Depositional Controls on Sulfur and Ash in Coal

J. C. Ferm, J. S. Esterle, T. A. Moore, G. A. Weisenfluh

Currently, the two most important measurements of coal quality are ash and sulfur content. Although some sulfur is introduced after coalification, the most sulfur and ash occur when coal is still in the peat stage. Other workers have shown that the major constraints on occurrence and distribution of sulfur in peat are the sulfate content of water coming in contact with peat and the acidity of water within the peat. Sulfate contents are higher in brackish and marine water than in fresh water. Peat water is very acidic due to decaying vegetal matter. Peat water retards the activity of sulfur-reducing microorganisms, and hence the precipitation of sulfur either as pyrite or organic sulfur. Contemporaneous or subsequent flooding by less acid water buffers peat acidity, allowi g sulfur-reducing bacteria to flourish. Floodwaters also increase the ash content of the peat by introducing mineral matter in suspension. Thus, low-ash low-sulfur coal reflects a depositional setting isolated from incoming floodwaters, especially brackish or marine water, but an environment sufficiently wet to prevent oxidation of organic material. Modern examples of such settings occur on the northwest coast of Samawak, Malaysia, and on the Atlantic coastal plain of the southeastern United States. Examples of coal seams interpreted as forming in similar settings are the Hance, Mudseam, and Hazard No. 8 seams in eastern Kentucky.

AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.