Tertiary coals from the La Jagua and Calenturitas coal mines in the Cesar sub-basin, Colombia were investigated for the first time using organic-petrological and organic-geochemical methods to assess the thermal maturity and palaeoenvironmental conditions during peat accumulation. Thirteen seams were sampled from the Los Cuervos formation at two mines. Evaluation based on organic petrological analyses and Rock-Eval pyrolysis reveals that the coals are thermally immature and of sub-bituminous B to sub-bituminous A rank with a predominance of type III, mixed with type II kerogen. 22S/(22S + 22R) homohopane, T-s/(T-s + T-m), MPI, MPR and 1-MP/9-MP ratios are very low. This low thermal maturity corresponds to high volatile matter contents. Petrographically, the La Jagua and Calenturitas coals are dominated by vitrinite, with low to high amounts of inertinite and low to moderate amounts of liptinite. The detailed petrographic data indicate a predominantly herbaceous plant input and oxidative conditions during deposition, mostly with strong tissue destruction. Further conclusions are deduced from petrographic ratios such as tissue preservation-, gelification-, groundwater(Ac)- and vegetation indices, ash- and sulfur contents, iso- and n-alkane distribution, and 17 alpha(H)-homohopane ratio. In summary, the data support a formation in tropical, ombrogenic, rather wet pests with high bacterial activity.