The United Kingdom North Sea contains a wide variety of reservoired oils, including large accumulations of heavy and heavy-medium (API < 25°) crudes trapped in Paleocene and lower Eocene sand bodies. Typically, these pools lie at depths of less than 6,000 ft (1,900 m), and they are characterized by having undergone extensive post-accumulation modifications. One of the prime causes of such changes is bacterial biodegradation, acting under temperature-critical control. Biodegraded heavy oils are notably common in the Tertiary of UKCS quadrants 3 and 9. All of these known accumulations appear to have been sourced from the Kimmeridge Clay Formation.

This paper reviews what is now known of the origin and distribution of these potentially important heavy oils and to analyze main controls on their accumulation and alteration. Tectonic deformation of the Tertiary sequence is of no small significance, and several heavy oil accumulations can be viewed as structurally trapped. Structural deformation has been a primary and highly important control in migration and eventual trapping, and this control can be confirmed as deep seated. An overview of basin development provides dynamic models both for migration and trapping and for understanding the movement of bacterially contaminated water into oil accumulation.

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