Petrological and geochemical characterization of Candiota coal seams, Brazil -- Implication for coal facies interpretations and coal rank

摘要

Seven coal seams, from cores of the borehole M-PV-C-101, Candiota Coalfield located in Rio Grande do Sul State, southern part of the Parana Basin were analyzed by coal petrographical and organic geochemical methods. The seams occur in the Rio Bonito Formation, Guata Group, Tubarao Supergroup of the Parana Basin and are of Permian (Artinskian-Kungurian) age. The composition, rank, coal facies and depositional environment characterization were made based on lithotype, maceral and microlithotype analyses. The data were then used in the definition of different types of facies-critical petrographical indices and were displayed in diagrams of coal facies. In addition, coal seams samples from Candiota Mine were analyzed by proximate, total moisture and gross calorific values and were classified using the United Nations-Economic Commission for Europe (UN-ECE) classification. In addition, Rock-Eval Pyrolysis, Liquid Chromatography, GC and GC-MS analyses were made to obtain independent parameters on coal rank and organic matter composition and environment of deposition. The results suggest deposition in open mires, with elevated contents of detrital macerals and clay minerals and also limnotelmatic coal facies. Coal-bearing sequence of coal seams represent a back-barrier setting. According to the International Classification (UN-ECE), Candiota coals are in the subbituminous (or low rank coal A) stage. Results from geochemical analyses showed that terrestrial organic matter is predominant as indicated by hydrogen and oxygen indices, carbon preference indices and the sterane distribution. Geochemical maturity parameters such as the 20 S/(20 S + 20 R) and alpha beta beta/(alpha beta beta + alpha alpha alpha) ratios for C_(29) regular steranes support the low level of rank (maturity) as indicated by the vitrinite reflectance measurements (0.41-0.45 percent) and gross calorific values (<24 MJ/kg, m, af).