Microdroplets initiate organic-inorganic interactions and mass transfer in thermal hydrous geosystems

Abstract

Organic-inorganic interactions regulate the dynamics of hydrocarbons, water, minerals, CO₂, and H₂ in thermal rocks, yet their initiation remains debated. To address this, we conducted isotope-tagged and in-situ visual thermal experiments. Isotope-tagged studies revealed extensive H/O transfers in hydrous n-C₂₀H₄₂-H₂O-feldspar systems. Visual experiments observed water microdroplets forming at 150-165 °C in oil phases near the water-oil interface without surfactants, persisting until complete miscibility above 350 °C. Electron paramagnetic resonance (EPR) detected hydroxyl free radicals concurrent with microdroplet formation. Here we propose a two-fold mechanism: water-derived and n-C₂₀H₄₂-derived free radicals drive interactions with organic species, while water-derived and mineral-derived ions trigger mineral interactions. These processes, facilitated by microdroplets and bulk water, blur boundaries between organic and inorganic species, enabling extensive interactions and mass transfer. Our findings redefine microscopic interplays between organic and inorganic components, offering insights into diagenetic and hydrous-metamorphic processes, and mass transfer cycles in deep basins and subduction zones.

Fig. 1. Formation and evolution of water/oil microdroplets near the interfaces from low to high temperatures in three different FSCT systems.

a1–a6 microdroplets evolution in a system with water and n-C₂₀H₄₂. b1–b6 microdroplets evolution in a system with water and liquid hydrocarbon from pyrolyzed n-C₂₀H₄₂. c1–c6 microdroplets evolution in system with water and crude oil. Microdroplets do not form at low temperatures. However, when the temperatures exceed 150–165 °C, high temperature and high pressure facilitate the creation of numerous water microdroplets at the oil–water interface. These microdroplets range in size from 5 µm to 30 µm and undergo continuous dynamic evolution. Small microdroplets converge into large microdroplets, which then burst to generate small microdroplets. These microdroplet persist until complete miscibility is achieved between water and oil at temperatures exceeding 350 °C.

Fig. 2. Electron paramagnetic resonance (EPR) spectra of hydroxyl free radicals (OH*) obtained in three different water–oil systems, with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the probe.

a EPR spectra obtained at 200 °C and 140 °C in system with water and n-C₂₀H₄₂. b EPR spectra obtained at 200 °C in system with water and liquid hydrocarbon from pyrolyzed n-C₂₀H₄₂. c EPR spectra obtained at 200 °C in system with water and crude oil. Detailed data have been deposited in Figshare.

Fig. 3. Yields of gases and gas chromatograms of liquid hydrocarbons after thermal experiments.

a yields of C₁–C₅, H₂, and CO₂ in different thermal experiments. The analytical uncertainties for the yields of gas products had a relatively small error of <0.5% (see “Methods”). b gas chromatograms of liquid hydrocarbons in experiments I–IV, the dashed curves in (b1–b4) represent the pattern of the main compositions of the liquid hydrocarbons generated in experiment I with only n-C₂₀H₄₂. Detailed data for (a) listed in Table 1.

Fig. 4. Leaching of feldspars and precepitation of secondary minerals in thermal experiments.

SEM images of the original K-feldspar grains used in the experiments (a), leached K-feldspar and authigenic minerals after the 14-d experiments (b–l). a surface of the original K-feldspar grains. b surface of the K-feldspar after experiment in the anhydrous n-C₂₀H₄₂ + feldspar system (II). c extensively leached feldspar (LF) in the C₂₀H₄₂ + H₂O + feldspar system (III). d extensively leached feldspar (LF) and euhedral boehmite (Bo) in the C₂₀H₄₂ + H₂O + feldspar system (III). e extensively leached feldspar (LF) in the C₂₀H₄₂ + D₂O + feldspar system (IV). f leached feldspar (LF) and euhedral kaolinite (Kao) in the C₂₀H₄₂ + D₂O + feldspar system (IV). g flower-like illite (muscovite) aggregates in the C₂₀H₄₂ + D₂¹⁸O + feldspar system (V). h lash-shaped boehmite in the C₂₀H₄₂ + D₂¹⁸O + feldspar system (V). i, j thin plate-shaped illite (muscovite) aggregates on the leached feldspar surface in the C₂₀D₄₂ + H₂O + feldspar system (VII). k subhedral plate-shaped illite (muscovite) on the leached feldspar surface in the C₂₀H₄₂ + H₂O + feldspar system (VIII). l small euhedral illite (muscovite) on the leached feldspar surface in the H₂O + feldspar system (IX). In the systems with only 20 mg feldspar, the feldspars were leached quite extensively (c–f), and secondary minerals, including kaolinite and illite, were precipitated on K-feldspar surfaces and were also detected in the water solutions. In the systems with 2 g feldspars, the feldspar grains were dissolved, and illite and muscovite precipitated on the K-feldspar surfaces (g–k) and were also detected in the water solutions.

