Case Study: Fault Seal Failure - Evidence from sub-Chalk reservoirs
This case study addresses the overpressure issues in the North Sea Central Graben study. One main low overpressure compartment at Jurassic Shoreface Sandstone level has been identified from the regional mapping and termed the Southern Low Overpressure Compartment. The compartment is located in UK Blocks 30/12b, 30/17 and 30/16 to the east of the Auk Ridge, which includes wells in the Fulmar-Halley-Clyde oilfields (Figure 1).
The presence of a pressure compartment with less than 1000psi overpressure at these depths is anomalous at 12,000’ (Figure 2).
Multi-well Pressure-Depth (P-D) plots generated using GeoPressure Technology PressureView 2.1 software for Blocks 30/12, 30/16 and 30/17 (Figures 3 and 4) and a single-well P-D plot for UK well 30/12b show the context for the Jurassic low overpressures.
• Palaeocene and Late Cretaceous pressures, where available, have overpressures in the region of 2,200 psi (15.2 MPa), consistent with the regional trend at these stratigraphic horizons.
• Rare pressure measurements in Late Jurassic turbidite sandstones in wells 30/17b-6 and –6ST have 3600 psi (24.8 MPa) overpressure much closer to the overpressure found elsewhere in the West Central Graben at similar depths in the Late Jurassic Shoreface Sandstone.
• Direct measurements in Triassic channel and associated sandstone reservoirs (especially in wells 30/12b-2, -3 and -4) show a trend of increasing overpressure with depth, and distance below and away from the boundary with the Late Jurassic Fulmar Sandstone.
In relation to overpressures in this compartment the observations are made:
(1) Late Jurassic Shoreface Sandstone overpressures are significantly lower than all overlying formations and particularly relative to the overlying turbidites encased in the Kimmeridge Clay Formation shales.
(2) Late Jurassic Shoreface Sandstone overpressures are lower than underlying Triassic sandstones, and well below their expected overpressures.
Issues addressed
(1) Lateral Seal Failure : Vertical v Lateral leakage
(2) Fault Sealing between compartments
(3) Dewatering into Late Jurassic shoreface sandstones
(4) Implications for migration
(5) Trigger mechanisms for seal failure
(6) Hydrodynamics in the Fulmar Field
Conclusions
Lateral seal failure explains the anomalously low overpressure in Late Jurassic shoreface sandstones in the vicinity of the UK Fulmar and Clyde oilfields (Figure 5). It may also have contributed to migration of hydrocarbons to the west at this time, to fill the Auk Field. Other undrilled structures to the west may also have received a charge.
The timing of failure appears quite recent, and helps explain dewatering signatures in Triassic and Jurassic strata above and below the Late Jurassic sandstones, as well as evidence for hydrodynamic tilt on the north flank of the Fulmar.
This case study is one of five included to address overpressure issues in the North Sea Central Graben study. The others include:
- Fracture Gradients-seal breach risk
- Late Jurassic intra-formational reservoir pressures
- Pressure Leak Points-regional significance
- Hydrodynamics in the North Sea Central Graben