Thermal history of the Upper Cretaceous-Paleocene Guaduas Formation, Eastern Cordillera Basin of Colombia: implications for timing of coalification

The Guaduas Formation, which spans from the Upper Maastrichtian to the Lower Paleocene, is the primary source of thermal and coking coal in the Eastern Cordillera of Colombia. This study analyzed 64 coal samples collected from the southern (Sutatausa), central (Guachetá), and northern (Samacá) sections of the Checua-Lenguazaque Syncline (CLS) coalfield, where this coal-bearing unit reaches approximately 1100 meters at its deepest point. Mean random vitrinite reflectance (VRr%) was integrated with numerical modeling of thermal histories, subsidence analysis, and estimations of eroded section thickness. The results reveal significant variation in VRr% values, which can be attributed to complex and contrasting tectonic and thermal histories, as well as prolonged residence times near the oil and gas generation window. The minimum estimated thickness of the eroded sedimentary succession is approximately 1000-1300 m, consistent with the maximum depth of the decompacted Guaduas Formation. In the Samacá section (northwards), two distinct VRr% populations were identified: one ranging from 0.8 to 1.0, and another from 1.2 to 1.5. These populations correspond to two different thermal residence periods, occurring at temperatures of approximately 120°C and 160°C, respectively, between 70 and 40 Ma. In the Guachetá section, thermal modeling of 18 samples indicates a rapid cooling phase between 70 and 55 Ma, followed by a reheating phase between 55 and 30 Ma. This was succeeded by a prolonged residence or slow cooling period at temperatures near 160°C, with VRr% values ranging from 1.4 to 1.8. For the Sutatausa section (southwards), a cooling pulse occurred between 70 and 58 Ma, followed by a heating event between 60 and 45 Ma. Subsequently, a prolonged residence period at temperatures close to 120°C was observed, resulting in VRr% values ranging from 0.6 to 1.0. The reheating phases of the organic matter align with periods of subsidence previously reported for the study area and the adjacent sedimentary basins (Middle Magdalena Valley and Llanos). At the same time, the most recent cooling pulse is associated with the post-Miocene exhumation of the Eastern Cordillera during the Andean Orogeny.