Good to Know Series – 02 – Coalification Process
In the previous episode, I explained how coal was formed. The overarching process that governs coal formation is known as coalification. In petroleum systems, a similar process is called maturation. Coalification is a diagenetic process in which chemical and physical changes occur over time in function of the temperature and pressure. During this transformation, organic matter progressively loses mobile components such as gases, water, and tars, becoming increasingly aromatic. The figure below illustrates the general chemical and physical changes that take place during coalification.
It is important to note that these changes do not occur uniformly throughout the process—some are more prominent at specific stages. For example:
From peat to lignite, the process is marked by moisture loss and volume reduction (dehydration).
At the high-volatile bituminous coal stage, hydrocarbons are generated (bituminisation).
At the low-volatile bituminous coal stage, hydrocarbons such as methane are expelled (debituminisation).
In the anthracite stage, the aromatic layers reorganize (graphitisation).
Beyond physical and chemical changes, petrographic transformations also occur as coal evolves. A lignite and an anthracite are vastly different rocks with distinct properties. However, before we explore these petrographic changes, it’s essential to understand what a maceral is—stay tuned for an upcoming episode on that topic!
You may have noticed terms like lignite, bituminous coal, and anthracite—these are coal rank categories that describe the properties of coal at different coalification stages. We will discuss coal ranks in a future episode.
If you have any questions or comments, feel free to email me at Sandra.Rodrigues@carbonmat.com.au.
Further Reading on the Coalification Process:
Taylor et al. (1998) – Organic Petrology. Gebrüder Borntraeger, Berlin-Stuttgart. (Chapter 3 covers coalification in depth.)
Levine, J.R. (1993) – Coalification: The Evolution of Coal as Source and Reservoir Rock for Oil and Gas. In: Law, B.E., & Rice, D.D. (Eds.), Hydrocarbons from Coal, AAPG Studies in Geology, v. 38, p. 9-77. Read more
Wagner, 2021 – Geology of Coal. Available on ScienceDirect (This reference from my colleague Nikki is freely accessible and contains excellent content.)
Stay tuned for more insights into coal science!
