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

Abstract

The exploration well ANH-CR-Montecarlo-1X, in the southern part of the Cesar-Ranchería basin in Colombia, cuts sedimentary rocks assigned to the Cogollo Group (Barremian-Cenomanian). The present study integrates 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 potential of these rocks as a conventional or unconventional hydrocarbon reservoir. The results of petrographic analysis revealed that the rocks correspond to quartz sandstones and greywackes and the carbonate rocks to mudstones and wackestones. These were affected by diagenetic processes such as compaction and mineral neoformations. The latter include minerals from the clays group that cover the grains of the rock framework, and other types of precipitates, such as silica, ferrous and non-ferrous carbonate that cement them. Some of these minerals present partial or total dissolution, for which secondary porosity is recognized. Chloritization and illitization of the argillaceous matrix and feldspars alteration to ferrous and non-ferrous carbonate are also observed, which affects the porosity and permeability of the rock. Additionally, during diagenesis micrite is recrystallized to sparite and carbonate precipitation ocurred in the calcareous rocks. The study also shows that the rocks were more affected by compaction than precipitation of the different types of cement, thus reducing their porosity and permeability. Based on these parameters the siliciclastic rocks in study area have a low potential as conventional reservoir but a good potential as non-conventional reservoir (tight sandstones). The geochemical studies in the calcareous rocks exhibit low content of total organic carbon (between 0 and 4.5% with an average of 0.71%), with kerogens type III and IV. They have high levels of organic matter conversion, indicative that the rocks did generate hydrocarbons, but currently are depleted and in a mature state, compatible with a low potential of hydrocarbon generation.