Now, about the amount of biomass involved ~ counting in the vast storehouse of living biomass in the rocks that make up the ocean floor, there's a whale of a lot of stuff down there. Hot, steaming lava could have "cooked" the critters out of the rocks where they reside and allowed them to simply flow to sandstone and other types of rocks, and then buried them.
I understand the concept of catastrophism, in fact I worked on a study in graduate school that helped that mechanism gain favor. This was the study of sedimentation and erosion caused by one event, a massive flood of the Big Thompson Canyon in Colorado following a thunderstorm. The gradualists just had a hard time holding sway after that.
However, the amounts of oil saturation of the shale beds suggested for this event seem to me to be outside the range of possibility. I would have to see the professional paper and their data to understand what they are trying to say, but I’m not convinced. This publication is not exactly a rock solid source, pun intended. I wouldn’t be surprised to find that the real study says something entirely different.