生物形成可能还是主要成因，但天然气在地墁高温高压情况下可以合成石油，然后石油沿地质断裂带到达地球表面。Hydrocarbons generated in the upper mantle could be transported through deep faults to shallower regions in the Earth’s crust, and contribute to petroleum reserves。德州10几公里深岩石层的岩油很难用生物形成来解释。