How are oil and gas generally formed in marine sediments?

• A. From dead marine organisms buried and subjected to heat and pressure over long times
• B. From volcanic activity
• C. From solar evaporation of seawater
• D. From biological processes in surface waters

Answer: (B)

Oil and gas come from the remains of microscopic marine organisms that settle with sediment on the ocean floor. Over millions of years, these organic-rich layers are buried deeper and subjected to heat and pressure, which transform the organic matter into kerogen and then, with sufficient warmth, into liquid oil or natural gas. The amount of heat controls whether oil or gas forms, and the hydrocarbons migrate through porous rocks until they get trapped in reservoirs. This is why marine sediments with plenty of preserved organic material become oil and gas sources. Other processes described—volcanic activity, solar evaporation, or surface biological activity alone—do not produce hydrocarbons in the way oil and gas form deep underground.