Overpressure and Seals in the North Sea - A Model Approach
Andrew G. Cavanagh, G. Couples, D. Darby, and R. S.
Overpressure prediction in the North Sea is complicated by fault seal assumptions. The computer code, BasinMod-2D, is used to simulate overpressure for a small-scale case study, the Puffin structure, located on the western flank of the Central Graben. This area is characterised by tilted fault blocks and syn-rift sedimentation of Upper Jurassic Fulmar sandstones with a post-rift cover of Cretaceous chalk and Tertiary mudstones. The model, a 10 km W-E section based on published seismic data, is tied to detailed well stratigraphy in 29/10 -2 with pressure data from RFT and mudweight.
The model fails to replicate observed overpressure data. The simulated overpressure is 20 MPa lower than the measured pressure of 85 MPa in the Puffin structure. The model fails to simulate hydrocarbon accumulation in the Fulmar sandstone reservoir, in contradiction to the observed bitumen indication of paleoaccumulation. The model also raises important questions as to the validity of uncalibrated parameter manipulation as a means of forward modelling fault seal, with specific reference to shale permeability.
We contend that these failures are not due to model limitations, but are the result of the size-scale of this case study. The inference is that overpressure and hydrocarbon creation and transmission is on a size scale larger than 10 km on geological timescales. Additionally, we infer that a fault seal at one time can leak hydrocarbons at another time. We consider that the drive for this comes from pressure increase during hydrocarbon generation in the deep kitchen.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California