Formation of limestone, coal, oil and gas
Chemistry of the atmosphere • Evolution of the Earth's atmosphere
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Overview of deposit formation
Sedimentary deposits form by accumulation of biological or chemical material followed by burial and compaction into rock or fuel. Biological remains or organic matter supply the raw material, and sedimentation buries the material so chemical and physical transformations occur. Geological time, burial depth and geochemical environment determine the final deposit type.
Limestone formation
Calcium carbonate (CaCO3) from shells, skeletons and corals accumulates on the seabed and forms carbonate-rich sediment. Progressive burial compacts the sediment and drives out water, and chemical cementation binds particles into limestone rock. Low oxygen at the seabed reduces decay of carbonate material, and continuous supply of marine organisms increases deposit thickness.
Coal formation
Plant material accumulates in waterlogged, anaerobic swamp environments where decay is slow and peat forms. Burial of peat by sediment increases pressure and temperature, which expels water and gases and converts peat into coal through chemical alteration (carbonification). Limited oxygen prevents complete decomposition, and rapid burial increases coal preservation and rank (from lignite to anthracite with greater depth and heat).
Crude oil and natural gas formation
Microscopic marine organisms and organic-rich sediments accumulate on the seabed and form organic-rich mud (source rock). Burial increases temperature and pressure, which chemically transforms organic matter into hydrocarbons (kerogen then oil and gas) through thermal cracking. Generated oil and gas migrate from the source rock into porous reservoir rocks and accumulate where impermeable cap rock traps the hydrocarbons, forming exploitable deposits.
Conditions and limiting factors
Temperature and pressure control the type of organic transformation: low heat preserves peat, moderate heat forms oil, and higher heat forms natural gas. Porosity and permeability of surrounding rocks control hydrocarbon migration and accumulation. Presence of an impermeable cap rock controls trap formation and prevents hydrocarbons from escaping to the surface. Biological productivity and rate of sedimentation control the quantity of initial material and therefore the size of eventual deposits.
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