The role of solid bitumen in the development of porosity in shale oil reservoir rocks of the Upper Cretaceous in Colombia

The La Luna Formation (Turonian–Santonian age) has been traditionally considered as one of the main hydrocarbon source rocks in the Middle Magdalena Valley Basin (MMV) in Colombia. At present this formation is also considered as an organic matter-rich reservoir of shale oil type (unconventional petroleum system) of interest due to its significant resources of recoverable oil. Given the interest in this formation, and the need to increase the knowledge of its physico-chemical characteristics, a series of petrographic, geochemical (TOC content and Rock-Eval pyrolysis) and textural analyses (picnometry and mercury intrusion porosimetry, MIP) were carried out to investigate two of its members: Galembo and Salada that were sampled from a core well perforated at the center of the basin. The organic fraction of both members, Galembo and Salada, is made up mainly of a kerogen of Type II typical of a marine paleoenvironment, solid bitumen and hydrocarbons. The content in organic matter of terrestrial origin is very scarce-to-null. These rocks have a degree of maturity that corresponds to the oil window, and their high hydrogen content in combination with a relatively high carbon content makes them excellent source rocks. The uneven distribution of the various types of organic matter along the Galembo and Salada Members, particularly of the solid bitumen that is concentrated at the lower levels of both members has resulted in large variations in porosity, pore size distribution and modal pore throat characteristics even with small variations (centimetric) in depth. The sedimentary levels in which kerogen is the predominant type of organic matter have a pore size distribution in the range of mesopores and macropores of small size, whereas the solid bitumen-rich levels show a predominance of pores of small pore size with a modal pore throat diameter below 10nm. The preliminary adsorption data obtained for all these samples (to be published in a study that will follow this), indicate that they contain some micropores, which is significant in the case of levels with a high content in solid bitumen.