Liassic Source Rocks from Three Wells from the Sea of Iroise (French Western Approaches) and Dorset (UK) Outcrops: Nature and Modeled Maturity History

Abstract

The first stage of exploration in the Western Approaches and Channel Basins was completed in the middle of the 1980s without any significant results (apart from one oil and one gas condensate show) although 12 wells were drilled in French waters. However, the on­ shore Wytch Farm oil field was developed, and produc­ tion was said to reach about 60000 bopd at the end of 1991. At the same time, the Kinsale Head and Ballycotton gas fields, offshore from southern Ireland, were deve­ loped with reported reserves of 1.4 Tcf and 73 Bcf respec­ tively. Significant amounts of hydrocarbons may thus be present in as yet undiscovered structural or stratigraphic traps. Present studies will benefit from the results of the first wells, as well as from the much improved quality of present seismic data with respect to those available in 1970. It is now also possible to gain a better understanding of the quality, distribution, and maturation history of the potential source rocks, the most widespread and ma­ ture of which are the Liassic black shales. Sections of the­ se shales have been sampled along the classic field expo­ sures of the Dorset coast, and from cuttings in three wells. The Dorset section is immature and the average organic content is 3 %. At Levneg 1 the Liassic section is about 1250 m thick, with the best potential in the Pliensbachian to Hettangian intervals. Present TOCs are low (about 1 %), but these rocks are overmature so that their initial organic content can be estimated to have been in the ran­ ge of at least 2 %. The present depth of these levels and the present heat flow cannot explain this present matura­ tion so that their tectonic history must also be taken into account. The IFP-CGG IS 88 seismic line clearly shows, at this locality, the inversion in Tertiary times of the basin where up to 4000 m of sediment hat accumulated in Me­ sozoic times. The relatively good Liassic source rocks at the Rea-Gwenn and Travank wells are immature and were deposited in a much less subsiding area. They ap­ pear to be more similar to the Dorset section. As for the area south of Wytch Farm, 1-D Matoil modeling shows that the generation of oil or gas stopped at Levneg as soon as the process of inversion was initiated, which im­ plies that, in this case, pre-Tertiary structural traps are more favorable targets than recently inverted structures.