Sequence stratigraphy of the Westphalian in the northern part of the

Abstract: A series of stacked fluvio-lacustrine sequences i the Westphalian ofthe northern part of the Southern North Sea can be correlated over areas of greater than 2500 km 2. Each sequence is similar in terms of thickness (100-300m) and the internal arrangement of lithofacies but they differ in colour and in the amount of sandstone and coal that they contain. A typical sequence consists of a and-rich lower unit, a mud-dominated middle unit which fines upwards towards its top and a mud-dominated upper unit which generally coarsens upwards. The main reservoir-prone intervals are confined to the lower-middle parts of sand-rich units and consist of coarse-grained braided or low-sinuousity channel deposits. These fluvial sandstones are laterally extensive and are neither typical of incised valley fills (in terms of stratal geometries) norof progradational systems tracts (in terms of facies distribution). Instead they appear to have been deposited uring periods of widespread fluvial aggradation caused by transgression as a result of relative lake-level rise. Each lower sand-rich unit is overlain by a dark mudstone with a high gamma ray expression which is interpreted as an initial (lake) flooding surface marking the onset of predominantly lacustrine and mire sedimentation associated with the development of th