Characterisation and correlation of the Triassic Mudstones in the Central North Sea

The Triassic of the Central North Sea is a continental succession that contains highly prolific hydrocarbon bearing fluvial sandstone reservoirs, partitioned by thick mudstones. Within the Triassic succession the mudstones act as top seals and bottom seals to hydrocarbon accumulations in the Judy, Joanne and Josephine Sandstone Members. This study integrates sedimentological, petrophysical and geophysical data to characterise the Julius Mudstone Member and investigate its semi-regional extent. The mudstone facies recognised in core are indicative of an ephemeral lake where short lived, shallow lacustrine conditions alternate with longer periods of marshy, palustrine conditions favourable for the formation of dolostones. Correlation of wireline log signatures has led to the recognition of lateral facies changes which have important implications both for the regional paleogeography and for the competency of the units as top seals. It is suggested that the deposition of the Triassic Mudstone Members occurred as a result of the retrogradation of fluvial systems during a change to more arid climatic conditions. Significant diachroneity is associated with these lithological transitions. The reduction in sediment supply during aridification is thought to have caused a change from an overfilled to an underfilled basin where halokinetic accommodation promoted the development of these ephemeral lake environments. The interpreted climatic trends recognised in the Triassic are compared to those of adjacent regions.