North Sea Viking Graben Pressure Study: Overpressure and fluid flow issues in a mature basin

Interpretation of overpressure in the North Sea Viking Graben, in terms of its distribution and subsequent implications for hydrocarbon trapping and future exploration.

The pressures in the Jurassic rocks in the North Sea Viking Graben are greatest in the centre of the basin where burial depths are deepest, and large portions of this prospective deep area remain undrilled. High overpressures create drilling difficulties and increase exploration costs. They are also a significant factor in hydrocarbon migration and trapping, with the highest overpressures creating breached traps. Where drilled and pressure tested, the Jurassic strata are heavily compartmentalized by faulting, although faults appear not to uniformly seal across the basin. At shallower stratigraphic levels, overpressure occurs more locally e.g. within some Cenozoic strata, for example in the East Shetland Basin, where pressure gradients are lower.

The dataset includes direct fluid pressures (RFTs, MDTs) plus Leak-Off data, Lost Circulations and Mud Weights for most wells.

Benefits:

• Reduced drilling risk because of improved definition of overburden and fracture gradients in the region.
• Enhanced exploration and development through a better understanding of the controls on pressure compartments and lateral drainage, and improved knowledge of the effects of overpressure distribution on trap integrity and hydrodynamics.

With the intent of improving safety and drilling success in the future, aiding well planning and minimizing exploration risk, this pressure study expands on preliminary observations that include:

• Controls on pressure compartments/cells
• Pressure prediction with high temperature and mixed lithologies
• Pressure reversals and lateral drainage
• Hydrocarbon column above minimum LOP trend vs. seal capacity

Deliverables:

• Digital database of pressure interpretations based on the IHS pressure database for 1362 wells in the North Sea Viking Graben.
• Maps of the distribution of overpressure across the area at all main stratigraphic horizons: Post Palaeocene, Palaeocene, Cretaceous, Late Jurassic, Middle Jurassic, Early Jurassic, Triassic and Pre-Triassic.
• Single- and Multi-Well Pressure-Depth plots for formation pressures and Leak Off/Limit Test data to illustrate the findings.
• Six case studies which emphasize interpretation of pressure data relevant to hydrocarbon exploration and production.

The study includes maps and plots illustrating the overpressures by geographical area and stratigraphic age, as well as a series of illustrated case studies dealing with topics such as:

• Seal integrity and breached traps
• Determination of fracture gradients
• Lithostatic gradients
• Lateral drainage
• Reservoir quality

An approach for predicting pressure in deep undrilled compartments is also included.

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