99/03569 Controls on the oil and gas potential of humic coals: Isakaen, G. H. et al. Org. Geochem., 1998, 29, (1–3), 23–44

Abstract

Colombia contains large coal reserves of Late Cretaceous to Cenozoic age in various sedimentary basins, but little is known about their potential to generate hydrocarbons. In Western Venezuela, coals are known to be potential source rocks for oil and the vicinity of these coals with coal-bearing basins in Colombia has prompted new studies in Colombian tropical coals to develop concepts about their hydrocarbon generating potential. The Paleogene Amagá Formation studied here, although of limited geographical extension, is an ideal field laboratory which contains low-rank coal seams, in which the transformation and preservation state of organic matter permits the assessment of environment and vegetation types existing at the time of peat formation.Several seams up to 1 m thick have been analyzed using both bulk seam samples and ply (or lithotype) samples. It is demonstrated that bulk seam samples are not representative for a proper evaluation of the hydrocarbon generation potential of seams. One needs to evaluate all lithotype assemblages within a seam. Proximate analysis and Rock-Eval pyrolysis provide a rapid and cheap way to assess the generation potential of each lithotype. The potentially interesting lithotypes can be further analyzed through the combination of petrographic and palynological observations. These methods permit the comparison of coal macerals with the equivalent palynofacies constituents, provided that a preparation technique especially designed for tropical, sub-bituminous, Tertiary coals is used.The palynofacies technique has proved to be more accurate than petrography in determining the relative proportion of the potentially oil-prone constituents/macerals, which can be hidden or difficult to identify in petrographic polished section. Moreover, the comparison between resinite oil observed in petrography and resins identified in palynofacies may yield indications of early hydrocarbon generation.This methodology provides a fully detailed inventory of coal properties with respect to hydrocarbon generation potential. The use of palynofacies is an additional tool to apprehend the floral composition and paleoenvironment of coals, thereby increasing the geological and geochemical database needed to evaluate their potential. Such a methodology may prove very important to assess the petroleum potential of thicker coal deposits in prospective petroleum provinces, such as the Cerrejon coals in Northern Colombia.