Influence of organic matter and mineral composition on carbonate source rock wettability: Implications for CO2 geostorage

• Organic matter influences the wettability of carbonate source rocks in CO 2 /brine systems. • Mineralogical composition affects wettability in both the presence and absence of organic matter. • Organic matter removal shifts wettability from CO 2 -wet to water-wet conditions. • Wettability changes are observed with increased pressure, modulated by rock mineralogy. • CO 2 storage capacity increases after organic matter removal, with notable effects on CO 2 column height. Wettability is critical in determining carbon dioxide (CO 2 ) behavior during geological sequestration in unconventional reservoirs. Unconventional reservoirs are compositionally heterogeneous, affecting CO 2 plume migration, containment security, and storage capacity during geological sequestration. Previous studies on CO 2 storage in unconventional reservoirs have primarily attributed to changes in organic matter content; however, this study examines how variations in the mineralogical and organic matter content combined affect wettability in CO 2 /brine systems under subsurface conditions. Three samples with varied mineralogy and TOC content were selected from a well drilled in Jordan source rocks—an immature analog of marine-derived, carbonate-dominated, Type IIS source rocks. Samples were analyzed using RockEval pyrolysis, X-ray diffraction, X-ray fluorescence, and Thermogravimetric analysis to characterize their organic and inorganic compositions. Samples were pyrolyzed at 600 °C to remove Volatile Organic Content (VOC), followed by a Brunauer–Emmett–Teller analysis and contact angle measurements (advancing: θa and receding: θr ) via a tilted-plate goniometer using the sessile drop method. The results revealed that θa and θr increase with pressure (e.g., from 55° to 70° at 0.1 MPa to 119° to 121° at 20 MPa for organic-rich samples), whereas temperature effects depend on mineralogy, likely due to CO 2 /shale interfacial energy shifts from the changing CO 2 density. Removing VOC reduced the brine contact angles (e.g., from 119° to 121° to less than 80° at 35 °C and 20 MPa), revealing a significant shift from CO 2 -wet to intermediate to strongly water-wet conditions. Additionally, the CO 2 column height considerably increased after the VOC removal, with quartz-rich samples exhibiting the greatest effects (e.g., from –1050 to 3536 m at 35 °C and 20 Mpa). These findings demonstrate how geochemical variability driven by changes in rock mineralogy and organic matter content at subsurface conditions can affect CO 2 storage capacity, plume migration, pore parameters, and competitive adsorption of CO 2 on rock surfaces.