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.

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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.

For several decades, some coal petrographic properties have been proposed as important parameters in the methane gas sorption processes. In this contribution, the petrographic variables (Vitrinite Ratio, Inertinite Ratio, the petrographic indexes (Gelification Index, Groundwater Index, Tissue Preservation Index, Vegetation Index, Vitrinite/Inertinite ratio, and the Vitrinite Reflectance were evaluated according to the maceral preservation and were related with the desorbed gas content. Twenty-five coal seams obtained from the drill cores of two wells in the Landázuri Area-Valle Medio del Magdalena basin were analyzed. The coal samples were grouped according to gas content using principal component analysis (PCA). The petrographic results were analyzed by linear regression and multiple regression. The Medium Volatile Bituminous to Low Volatile Bituminous coals from Landázuri 1 are twice as high in gas content that the High Volatile Bituminous A to Medium Volatile Bituminous coals from Landázuri 2. The volume percentage and the preservation degree macerals are related closely to the gas content and the pore's size involved in the sorption process. The Inertinite is the maceral group related with the highest gas content groups in Landázuri (600 SCF-Standard Cubic Feet/ton, 300 SCF/ton), while the other groups show the correspondence with the vitrinite macerals. The syngenetic and diagenetic origin of the pyrite contributes microporosity to the desorption process, while the pyrite epigenetic by its size reduces it. The petrographic indexes reveal that the Upper Cretaceous coals were developed in swampy or lacustrine continental basins- limnic facies.

Coal petrological characteristics along the Piedemonte Llanero and the reconstruction of the deposit environment were obtained from macerals and micro-lithotypes analysis since these data provide information about the processes and prevalent conditions during the peat formation. We analyzed seams from Cenomanian to Miocene geological units (Chipaque Formation, Palmichal Group, Arcillas del Limbo Formation, and San Fernando Formation). Coal range decreases gradually from high-volatile C bituminous (HVCB) in the Chipaque Formation to sub-bituminous C in the San Fernando Formation. The coals are enriched in macerals of vitrinite, whereas the liptinite and inertinite concentrations vary according to the stratigraphic position. The micro-lithotypes are bi-maceral and tri-macerals, being the highest concentrations of clarite and vitrinertoliptite. The results of the facies analysis show that the peat in which the coals developed is mainly of arboreal and herbaceous affinity (rich in lignin and cellulose). Peats are ombrotrophic (rainfed) to mesotrophic (transitional or mixed mires) with variations in the flooding surface and influxes of brackish water. Good tissue preservation is inferred from the wet conditions in forest swamps with few humification and gelation. According to the micro-lithotypes composition, the peat environment was deduced as estuarine system, evolving to lacustrine environment of the deltaic system, both restricted by changes in sea level, which are evidenced by oxic and anoxic periods in the analyzed sequence.