Coal petrology analysis and implications in depositional environments from Upper Cretaceous to Miocene: a study case in the Eastern Cordillera of Colombia

Abstract

The Piedemonte Llanero basin is located on the eastern side of the Eastern Cordillera of the Colombian Andes. It has been the subject of numerous geological studies carried out for the oil sector, mainly. This study presents the coal-petrographical features of 15 coal seams of four geological formations from Late Cretaceous to Middle Miocene (Chipaque Formation, Palmichal Group, Arcillas del Limbo Formation, and San Fernando Formation). Analysis of 33 samples indicates enrichment in vitrinite, while liptinite and inertinite concentrations vary according to the stratigraphic position. Reflectance indicates that the coal range gradually decreases from highly volatile bituminous C (Chipaque Formation) to subbituminous C (San Fernando Formation). The microlithotypes with the highest concentrations are clarite and vitrinertoliptite.Maceral composition and coal facies indicate changes in the depositional conditions of the sequence. The precursor peat from Late Cretaceous to Late Paleocene accumulated under limnic conditions followed by telmatic in Late Eocene- Early Miocene. The coal facies indices show wet conditions in forest swamps with variations in the flooding surface, influxes of brackish water and good tissue preservation. The tectonic conditions along the Piedemonte Llanero basin is evident, from post-rift to foreland basin, evidenced by oxic and anoxic periods reflected in the maceral composition and its morphology. The coal environment corresponds to an estuarine system started in the Chipaque Formation evolving to the lacustrine conditions in the San Fernando Formation.