Assessing Seal Capacity in North Sea Central Graben Mudrocks

Richard E. Swarbrick¹ and Richard W. Lahann^2
1 GeoPressure Technology Ltd, Durham, United Kingdom
² GeoPressure Technology Ltd, Houston, TX

Seal breach risk assessment requires knowledge of reservoir fluids, pore pressures and seal fracture pressures. Fluid pressure measurements and Leak Off Test data from 141 wells in the Central North Sea High Pressure High Temperature (HPHT) area were analysed to assess the top reservoir seal capacity of the Upper Jurassic shore face sandstones. The aquifer overpressure was determined from unambiguous water gradients established in 84 wells, supplemented by examination of pressure measurements from other wells. The distribution of overpressure was mapped onto a tectonic base map, revealing the pressure compartmentalization.

Fracture gradients at top reservoir were established from an algorithm developed by examination of Leak Off Tests (LOTs) and fluid pressures estimated at each Leak Off Test depth. Both aquifer and hydrocarbon seal capacities were determined for each well location by comparison of aquifer/hydrocarbon pressures and calculated fracture pressures.

The study shows that the fracture gradient has an upper limit of the lithostatic pressure at depths shallower than 3.8 km, but the LOT can exceed the lithostatic pressure at greater depths. Fluid pressure contributes to fracture gradient magnitude due to pore pressure-stress coupling. Previous studies in the area propose using the lower bound of the LOP trend as the upper limit to hydrocarbon sealing: missed exploration opportunities result from this approach. Analysis of the seal capacity at top reservoir does not separate dry holes from hydrocarbon discoveries, suggesting the top reservoir seal is not the controlling sealing lithology in this part of the North Sea.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005