The Kimmeridge Clay Formation of The North Sea

Abstract

The Kimmeridge Clay Formation and its equivalents of Callovian to early Ryazanian age are by far the most important oil source rocks of the northern North Sea area. The source rocks comprise marine claystones which were deposited in waters of increased salinity with short episodes of high salinity, during which uranium-enriched shales accumulated. Much of the sedimentary sequence in the inner parts of the basin comprises mass-flow deposits and storm-activated reworked deposits. The mineralogy and quantity and quality of the organic content is extremely varied, both regionally and in vertical sequence. A comparison of the biomarker and other geochemical characteristics leads to a conclusion that at least eight definable kerogen types contribute to the organic content, and a knowledge of their distributions is necessary before oil to source rock correlation can be performed with confidence. However, such interpretations require a large database and computer-based multivariate analysis. It is necessary for the petroleum geologist to understand how these variations can be explained by palaeogeographical factors so that the quantity and quality of oil-generating organic matter can be mapped with confidence. Thus, more precise estimates can be made of the quantities of oil and gas which have been generated in the un-drilled kitchen areas and, where migrated oil is discovered in the adjoining structures, its provenance can be correlated with the kitchen source rock or shown to be from an alternative source.