Overpressure in the Central North Sea

The Central Graben of the North Sea is characterised by high levels of overpressure. This causes drilling problems and may control the migration and entrapment of hydrocarbons. Pressure measurements from repeat formation tests, mud weight and drilling gas levels have been compiled and interpreted, and integrated with the structural, lithostratigraphic and diagenetic framework of the basin. Interpretation of this data reveals that Jurassic sandstones in the region are divided into a boxwork of pressure cells. These are bodies of rock which are internally in hydraulic communication but externally isolated from adjacent cells by pressure seals. Pressure seals in the region are lithological and vary with depth, in contrast to previous hypotheses of diagenetic or temperature controls. The magnitude of overpressure in a pressure cell is controlled by the structural position of the cell, with high overpressures (close to the lithostatic pressure) occurring in structurally-elevated cells on an axial horst. Lateral hydraulic communication between deep regions and structurally-elevated positions increases the fluid pressure in the permeable sandstones in the elevated regions. This leads to focused vertical fluid flow through the thin aquitard at these elevated regions, which are termed "Leak Points". Here, the pressure seal occurs at the top of the permeable Jurassic sandstone, while in adjacent off-structure regions the pressure seal occurs in the Kimmeridge Clay Fm., the region's petroleum source rock. Analysis of density and sonic log data demonstrates excess mudstone porosity in the overpressured cells, suggesting that disequilibrium compaction is a cause of overpressure in the region. The location of pressure seals within gas-mature source rocks suggests hydrocarbon generation may also play an important role in causing overpressure. Quantitative computer modelling of the basin supports the data-driven model, emphasising lateral flow in Jurassic sandstones beneath the pressure seal and focused vertical flow across formations at structurally-elevated points. The model shows that rapid Cenozoic sedimentation, coupled to low permeability of the mudstone- dominated basin, has led to disequilibrium compaction due to restricted fluid flow. The model suggests that overpressuring began at 40 Ma in Jurassic sandstones of the Graben axis. A link between overpressure-controlled fluid flow and the K-Ar dates of authigenic illite in the Jurassic sandstones is delineated. Measured illite dates coincide with modelled periods of declining fluid flow. A link to the incursion of organic acids from adjacent mudstones into the sandstones is proposed, with the supply of organic acids controlled by the compaction and overpressure in the basin. The distribution of porosity within Jurassic sandstones is examined and is shown to be controlled by the complex distribution of overpressure. Overpressure supports the development of secondary porosity. Products of mineral dissolution are preferentially removed from Leak Points due to enhanced vertical fluid flow. This results in 10-12% excess porosity at 4.6 km depth. Overpressure is a dynamic system, with basin-scale structures such as pressure cells being caused by microscopic changes in pore size. The basin-scale overpressure systems in turn effect pore-scale alterations on the basin. Overpressure is an expression of the complex dynamic interaction of coupled hydrogeological and geochemical processes active throughout the evolution of the basin