Petrographic characterization, variations in chemistry, and paleoenvironmental interpretation of Colombian coals

• Published in: International journal of coal geology 2020-07, Vol.227, p.103516, Article 103516
• Main Authors: Patricia, Gómez Rojas, Olga, Blandón, Astrid, Perea, Carlos, Mastalerz, Maria
• Format: Article
• Language: English
• Subjects: Coal; Coal rank; Colombia; FTIR; Maceral chemistry; Maceral composition
• ISSN: 0166-5162; 1872-7840
• DOI: 10.1016/j.coal.2020.103516

This study focuses on variations in petrographic composition and chemistry of macerals from different coalfields in Colombia. The coal-bearing formations in Colombia occur in the age interval from the Maastrichtian in the Upper Cretaceous to the Paleogene and Neogene (Paleocene, Eocene, Oligocene, and Late Pliocene) up to Early Pleistocene. The coals range in rank from lignite to semianthracite. Petrographically, the coal is composed dominantly of macerals of the vitrinite group (>50% by volume). The coals from the regions of Antioquia, Córdoba, and Caldas are characterized by relatively high proportions of liptinite (up to 30 vol%). The inertinite contents vary up to 30 vol%, and the coals from the regions of Boyacá and Guajira contain the highest quantities of this maceral group. The Colombian coals studied have low moisture, except for the coal from Caldas being lignite type A. The sulfur content ranges between 0.28 weigh % in coal from Guajira to slightly above 2% in coal from Córdoba and Cauca. The coal facies diagrams suggest that palaeomires developed mostly under limno-telmatic to wet forest mire conditions. The ratios of microlithotypes suggest that the original peat mires evolved under fluvial, upper deltaic and brackish settings. Overall coal petrography data suggest that peat-forming vegetation and water tables in palaeomires experienced significant variations between the Upper Maastrichtian to Pliocene - Early Pleistocene period. Variations in chemistry of coal macerals reflect to large extent the coal rank variations. Specifically, with the increase in vitrinite reflectance (Ro), the contents of aliphatic functional groups decrease whereas those of the aromatic groups increase. The values of CH2/CH3 ratios also decrease with increasing coal rank (2.24 in Boyacá and 0.97 in Cauca). Expectedly, spectral differences between individual macerals are most distinct in the low rank coals (0.48% Ro). •Colombian coals vary in rank from subbituminous to semianthracite.•Coal was forming in a range of environments from fluvial, upper deltaic to brackish settings.•Coal chemistry depends to large extent on rank.•Spectral differences between macerals are most distinct in the low rank coals.