A source rock evaluation of the Mesozoic Sediments of the Well Hyllebjerg-1 Danish Subbasin

Abstract

The source rock potential of Mesozoic sediments (cuttings) from the Hyllebjerg 1 well, Danish Subbasin, has been assessed using a number of different petrographical and organochemical methods. Upper Jurassic sediments (Bream Formation) equivalent to the principal source rocks of the North Sea graben structures (Kimmeridge Clay Formation and lateral equivalents) do not show similar prominent source rock characteristics in this well, although a higher proportion of algal material is observed. Sediments with the most promising source rock characteristics for liquid hydrocarbons were· detected mainly in the lower- Jurassic sequences of the upper Fjerritslev Format ion (F-4 and upper F-3 Member) and in one horizon in the Upper Cretaceous Vedsted Formation which showed a good quality composition and a relatively high content of organic matter. But these sediments may be excluded as actual source rocks in this well as maturity (assuming the threshold value near 0. 60 % R ) is first reached at approximately 8500' 0 depth i.e. at the top of the Gassum Formation (Upper Triassic/ Lower Jurassic). The conditions may only by slightly different off - structure is this area, as the F-4 and F-3 Member sequence according to seismic sections is found at approximately the same depth. But the depth to ( and the thickness of) the Fjerritslev Formation is increasing towards the SE into the rimsynclines of the saltdomes nearby. While sufficient maturity is reached in the deeper part of the well, no commercial accumulations of hydrocarbons were encountered. This is attributed to the mainly reworked, unfavourable type of organic matter and the generally decreasing organic content downwards in the well, approaching the lower 1 imi t for potential source rocks ( set at O, 5% TOC). However, generation and migration of small amounts of gaseous hydrocarbons from Gassum Formation sediments containing more humic-influenced organic matter with only minor reworking cannot generally be excluded either here or elsewhere in the basin. Some more attention should also be paid to the Vinding Formation sediments which contain some algae- ri eh ( Botryocous-type) oil-prone horizons of more favourable source rock conditions. Mature sediments are found at shallower depths ( 8500 ') in this well in the central part of the basin compared to the more marginal areas (8900') where a slightly higher geothermal gradient in Jiyllebjerg 1 ( 28°C/km uncorrected) is seen compared with the marginal areas (23.5°C/km uncorrected) away from the basinal depocenter. The basinal depocenter also has a higher heat flow.