Just as I was thinking about writing this, I attended an excellent luncheon talk by Barry Jay Katz (Chevron Fellow) on Anatomy of a Source Rock where he showed the following plot. He posed the question "Where is the cross-over?" I'm pretty sure I've drawn this correctly, but memory may fail me, so you might want to find a backup source, but the point is that I'd been thinking about cross-over points already, and had even scribbled out the following plot immediately after a me