Oil is not just a fuel source humans extract. It plays several natural roles in Earth’s systems, from feeding entire ecosystems of microbes to maintaining underground pressure that keeps the land surface stable. Most of these functions are invisible, happening deep underground or on the ocean floor, but they’ve been shaping the planet for hundreds of millions of years.
Oil Feeds Deep-Sea and Underground Ecosystems
One of oil’s most surprising roles is as a food source. Entire communities of microorganisms have evolved to consume petroleum compounds as their primary energy source. NOAA identifies several families of marine bacteria, including Marinobacter, Pseudomonas, and Alkanivorax, that eat compounds found in crude oil. At least seven known species survive on oil alone, with nothing else in their diet.
These organisms aren’t just oddities. They form the base of food chains around natural oil seeps on the ocean floor, where crude oil bubbles up through cracks in the earth’s crust. Worms, clams, and other bottom-dwelling creatures cluster around these seeps, sustained by the microbial communities that thrive on the hydrocarbons. Research from the Mid-Atlantic Ridge has identified at least 26 deep-sea bacterial strains capable of using crude oil as their sole carbon and energy source, suggesting these ecosystems are widespread across the world’s oceans.
Underground oil reservoirs host their own extreme microbial communities. These environments feature high temperatures, intense pressure, toxic conditions, and very little water, yet life persists. Researchers have detected hyperthermophilic microorganisms (heat-loving microbes) in reservoirs with temperatures up to 131°C. Specialized groups of bacteria and archaea, an ancient branch of single-celled life, have been found thriving in oil fields above 70°C. Only above roughly 80°C do oil reservoirs become too extreme even for these hardy organisms.
Natural Oil Seeps and Ocean Chemistry
Oil doesn’t stay locked underground forever. It continuously leaks into the ocean through natural geological seeps, cracks in the seafloor where petroleum migrates upward over millennia. According to a National Research Council report, approximately 160,000 tonnes of petroleum enter North American waters through natural seeps each year, contributing about 5 million gallons annually with wide variation from year to year.
This might sound alarming, but the ocean has adapted. The oil-eating bacteria described above act as a natural cleanup system, breaking down hydrocarbons before they can accumulate. This process has been running for millions of years, long before humans began drilling. The bacteria convert petroleum compounds into simpler molecules, effectively recycling carbon that was locked in rock back into the biological world. Natural seeps also release methane and other gases that feed additional microbial communities, creating productive biological hotspots on what would otherwise be barren stretches of deep ocean floor.
Oil Stores Carbon Underground
Crude oil is concentrated carbon. The petroleum trapped in underground reservoirs represents organic material, ancient algae, plankton, and other organisms, that was buried and compressed over tens to hundreds of millions of years. By locking this carbon deep in the earth’s crust rather than leaving it in the atmosphere, oil deposits function as a long-term carbon storage system.
This is part of what geologists call the slow carbon cycle. Carbon moves between the atmosphere, oceans, living things, and rocks over timescales of millions of years. Oil and other fossil fuels represent carbon that was pulled out of circulation during ancient periods when the planet’s climate and atmosphere were very different from today. As long as that carbon stays underground, it has no effect on the current atmosphere. The climate concern with burning fossil fuels is precisely that it releases this stored carbon back into the air on a timescale of decades rather than millions of years, overwhelming the slow cycle’s ability to rebalance.
Oil Maintains Underground Pressure and Stability
Beneath the surface, oil isn’t sitting in underground lakes. It fills tiny pore spaces within sedimentary rock, and the pressure it exerts plays a structural role. In a confined underground reservoir, the weight of all the rock above (called geostatic pressure) is supported partly by the fluid pressure of the oil and water in the pores and partly by the rock grains pressing against each other.
When that fluid is removed, the balance shifts. The full weight of the overlying rock transfers onto the rock skeleton itself, compressing it. This is why extracting oil, gas, or groundwater can cause land subsidence, a gradual sinking of the ground surface. The U.S. Geological Survey notes that fluid withdrawal by humans has caused noticeable subsidence under certain geological conditions. In its natural state, the pressurized oil helps keep the rock structure inflated and the surface stable.
This pressure also influences how underground water moves, where natural gas migrates, and how geological formations behave during earthquakes. Oil-filled rock responds differently to seismic stress than dry rock, and the presence of pressurized fluids in fault zones can affect how and when faults slip.
Oil Shapes Geology Over Deep Time
Oil’s formation and migration reshape the rocks it passes through. As petroleum forms from buried organic material under heat and pressure, the chemical reactions involved alter the surrounding minerals. Oil migrating upward through rock over millions of years can dissolve certain minerals and deposit others, changing the porosity and permeability of entire geological formations.
At natural seeps, oil reaching the surface or seafloor creates distinctive geological features. Tar pits like the famous La Brea deposits in Los Angeles form where heavy crude reaches the surface and pools. On the ocean floor, seeping hydrocarbons can combine with water under high pressure and low temperature to form gas hydrates, ice-like structures that lock methane into the sediment. These hydrate deposits contain enormous quantities of carbon and influence seafloor stability along continental margins worldwide.
Oil also plays a role in mineral formation. The chemical-reducing environment around petroleum deposits, where oxygen is scarce, promotes the formation of certain sulfide minerals and can concentrate metals. Some ore deposits owe their existence to the chemical conditions created by nearby oil accumulations over geological time.