High magnitude of overpressure is a characteristic of the deep, sub-Chalk reservoirs of the Central North Sea. The Upper Cretaceous Chalk Group of the Central North Sea comprises both reservoir and non-reservoir intervals, the former most commonly identified near the top of the Tor Formation. The non-reservoir Chalk has been extensively cemented with fractional gross porosity up to 0.18, compared to 0.2 to 0.4 in the reservoir intervals whose permeability is in the milliDarcy to microDarcy range (10-15 to 10-18 m2). Non-reservoir Chalk permeability is in the nano to micro-Darcy range (10-18 to 10-21 m2) depending on depth of burial, but comparable to shale permeability. Hence the ability of deeply buried Chalk to prevent dewatering and allow overpressure to develop is similar to shales. Direct pressure measurements in the Chalk are restricted to the reservoir intervals, plus rare fractured Chalk. These pressures normally lie on a pressure gradient which links the pressures found in the Lower Tertiary reservoir above the Chalk and the Jurassic/Triassic reservoir pressures below, suggesting a pore pressure profile of constantly increasing overpressure with increasing depth. Mud weight profiles through the Chalk, by contrast, show a tendency to much lower pressures than indicated by these direct measurements, implying wells are routinely drilled underbalanced. Gas readings and “kicks” are consistent with the higher pressure regime than indicated from mud weights. The Chalk is therefore considered the main pressure transition zone to high pressures in sub-Chalk reservoirs.
Analysis of dry holes and hydrocarbon discoveries relative to their aquifer seal capacity (difference between water pressure and fracture strength) shows that the best empirical relationship exists at Base Chalk, rather than Top Reservoir where the relationship is traditionally examined. Overpressures for the main Jurassic/Triassic reservoirs in the Central North Sea has been extracted from a comprehensive pressure database and used to map pressure cells across the study area. The overpressure values have also been extracted from isolated reservoirs within overlying turbidites of the Heather and Kimmeridge Clay Formations. The results indicate hydraulic connectivity through these shale-dominated intervals, from main reservoir (e.g. Fulmar/Piper) at least to Base Cretaceous Unconformity. Using a database of 67 wells, and extending the aquifer gradients to Base Chalk leads to a risking threshold at 750 psi aquifer seal capacity, with 94% discovery rate above and 35% below, and 100% dry holes where the aquifer seal capacity is zero (i.e. predicted breached trap). Applying this relationship at Base Chalk reveals the geographical distribution of breached traps in the Central North Sea high pressure area, pressure cell “leak points” and “protected traps” with enhanced confidence of successfully locating trapped hydrocarbons.
The evidence is mounting for the North Sea Chalk to play a significant role in the development of the high overpressures found in deeper reservoirs, especially in the Central North Sea area. Earlier emphasis on the rock properties of the volumetrically minor reservoir chalk may have led to the belief that the Chalk did not contribute to trapping of overpressure beneath. The results revealed in a paper written by Ikon GeoPressure and Durham University aim to dispel this assessment.
Please request a copy of a paper on this subject, written by Ikon GeoPressure and Durham University.
Role of the chalk in development of deep overpressure in the Central North Sea
Swarbrick, R.E., Lahann, R.W., O'Connor, S.A. & Mallon, A.J. 2010 Role of the Chalk in development of deep overpressure in the Central North Sea In Edited by B. A. Vining and S. C. Pickering Petroleum Geology: From Mature Basins to New Frontiers Proceedings of the 7th Petroleum Geology Conference The Geological Society. P 493-507.