It’s time for some memories from old days…a few years back, from, probably, the closest petroleum geologists get to pilgrimage: Kimmeridge Bay, a troll’s stone throw east of Weymouth. Still on the English Channel Coast, we jump a bit down again, to the Upper-Upper Jurassic. Kimmeridge Bay is home to the shale above all shales, the one shale to feed them all – all the the oil fields, that is. Its dark, dirty shale is the source rock for most of the oil in the North Sea.
In the Upper Jurassic, the North Sea was a shallow sea within a rift zone. The spasms that heralded the opening of the Atlantic Ocean here had been going on for quite some million years, and created a seaway surrounded by steep faults. (The rift later jumped westwards and opened the Atlantic Ocean in its present position. Had it continued in the North Sea, the British Islands might have ended up in splendid isolation somewhere in the Atlantic Ocean, far from the annoying EU).
During the Kimmeridgian – yes, that period on the time scale got its name from the bay – the basin-bounding faults had created some quite isolated basins, with little water circulation. Low circulation means little oxygen at the bottom, which in turn means that organic matter sinking to the bottom does not rot, but is preserved and covered by more sediment. Most of this organic matter is dead algae. No, oil doesn’t come from dead dinosaurs. There were not enough dinosaurs, but zillions of algae, probably turning the sea surface into green slime for long times.
Add time, burial and heat down in the deep North Sea basins, and the algae break down to oil. Add more burial and heat, and the oil breaks down to gas. In Kimmeridge bay, the rock hasn’t been buried enough to do the final cracking to oil, but parts of it is actually so rich that it burns if put onto a fire!
How to get there
By car: Folow M27, then A31 from Southampton, and around Bornemouth. Then, A350, onto A35 and then A351 to Wareham. From Stoborough, Grange Road towards Kimmeridge.