Chemostratigraphy of upper Jurassic reservoir sandstones, Danish Central Graben, North Sea

摘要

A chemostratigraphic study of Upper Jurassic sandstones in the northern Danish Central Graben has been undertaken within the framework of a well-defined stratigraphic/sedimentological model based particularly on cored well sections. Two reservoir sandstone units are recognised, the transgressive marginal marine to shoreface sandstone of the Gert Member and the regressive to transgressive shoreface sandstone of the Ravn Member. Both members belong to the Heno Formation, which is equivalent to the Fulmar Formation (UK) and the Ula Formation (Norway). Multivariate analysis of geochemical data from 264 core samples from 8 wells reveals the distinction between the two reservoir sandstones (Gert and Ravn members) and the two offshore claystones (Far-sund and Lola formations). Specific elements have proven to be important for this separation and these elements demonstrate differences even in 2-dimensional cross plots. The Farsund Formation is characterised by higher V, U and P2O5, and lower MgO and K2O when compared with the Lola Formation. The Gert Member typically has higher maximum amounts of Cr and TiO2 than the Ravn Member. The high Cr and TiO2 content (probably from chrome spinel and Ti-minerals) might be related to a source of exposed Carboniferous sediments in the Gert Ridge area. The Ravn Member is characterised by higher Na2O, P2O5 and Th contents than the Gert Member, which may reflect a higher content of plagioclase (Na2O) and a changed heavy mineral assemblage. The Mid North Sea High is a likely source for the heavy mineral suite that characterises the Ravn Member. The Rita-1 well, situated closest to the Mid North Sea High, seems to have been more influenced by this source than the other wells. In the Hejre area, a volcanic source supplying K-feldspar may be responsible for the relatively high K2O/Al2O3 observed in both the Gert and Ravn members. Hence in addition to differentiation between the two reservoir sands and between the two offshore claystones, this study also illustrates the use of geochemical data for evaluation of source characteristics and dominance of different sediment source areas.