Heavy mineral sorting in downwards injected Palaeocene sandstone, Siri Canyon, Danish North Sea

摘要

Post-depositional remobilization and injection of sand are often seen in deep-water clastic systems and have been recently recognised as a significant modifier of deep-water sandstone geometry. Large scale injectite complexes have been interpreted from borehole data in the Palaeocene Siri Canyon near the Danish Central Graben of the North Sea hydrocarbon province. The emplacement of large scale injectite complexes has been commonly attributed to seismic activity and consequent sand liquefaction. However, due to very small differences in textural and compositional properties, and the lack of depositional structures in deep-water sandstones, the distinction between "in situ" and injected or remobilized sandstones is often ambiguous. Large scale heavy mineral sorting (in 10m thick units) is observed in several reservoir units in the Siri Canyon and has been interpreted to represent the depositional sorting. In this study we describe an example of effective shear-zone sorting of heavy minerals in a thin downwards injected sandstone dyke which was encountered in one of the cores in the Cecilie Field, Siri Canyon. Differences in sorting pattern of heavy minerals are suggested as a tool for petrographic/geochemical distinction between "in situ" sandstones and their related injectites, especially where primary sedimentary structures are removed by fluidization or minor remobilization.