Paleoenvironments of coals using organic petrography and their relationship with physicochemical properties, guaduas formation, Checua-Lenguazaque syncline

Abstract

The coal is an important mineral resource used worldwide in the production of coke and thermic energy, the Checua-Lenguazaque syncline in the major zone where coals are exploited in the central zone of Colombia (Cundinamarca and Boyacá provinces) with this purpose; so understand their origin and processes that affected their quality is very important to development a sedimentary model including the thermal history and the depositional environment; these aspects can be evaluated in coals with the use of the organic petrography through the reflectance of vitrinite and maceral reading tests. Vitrinite reflectance indicated that rank increase with depth causing textural, physical and chemical changes along the coalification process varying the content of moisture, volatile matter and fixed carbon. The samples were classified with the ASTM D388-12 norm (High volatile bituminous coals to low volatile bituminous coals) and the ISO 11760 norm (Bituminous coals type C to bituminous coals type A). The Baker equation allowed the determination if the achieved peak temperature for each coal and determine the thermal gradient in the basin concluding that there were different paleogeothermal gradients along the area being more intense in the Samacá section (100°C/Km) than Sutatausa (52°C/Km) and Guachetá (82°C/Km) sections. The maceral reading showed a predominance of macerals of the vitrinite group indicating a good preservation of precursor organic matter, and it allowed the calculate of petrographic indices (GI, TPI, Diessel diagram) (VI, GWI, Calder diagram) to stablish the possible depositional model indicating that coals were deposited in three major environments: Marshes, wet forest swamps and back barrier zones with a fluctuating water level as result of marine incursions increasing the content of framboidal pyrite and calcium; likewise nutrients of peatbog came from rain falls according with the hydrological behavior. The ternary diagrams proposed by Mukhopadhyay and Singh & Singh, give and idea of the predominant vegetation (Forest or reeds), water level, oxic conditions and degradation, showing a preferential behavior to a transition of forest and reed swamps with a medium water level and anoxic conditions, then the paleoenvironments is directly related with the ash and sulfur contents in coals in the study area and they present a correlation with the chemistry water and its level in the peatbog.