There are several ways, including:
That all may be well known (not to me), but it was not clear from the article what type of material was encountered. I am sure with a little “digging” we can find out from the crew who drilled it what they went through. Unless they do not want competitors to know what is involved?
How many pounds mass of animal and plant matter is required to make a lb of oil? And how did it get so deep? I have seen the theories about plate tectonics being the cause of organic matter being drawn into the depths, but does it really compute?