenites, whereas illite is rare. Chlorite is common, together with calcite, in quartzose lithic arenites. Abundant smectite and calcite with minor zeolites are present in the volcanic arenites, which suggests a strong relation between mineralogy of the detrital grains and that of associated cements. Unstable detrital grains released ions to pore waters, which subsequently precipitated clay and calcite cements. Induration increases and maximum porosity decreases from east to west across Svalbard, owing both to higher temperature, caused by progressively greater burial depth and steeper geothe mal gradients, and to tectonic stress in westernmost areas. Early Cretaceous volcanic activity may have provided abundant chemically unstable detritus to basins in the southern part of the Barents Shelf. As a major consequence, clay cements could form during diagenesis, adversely affecting sandstone reservoir parameters. However, the development of secondary dissolution porosity could improve reservoir characteristics of those sandstones which contain chemically unstable minerals such as feldspar grains and calcite cement.