Coalification patterns and thermal-depositional history of the Upper Cretaceous-Paleogene coal in the Eastern Cordillera Basin, Colombian Andes

Abstract

Understanding coalification patterns and thermal history in coal-bearing sequences is critical for basin analysis. In most basins, coalification is linked to maximum burial temperatures. However, syn/post-deformational thermal overprinting may occur in tectonically active settings. This study evaluates the coalification history of the Cretaceous-Paleogene Guaduas Formation in the Colombian Andes, a 1000 m thick sequence with extensive coal seams and significant gas generation potential. A comprehensive methodology was applied, including a geological survey, open system pyrolysis (Hawk), organic petrology, and thermal history modeling of 118 coal samples. The gas content was also determined using the desorption method. Total Organic Carbon (TOC) varied from 19.29 wt% to 82.73 wt%, with Tmax values between 430 °C and 494 °C. The random vitrinite reflectance (VRr%) ranged from 0.46 % to 1.65 %, indicating coal rank from Sub-bituminous A to Bituminous A (ISO 11760 classification). Paleotemperatures ranged from 72.5 °C to 175.9 °C, with gas content varying between 6.01 ft³/ton and 55.23 ft³/ton. Paleogeothermal gradients ranged from 63.4 °C/km to 94.9 °C/km. Coal rank generally increases with depth, highlighting burial as the primary coalification driver. However, variations in VRr% within individual seams suggest irregular coalification linked to multiple deformation phases. An increase in paleotemperature toward the northern sector correlates with enhanced tectonic activity. Burial modeling indicates over 4100 m of sedimentary overburden, followed by significant Cenozoic erosion after the Andean uplift. The organic matter (type II-III) and the combined influence of burial and tectonic processes played a fundamental role in organic matter maturation and the development of gas generation potential.