I suspect the purple colour is interpreted as a volcanic...

I suspect the purple colour is interpreted as a volcanic intrusion. I am not sure how reliable the interpretation is though, it is not overly convincing for mine. In any case if it was a volcanic intrusion, the heat effect would be localised and would possibly compromise the source rock in that immediate area. It would not affect regional source rock maturity. The volcanics on which the source rock is deposited is of interest though. This area is thinned crust and is thought to come with an elevated heat-flow because of that. This is why Total went exploring in the deep. Higher heat flow with better source rocks and cleaner reservoirs.