How does depth of burial affect coal formation?

The process of coalification is primarily driven by the physical conditions associated with depth of burial rather than microbial activity, which ceases near the surface.1

As organic material is buried, it is subjected to increasing temperature due to the geothermal gradient, which typically averages 30 °C per kilometer. This rise in temperature is the primary factor responsible for the transformation of coal through its various ranks—from lignite and subbituminous to bituminous and anthracite. If coal is not buried deeply, it may never reach a higher rank, regardless of its age.1

Conversely, the increasing pressure resulting from the weight of overlying sediment is not considered a driver of coalification. In some cases, high overburden pressure may even hinder the process by trapping volatile compounds like methane that need to escape. However, pressure does influence the physical characteristics of the coal, specifically its porosity and moisture content.1