Late Jurassic Stratigraphy of the North Sea
The stratigraphy during the Late Jurassic and the Early Cretaceous in the northern North Sea is shown left. The oldest rocks shown, those of the Brent Group in the Middle Jurassic, are predominantly shallow marine and coastal sandstones (Richards et al., 1993; Husmo et al., 2003), and hence form the main reservoir rock in the northern North Sea. The Brent Group was deposited at a time of tectonic quiescence during the last stages of thermal subsidence following Triassic rifting (Yielding et al., 1992). However, rifting recommenced during the deposition of the youngest formation within the Brent Group, the Tarbert Formation (Davies et al., 2000; McLeod et al., 2002), and continued throughout the deposition of the Humber Group in the Late Jurassic (Fraser et al., 2003).
The Humber Group consists of the Heather and Kimmeridge Clay Formations. Both are dominantly shale and mudstone units, and the two are hard to distinguish on lithological criteria alone (Richards et al., 1993). The Heather Formation is taken to represent deposition in an aerobic marine environment, whereas the Kimmeridge Clay Formation was deposited in anoxic, hemipelagic facies (Richards et al., 1993; Fraser et al., 2003). Hence the Brent and Humber Groups represent an overall transgressive sequence with increasing water depths. Because of the anoxic conditions, the Kimmeridge Clay Formation retained a high degree of organic material, and thus is the principle hydrocarbon source rock in the northern North Sea.
Deposition of the Humber Group was followed in the Early Cretaceous by that of the Cromer Knoll Group. This transition marked the end of active rifting in the northern North Sea and the start of thermally-controlled subsidence. The Cromer Knoll Group is composed mainly of marls and carbonaceous mudstones, and is interpreted to be a hemipelagic deposit which formed in an aerobic environment (Johnson & Lott, 1993; McLeod, 2000; McLeod et al., 2002; Copestake et al., 2003).
Three main seismic reflections were mapped for this study. The first, the Top Brent horizon, represents the top of the Brent Group. This reflection event is bright due to the density contrast between the sand-dominated Brent Group and the more shaly overlying Humber Group. This reflection approximately marks the top of the pre-rift succession in the northern North Sea. Although rifting actually commenced during the deposition of the Tarbert Formation, this formation is not areally extensive across the ESB, and is very thin (less than 75 m, Richards et al., 1993; Young et al., 2001, McLeod et al., 2002}. The Tarbert Formation is therefore difficult to map because the thickness of the formation approaches the seismic resolution of the data, and it is only locally present (generally in the deepest parts of the Late Jurassic half-graben). Therefore, the top of the Brent Group was mapped as this reflection is continuous, bright and regionally extensive.
The second major reflection mapped was the base of the Kimmeridge Clay Formation (the 'Base Kimmeridge Clay horizon'). This reflection is less bright and more discontinuous than the Top Brent reflection because of the similar lithologies of the Heather and Kimmeridge Clay Formations. The age of this horizon is not well constrained. Within the ESB as a whole, the boundary between the Heather and Kimmeridge Clay Formations is taken to be at the Oxfordian--Kimmeridgian boundary (Richards et al., 1993; Husmo et al., 2003}. However, McLeod et al. (2002) interpret this boundary to lie within the Middle Oxfordian in the Strathspey-Brent-Statfjord area of the North Sea. As this project is concerned with the entire ESB, the age given by Richards et al. (1993) and Husmo et al. (2003) is used here.
The third surface to be mapped in detail is often called the Base Cretaceous Unconformity. However, this horizon does not actually lie at the base of the Cretaceous, but is Ryazanian in age (Richards et al., 1993; McLeod et al., 2002; Fraser et al., 2003). Neither is it a regional unconformity. In the hanging walls of Late Jurassic faults, the stratigraphic column as shown in the figure above is complete. Near the fault footwalls, however, the Humber Group is commonly overlain by the Shetland Group (which overlies the Cromer Knoll Group), and thus the 'Base Cretaceous' surface is only locally unconformable. Therefore, to avoid confusion, this horizon is termed here the 'Base X horizon', and marks the top of the Kimmeridge Clay Formation. This surface is readily identifiable in the seismic data as a very bright, highly continuous reflection, owing to the density contrast between the shaly Kimmeridge Clay Formation and the carbonate-rich Cromer Knoll Group.
© Copyright 2004. All Rights Reserved. Jonathan Bryon.