>in the 1970s, the oil companies decided they would no longer look for deposits in layers dating to the 50 million years after the Permian Triassic extinction -

>in the 1970s, the oil companies decided they would no longer look for deposits in layers dating to the 50 million years after the Permian Triassic extinction -- there is nothing for them to find at that level You’re pretty spot on. Oddly, the Permian is one of the prolific source rocks in the US now!