A scale referred to as "Level of Organic Metamorphism" (LOM) describes how far the thermal metamorphism of sedimented organic matter has progressed during subsurface burial. It represents a single, continuous numerical scale which is applicable to the entire thermal range of interest in the generation and destruction of petroleum. It is based on coal rank and is convenient for interrelating other useful scales of organic metamorphism.

A relation of temperature to time for petroleum generation is based on LOM values of sedimentary rocks in the 9- to 16-LOM range with reasonably dependable maximum temperatures and effective heating times. The relation is nearly equivalent to a doubling of the reaction rate for each additional 10°C, and the apparent activation energies increase from about 18 to 33 kcal/mole as LOM increases from 9 to 16.

The principal metamorphic stages of petroleum generation and a zone of initial oil-source-rock maturity are superimposed on graphs of LOM versus depth for two wells to illustrate the prediction of specific depths where oil, gas condensate, and high-temperature methane would be generated in petroleum source rocks at any given location.