CONTRASTING SIGNATURES OF COAL METAMORPHISM DUE TO DIKE AND SILL INTRUSIONS: AN EXAMPLE FROM THE RATON BASIN, CO AND NM

The Raton Basin, CO and NM contains Cretaceous and Tertiary coal beds. Methane originating from the coals occurs throughout the basin, stored in coal beds and surrounding sandstones. Igneous activity is present throughout the coal-bearing formations as sills and dikes that average 1 meter thick. Sills preferentially intrude along coal beds and are laterally extensive. Some outcrops expose multiple intrusions only a few meters apart. Baked country rock pods also occur in the absence of visible intrusions, suggesting that igneous activity is much more prevalent than surface maps reveal.

This study focuses on understanding the signature of coal metamorphism as a result of igneous activity and its role in coal maturation and methane generation. Coal profiles across several intrusions were studied using vitrinite reflectance, organic petrography and carbon isotopes. The extent and style of metamorphism of coal as a result of igneous activity is dependent on several factors such as coal rank at the time of intrusion, intrusion geometry, thickness, and spacing.

Vitrinite reflectance values always increase towards intrusions, from background values around 1% R_o (depending on location) to peak values of 5% to 6% R_o in coked contact zones, where pyrolytic carbon is often found. Changes in δ¹³C values depend on intrusion geometry: bulk coal becomes enriched in ¹³C by approximately 1‰ approaching dikes, but depleted by approximately 1.5 ‰ approaching sills. Values vary for each outcrop and the total range is from -27.4‰ to -25.2‰. As coal is heated by an intrusion volatiles are driven off which should result in enrichment of the coal in ¹³C as observed at the contacts of dikes. The depletion of ¹³C across sills may be the result of trapped pyrolysates or mineralized carbon as suggested by others and our preliminary petrographic observations.

The difference in isotopic patterns as a function of geometry may be due to the material intruded in each case. Sills are in contact primarily with coal while dikes are in contact with a small amount of coal and mostly organic shale, which has a higher thermal conductivity than coal. Heat may travel away from the contacts of dikes much more quickly than from sills. The longer heating duration for sill contacts may enhance the trapping of pyrolysates or mineralized carbon relative to dike contacts.