Diagenesis and Thermal Maturity of the Cogollo Group rocks in the ANH-CR-Montecarlo-1X well, Cesar-Ranchería Basin, Colombia

Abstract

In ANH-CR-Montecarlo-1X well located in the southern sector of the Cesar-Ranchería basin, Colombia, rocks of the middle Cretaceous outcrop, which have been defined as belonging to the Cogollo Group. The present study concerns with the diagenetic evolution and thermal maturity of this geological unit, integrating petrographic techniques (thin section microscopy and scanning electron microscopy), geochemistry (total organic carbon and pyrolysis rock-eval) and basic petrophysics, to establish the thermal maturity and the potential of rocks as reservoirs of conventional and unconventional hydrocarbons. The results of petrographic and diagenetic analysis revealed that the rocks compositionally correspond to quartz sandstones and graywackes and carbonate rocks to mudstones and wackestones; which were affected by diagenetic processes such as compaction, mineral neoformations distinguishing minerals from the group of clays that cover the grains of the framework, and other types of precipitates of cements such as silica, ferrous and non-ferrous carbonate, some of these present partial and/or total dissolution, for which secondary porosity is recognized, also metasomatisms, where processes of chloritization and illitization of the argillaceous matrix are observed, alteration of feldspars to ferrous and non-ferrous carbonate, which affects the porosity and permeability of the rock; and recrystallization from micrite to sparite and carbonate precipitation in calcareous rocks. The study also showed that the rocks were more affected by compaction than precipitation of the different cements. The research contributes to the understanding of the impact of diagenetic processes on porosity, as well as their spatial and temporal distribution, providing diagenetic paragenesis for both siliciclastic and carbonatic rocks. When we classifying siliciclastic rocks as potential reservoir rocks, low potential results were obtained as conventional reservoirs but has good potential as non-conventional reservoirs (tight sandstones), this respect to porosity and permeability data. The geochemical studies in the calcareous rocks exhibited low to good content of total organic carbon, overmaturity state and a low generation potential with type III and IV kerogens.