The correct answer is - marine organisms.
Both, the oil and the natural gas are formed on the basis of dead marine organisms. When the marine organisms were dying, they were falling in depths and onto the sea/ocean floor. The sea floor is covered with very soft sediments, and if they were not eaten by scavengers, or they have been covered with another layer of sediments, they have been relatively well preserved. As they were decomposing, the isolation, temperature, and pressure, managed to transform the dead remains of the marine organisms into oily sticky liquid that over time gained the characteristics that we see now in the oil and the natural gas.
