Elemental composition and mineralogy of NW Venezuelan Guasare coals: Provenance study and role of illite in methane sorption capacity

Abstract

A series of coal seams (groups 4 to 11) of Paleocene age in the Marcelina Formation have been open pit exploited in two mines (Paso Diablo and Mina Norte) in the Guasare Coalfield (NW Venezuela). An investigation has been conducted on the elemental composition, mineralogy, and coalbed methane (CBM) in Guasare coals. For this study, 46 core coal samples of 16 seams (groups 2 to 7) were collected from 16 exploratory boreholes that were drilled in both mines. It was also considered unpublished proximate-ultimate and gross calorific data of other 66 coals of 31 seams intersected in most of these wells. The coals under study showed low ash yields (3.4 wt % on average) and were classified as high volatile bituminous A. Mean values of trace element and lanthanide (REE) contents in Marcelina coals generally indicate depletion compared to average values for world hard coals. The statistical evaluations indicate that elements (except Se) show inorganic affinity or intermediate association and mixed mode of occurrence. La/Sc, Th/Sc, La/Co, and Th/Cr ratios support a felsic or metamorphic felsic source for the mineral matter present in Marcelina coals. Most coal samples exhibited similar normalized REE patterns, showing light REE enriched, as well as negative Ce and Eu anomalies. The comparison of REE patterns to those of some nearby granitoids suggests that Marcelina coals received a supply of sediments from theses granitic source rocks. The mineralogical compositions of representative coal samples are notably similar, kaolinite and quartz were noted as the major mineral species in all coal seams. The measured gas volumes available for CBM production from Marcelina seams vary approximately between 5 and 11 cm3/g. Variation in the illite content has an appreciable positive effect on gas adsorption capacity of Guasare coals. Lastly, a numerical model of the gas generative potential of the coals in the Marcelina Formation was also developed.