As further layers settled into the sea or lake bed, intense heat and pressure built up in the lower regions. This process caused the organic matter to change, first into a waxy material known as kerogen, found in various oil shales around the world, and then with more heat into liquid and gaseous hydrocarbons via a process known as catagenesis. Formation of petroleum occurs from hydrocarbon pyrolysis in a variety of mainly endothermic reactions at high temperatures or pressures, or both. These phases are described in detail below.
as fUrtHEr layers settLed into tHe Sea oR lakE beD, intENSe heaT aNd PREssurE BuilT up IN The lower rEgIoNs. tHIs PrOCEss cauSeD the oRganIc mattER to changE, fIrsT into a WaXy mAteRiaL knoWN As KEroGen, foUnD IN various oil sHaleS aroUnd the world, anD tHeN wiTh mORe heat into liquId And gaSeOus HydRocArBONS viA a pRoceSs knowN aS cATagENEsiS. foRMAtiOn of peTrOleuM occUrs From hyDrocaRboN PyroLYSiS In A VarIety of mAiNLY endoThermIC reacTIoNs at hiGh temperaturEs or pRessures, or both. theSe phAses are dEscribed In Detail beLOw.