Role of the Chalk in development of deep overpressure in the Central North Sea

Abstract

A high magnitude of overpressure is a characteristic of the deep, sub-Chalk reservoirs of the Central North Sea. The Upper Cretaceous chalk there comprises both reservoir and non-reservoir intervals, the former volumetrically minor but most commonly identified near the top of the Tor Formation. The majority of nonreservoir chalk has been extensively cemented with average fractional gross porosity of 0.08, and permeability in the nano-to microDarcy range (10 218 -10 221 m 2 ), and sealing properties comparable to shale. Hence deeply buried chalk is comparable to shale in preventing dewatering and allowing overpressure to develop. Direct pressure measurements in the Chalk are restricted to the reservoir intervals, plus in rare fractured chalk, but reveal that Chalk pressures lie on a pressure gradient which links to the Lower Cenozoic reservoir above the Chalk and the Jurassic/Triassic reservoir pressures below. Hence a pore pressure profile of constantly increasing overpressure with increasing depth is indicated. Mud weight profiles through the Chalk, by contrast, show many borehole pressures lower than those indicated by these direct measurements, implying wells are routinely drilled underbalanced. The Chalk is therefore considered the main pressure transition zone to high pressures in subChalk reservoirs. In addition to its role as a regional seal for overpressure, the Base Chalk can be shown to be highly significant to trap integrity. Analysis of dry holes and hydrocarbon discoveries relative to their aquifer seal capacity (the difference between water pressure and minimum stress) shows that the best empirical relationship exists at Base Chalk, rather than Base Seal/Top Reservoir, where the relationship is traditionally examined. Using a database of 65 wells from the HP/HT area of the Central North Sea, and extending the known aquifer gradients from the Fulmar reservoirs via Base Cretaceous to Base Chalk, leads to a risking threshold at 5.2 MPa (750 psi) aquifer seal capacity. Discoveries constitute 88% of the wells above the threshold and 36% below, with 100% dry holes where the aquifer seal capacity is zero (i.e. predicted breached trap). This relationship at Base Chalk can be used to identify leak points which control vertical hydrocarbon migration as well as assessing the risk associated with drilling high-pressure prospects in the Central North Sea.

Enhanced porosity preservation by pore fluid overpressure and chlorite grain coatings in the Triassic Skagerrak, Central Graben, North Sea, UK

“…2) are the main reservoir seals for the sub-Chalk reservoirs in the Central Graben, North Sea (Mallon & Swarbrick 2002Swarbrick et al 2010). These highly cemented and mechanically compacted Chalk units have the potential to seal high overpressure in the underlying highly pressured reservoirs (Mallon et al, 2005;Mallon & Swarbrick 2008;Swarbrick et al 2010). After the North Atlantic Ocean opened, the Eocene and younger Hordaland and Nordland groups, comprising up to 2500 m of predominantly siltstone and shale, were deposited.…”

“…The lithological unit types used in this model are mainly PetroMod default lithology types or mixed default lithology types based on well log descriptions and core analysis reports for the Central Graben lithologies. The only exceptions are the modelled Chalk units, which were modified to match the specific characteristics of the North Sea non-reservoir chalk (Mallon & Swarbrick 2002;Mallon et al 2005;Swarbrick et al 2010).…”

Integrating a hydrodynamically‐titled OWC and a salt‐withdrawal depositional model to explore the Ula Trend

“…If we add this total to the 2087 psi overpressure in the Chalk, we are suggesting that the ‘original’ Ula overpressure is 3595 psi. Using this value would put the top seal at high risk of failure using the methodology outlined in Swarbrick et al. (2010).…”

“…Using a simple loading model, the additional overpressure owing to this extra weight is calculated by the difference between the overburden gradient (i.e. weight of the sediments, 3.64 psi m )1 using data from Swarbrick et al 2010) and the hydrostatic gradient (1.46 psi m )1 ), multiplied by 692 m, giving 1508 psi of overpressure. If we add this total to the 2087 psi overpressure in the Chalk, we are suggesting that the 'original' Ula overpressure is 3595 psi.…”

Importance of vertical effective stress for reservoir quality in the Skagerrak Formation, Central Graben, North Sea

“…The syn-rift sediments are mainly siliciclastic Triassic and Jurassic sediments with a cumulative thickness of 1000-4000 m. The post-rift sediments comprise the Cretaceous to Holocene successions, of up to 4500 m in total thickness, which are dominated by shale, sandstone, silty-sandstone and thick Upper Cretaceous chalk units including the Ekofisk, Tor and Hod Formations (Figure 2) (Goldsmith et al, 2003). These highly cemented and compacted chalk units provide the main seal for the highly overpressured sub-Chalk reservoirs in the Central Graben, North Sea Swarbrick, 2002, 2008;Swarbrick et al, 2010). The focus area for this study includes the Heron (well 22/29-5RES1) and Skua (well 22/24b-7) fields from the Heron Cluster in UK quadrant 22, the Jade (well 30/2c-4) and Judy (wells 30/7a-7, -8, -9, 11Z, -P3 & 30/13-5) fields from the Josephine Ridge in UK quadrant 30, and the Cod (well 7/11-7) and Gaupe (well 6/3-1) fields in Norwegian quadrants 7 and 6, respectively (Figure 1).…”