Answer :
the answer is comparison and contrast; cause and effect. In the beginning, the author is giving differences and similarties between petroleum and coal,
which you can see from this sentence :
. . .Both are vital sources of energy for the modern world and both . . . .
. . .However, petroleum was mainly formed from the remains of ocean-dwelling microorganisms. . . .
After that, the author moves the discussion into talking about the cause and effect of each.
which you can see from this sentence :
. . .Both are vital sources of energy for the modern world and both . . . .
. . .However, petroleum was mainly formed from the remains of ocean-dwelling microorganisms. . . .
After that, the author moves the discussion into talking about the cause and effect of each.