Fig. 5. Isotopic compositions of hydrocarbons, CO2, water, and clay minerals in the different anhydrous and hydrous systems with and without tracing isotope of D and 18O.

a δD of different gaseous and liquid hydrocarbons in hydrous systems with and without D₂O. b ¹⁸O of water and CO₂ in different hydrous experiments. c δD of water and clays in in different hydrous experiments. The analytical uncertainties for the determination of δD and δ¹⁸O were better than 2.0‰ and 0.1‰, respectively (see “Methods”). Detailed data are listed in Supplementary Tables 3–5.

Fig. 6. Deuterium (D)-NMR (nuclear magnetic resonance) of liquid oils and water after experiments.

a D-NMR of liquid organics in systems with and without D-labeled water. Peaks at 0.5–1.7 ppm, 1.7–2.5 ppm, and 6.7–7.0 ppm represent deuterium in alkanes, oxygen-containing organics (ketones), and organics with benzene rings, respectively; b D-NMR of water in systems with and without D-labeled n-eicosane. Detailed data have been deposited in Zenodo.

Fig. 7. Schematic diagram showing the pathways of microdroplet-induced interactions between alkane and water at elevated temperatures.

Step-1 represents the formation of water microdroplets in the alkane phase; step-2a represents the formation of water-derived free radicals based on the water microdroplets; step-2b represents the formation of alkane-derived free radicals; step-3 represents the recombination of different free radicals to form different species.

Fig. 8. Schematic diagram showing the pathways of mineral alteration in thermal systems with water and K-feldspar.

H⁺ formed via water ionization reacts with feldspar, inducing various mineral alterations. The dissolution and precipitation of minerals like feldspar, boehmite, and illite / muscovite, along with the ionization and reformation of water molecules, entail the release or consumption of ions and solutes (H⁺, OH⁻, Al³⁺, K⁺, and SiO_2(aq)) into or from the water solution. These processes potentially lead to H/O exchange between water and minerals. The L and B sites represent the Lewis acid sites and the Brønsted acid sites in the aluminosilicate minerals, respectively (re-use permission has been obtained from Elsevier).

Fig. 9. Schematic diagram showing the microdroplet-induced model for organic-organic interactions and mass transfer among different species in thermal alkane–water-feldspar hydrous systems.

a Low temperature stystem without formation of microdroplets near the interface btween alkane and water zones. b Formation of microdeoplets near the alkane-water interface at elevated temperatures higher than 150 °C. c Occurrence of free radical reactions, ion reactions and mass exchanges in diffferent zones of the thermal alkane-water-feldspar system. In the water zone, water-derived ions react with minerals, lead to continuous mineral dissolution and precipitation, as well as H/O exchange between water and minerals. In the oil zone, high temperature facilitates the formation and recombination of alkane-derived free radicals, resulting in free radical thermal-cracking and cross-linking reactions. In the water–oil mix zone, water microdroplets form near the water–oil interface, trigging the formation of water-derived free radicals and initiating the organic–inorganic interactions between water and oil. The formation and recombination of water-derived and alkane-derived free radicals result in H/O exchange among water, alkanes, organic acids, CO₂, and H₂. These processes, with water serving as a bridge, erase the conventional boundaries between oil and minerals, facilitating the transfer of alkane/water/mineral-derived H/O among the newly formed organic and inorganic compounds.