Hypothesis: Millions of tons of CO2 are stored in CO2 geological storage (CGS) formations (depleted oil reservoirs and deep saline aquifers) every year. These CGS formations naturally contain small concentrations of water-soluble organic components in particular humic acid (HA), which may drastically affect the rock wettability - a significant factor determining storage capacities and containment security. Hence, it is essential to characterise the effect of humic acid concentration on CO2-wettability and its associated impact on storage capacity. Experimental: To achieve this, we measured advancing and receding contact angles at reservoir conditions using the pendant drop tilted plate method for various humic acid concentrations (1, 10, and 100 mg/L) as a function of pressure (0.1-25 MPa), temperature (303-333 K), and brine salinity (0-0.3 M NaCl). Further, the influence of humic acid adsorption on the mineral's surface was examined by several independent techniques. Results: Our results demonstrate that humic acid significantly changes rock wettability from water-wet (0-50 degrees) towards CO2-wet (90-110 degrees). An increase in pressure, temperature, and salinity had a similar effect. Humic acid adsorption also increased the surface roughness of the substrates. We conclude that even trace amounts of humic acid (i.e. 1 mg/L), which exist in storage aquifers, significantly increase CO2-wettability and thus reduce structural and residual trapping capacities. Therefore, it is pertinent to account for these humic acid concentrations to de-risk CGS projects. (C) 2020 Elsevier Inc. All rights reserved.