I would not worry to much about where the algae and plants came from. I would worry about if worrying how they got hundreds and thousands of feet under the earth surface.Tell us where did the algae and plants come from.
“Oil drilling depths have increased over the last six decades, from an average of 3,635 feet (1,108 meters) in 1949 to 5,964 feet (1,818 meter) in 2008, reflecting the depletion of easily accessible oil reserves.”