The present study consists in the evaluation of Cretaceous rocks from the Middle Magdalena Valley (MMV) basin, Colombia, in the Aptian - Albian and Cenomanian - Campanian intervals, in order to establish if they can be considered as UShR (Unconventional Shale Reservoir). For this purpose, organic geochemistry, organic petrography, palynofacies and textural analyses (porosimetry, N2 and CO2 adsorption isotherms) were carried out. TOC-Leco and Rock Eval pyrolysis data were analyzed for 4235 samples, organic petrography analyses to 104 samples and palynofacies analyses to 469 samples; these samples are distributed in 6 wells of the MMV Basin (Wells EST-1, EST-2, EST-3, EST-4, EST-5 and EST-6). In addition, a textural study (porosimetries, pycnometries, adsorption of N2 at -196ºC and CO2 at 0ºC) was carried out in a series of samples artificially matured by hydropyrolysis to reproduce the evolution of organic matter. The Aptian-Albian interval has high organic content (>4%TOC) and type II kerogen with local contributions of type III kerogen. The analysis of thermal maturity in the northern part of the study area suggests variations from the peak of liquid hydrocarbon generation to the volatile oil window; in the northwest, they vary from the volatile oil window to the wet gas window; and in the south/southeast sector, they vary from the wet gas window to the dry gas window. The evaluation of the presence of hydrocarbons (liquid and gaseous), allowed the identification of a 100 feet (30.5 m) interval in well EST-6 with characteristics of producible hydrocarbons, and in well EST-1 values of interest were identified (67 to 221 scf/ton), with characteristics of a potential gas reservoir. The Cenomanian - Campanian interval, has very good to excellent organic contents (TOC>2%), type II kerogen and a specific interval with type II/III kerogen; thermal maturity is at the early stage in the Coniacian - Campanian interval, and at or near the generation peak, in the Cenomanian - Coniacian interval. Regarding the presence of hydrocarbons, it is suggested the presence of intervals of interest as producible reservoirs in the Coniacian - Campanian interval, probably as a result of the mixture of migrated hydrocarbons with lesser amount of hydrocarbons generated by the rock itself. Likewise, intervals of producible hydrocarbons were identified in the Cenomanian - Coniacian, towards the middle and top of this sequence, and it is supposed that in this case the rock corresponds to an UShR.

The sedimentary environment proposed from palynofacies studies for the Aptian - Albian correspond to a carbonate platform, with fluctuations in the mid to proximal shelf marine environment, while for the Cenomanian - Campanian it corresponds to fluctuations in a distal marine platform and a suboxic-anoxic proximal basin, except for the Middle Turonian, where the basin is shallowed, and marine material with terrestrial input is deposited in a mid to proximal shelf environment. According to the textural analysis of the hydropyrolized rock, the pore volume gradually increases with the thermal evolution of the organic matter. This increase is pronounced in macropores and mesopores with sizes between 5.5-12000 nm. The nitrogen adsorption results indicate a gradual increase in total adsorbed volume (micro-mesopores), as well as, BET specific surface area with increasing thermal maturity. As for the results of CO2 adsorption isotherms, a substantial increase (more than threefold) in micropore volume and Dubinin-Radushkevich specific surface area can be observed between the original sample and the dichloromethane washed sample. This trend is also observed with with increasing thermal maturity. Additionally, a solid bitumen study was performed from which a scatter plot was built between vitrinite reflectance and solid bitumen reflectance, obtaining the equation Roequi =0.8607Robs + 0.2358, where Roequi is the equivalent vitrinite reflectance from solid bitumen and Robs is the measured solid bitumen reflectance.

Tesis doctoral defendida en el Departamento de Geología de la Universidad de Oviedo, 2021

Peer reviewed