Petrographic and Petrophysical Characterization of Detrital Reservoir Rocks for CO2 Geological Storage (Utrillas and Escucha Sandstones, Northern Spain)

The aim of this article is to provide a qualitative and quantitative description of Lower–Upper Cretaceous detrital rocks (Escucha and Utrillas sandstones) in order to explore their potential use as CO2 reservoirs based on their petrographic and petrophysical characteristics. Optical microscopy (OpM) and scanning electron microscopy (SEM) aided by optical image analysis (OIA) were used to get qualitative and quantitative information about mineralogy, texture and pore network structure. Complementary analyses by X-ray fluorescence (XRF) and X-ray diffraction (XRD) were performed to refine the mineralogical information and to obtain whole rock geochemical data. Furthermore, mercury injection capillary pressure analysis (MICP), the gas permeameter test (GPT) and the hydraulic test (HT) were applied to assess the potential storage capacity and the facility of fluid flow through the rocks. Both of these factors have an outstanding importance in the determination of CO2 reservoir potential. The applied petrophysical and petrographic methods allowed an exhaustive characterization of the samples and a preliminary assessment of their potential as a CO2 reservoir. The studied conglomerates and sandstones have a porosity range of 8–26% with a dominant pore size range of 80–500 μm. The grain skeleton consists of quartz (95%), very minor potassium feldspars (orthoclase) and a small amount of mica (muscovite and chlorite). According to these preliminary results, among the studied varieties, the Escucha sandstones have the most favorable properties for CO2 geological storage at the rock matrix scale.