Submarine lobe complexes as CO2 storage sites: the role of mineralogical heterogeneity and mineral trapping

Submarine fans are volumetrically the largest and most extensive sedimentary bodies on Earth, but current understanding of their behavior as fluid reservoirs is largely based on the properties directly relevant to hydrocarbon extraction. CO2 injection, migration and storage are dependent on many of the properties important in hydrocarbon extraction, but reactions between CO2, brines and reservoir rock make the role of pore- to reservoir-scale heterogeneities substantially different. Mineral trapping of injected CO2 is viewed as the final step in CO2 trapping, following physical, residual and solubility trapping. Its importance is less well understood and operates on a longer timescale than physical trapping. Here we investigate the role of mineralogical heterogeneity within deep-marine facies and lobes, using examples from subsurface CCS targets and outcrop analogues, and consider sedimentological factors affecting the likelihood of permanent mineral trapping. Dissolution of CO2 within reservoir pore-water (affected by mineral and brine chemistry), acidifies the brine and promotes mineral trapping reactions. Quartz-rich reservoirs are considered largely unreactive, requiring 1000s of years for any important level of mineralization to occur, while reservoirs rich in mafic and carbonate minerals are more reactive over shorter time scales. Mineralogy in deep-marine sedimentary environments is fundamentally governed by sediment provenance, which is then modulated by the hydrodynamic behavior of sediment grains within gravity-driven flows. Factors impacting hydrodynamic behavior include grain size, shape, density and electrostatic attraction. Results show that mineralogical fractionation is expressed at laminae-scale within beds, and lobe-scale within fans, where reactive minerals are distributed around distal frontal and lateral fringes. Behavior of CO2 at these scales is investigated using reactive transport modelling of characteristics reservoir facies and mineralogy to develop reservoir-scale conceptual models. Lobe fringes may represent the ultimate pinchout of CCS reservoirs, representing zones of enhanced reactive mineralogy, supporting their long-term viability as safe storage sites.