The Central Graben: A Dynamic Overpressure System.

Gordon M. Holm

The Upper and Middle Jurassic sediments of the Central Graben in the North Sea contain highly overpressured aquifers, with pore pressures which approach the fracture gradient. These overpressures, whilst partly due to compactional effects, may principally be derived from generation of hydrocarbons from the Kimmeridge Clay source rock, which is currently oil and gas generative.

This overpressuring mechanism works as a Dynamic System, with the reservoir seal acting similar to a valve on a pressure cooker. The fluid volume (and pressure) increases until micro-fractures develop in the seal. With fluid escape, there is a proportional reduction of the fluid pressure in the aquifer until the micro fractures reseal. This process is intermittent and will continue as long as the source rock is actively producing hydrocarbons.

The Dynamic Overpressure System has significant implications for hydrocarbon prospectivity. The conventional model for hydrocarbon emplacement is early oil fill, and then subsequent displacement by gas, with the oil escaping at the spill point. In the Dynamic Overpressure System, the gas will start to fill and initially displace the oil but when the pore pressure exceeds the fracture pressure, the gas can leak through the roof seal. Thus oil may be preserved in structures which are connected to a late stage gas generative kitchen, as the gas can 'bypass' the oil, by partial leakage through the seal.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995