GoKawiilChemical composition of the target subsurface basalts
Cuttings collected from the injection and production wells were characterized by X-ray fluorescence (XRF) to estimate their chemical compositions. The XRF measurements were performed at the laboratories of Isotope Tracer Technologies Europe (IT2E) in Milan, Italy and at King Abdullah University of Science and Technology (KAUST) in Thuwal, Saudi Arabia.
At the IT2E laboratory, the samples were pulverized with an agate mortar and pestle and their volatile contents were determined by the loss on ignition method. The powders were then dried in an oven at 110 °C overnight, heated in a muffle oven to 1,000 °C, mixed with powdered boric acid and compressed into pellets. The pellets were loaded onto an ARL automatic ADVANT XP spectrometer equipped with a Rh front-window X-ray tube. Analyses were performed using an applied power of 3.0 kW. The count times on identified peaks were 10 s for major elements and 40 s for trace elements. Matrix correction and interelement effects were accounted for using the method of Lachance and Traill30. Analytical uncertainties were determined by analysis of international standards with known compositions. For the major elements, the uncertainties in Si, Ti, Ca and K are less than 3%, whereas for Al, Mn, Mg, Na and P, the uncertainties are less than 7%.
The samples analysed at the KAUST laboratories were first ground to a powder. Approximately 1 g of each sample was mixed with 9 g of XRF Scientific X-ray flux powder composed of 66% lithium tetraborate and 34% lithium metaborate. This mixture was melted at 1,050 °C in an Eagon 2 fusion instrument and poured to form a homogenous pellet that was analysed on a Bruker S8 TIGER machine. The detection limit was <150 ppm for all elements and the uncertainty was <1% for the major elements.
The mineral phases present in the ground well cutting samples were characterized by X-ray diffraction (XRD) analyses conducted at the Exploration Core Labs Department (ECLD) of Saudi Aramco and at IT2E. The XRD analyses at ECLD were run on powders ground with an agate mortar and pestle using a Rigaku Ultima IV powder X-ray diffractometer with CuKα radiation (40 kV, 40 mA) over the 3°–70° (2θ) interval, with a step size of 0.02° increment and a scan speed of 1° s−1. We interpreted the XRD patterns using X’pert HighScore software using specific crystallographic information files for Rietveld refinement and conducted cluster analysis using JADE Pro and its toolkit.
The XRD analyses at IT2E were performed on samples dried at 60 °C and then ground in an agate mortar and pestle. Each sample was then loaded on a onto a polymethylmethacrylate (PMMA) sample holder and placed in a Bruker D8 ADVANCE DaVinci automatic powder diffractometer, equipped with a LYNXEYE detector set to discriminate CuKα 1,2 radiation. The interpretation of the diffractogram for phase identification was carried out by comparison with crystalline phases of the PDF-2 International Centre for Diffraction Data (ICDD) and Crystallography Open Database (COD) databases. A preliminary semi-quantitative estimate of weight fractions was carried out by using the normalized reference intensity ratio method (also known as the Chung method31). This method uses scaling factors assigned according to the heights of the characteristic peaks of the different phases. No internal standard was added, so the presence of any amorphous phase was not checked. Quantitative phase analysis of each sample was achieved by the Rietveld profile fitting method as implemented in the Bruker TOPAS V.5 program. This is based on the fundamental parameters approach32. The crystal structure models of crystalline phases considered in the XRD profile fitting for andesine plagioclase, augite pyroxene, clinochlore chlorite, montmorillonite, quartz, laumontite zeolite, calcite and richterite amphibole were taken from the Inorganic Crystal Structure Database (ICSD) Release 2021-2 (ref. 33) (Supplementary Table 4). Unit cell parameters, scale factors and crystal sizes were allowed to vary for all phases. Atomic coordinates and atomic displacement parameters were fixed while site occupancy factors of octahedral cations and extra-framework species were adjusted, with restraints, in richterite and laumontite, respectively, to account for the crystal-chemical variations of these phases compared with the model. Rietveld profile fitting allowed testing of the presence of mineral phases preliminarily identified and semi-quantified by the reference intensity ratio method34. The resulting chemical and mineralogical compositions of the subsurface drill cuttings are provided in Extended Data Tables 1 and 2.
Chemical analysis of production well fluid samples
Fluid samples were regularly collected from a dedicated outlet port located at the production well. Fluid temperature, pH at the reservoir temperature of 45 ± 0.5 °C and total dissolved solids were measured directly at the fluid sampling port using a Myron L Ultrameter II 6PFC multimeter. The pH electrode was regularly calibrated using Mettler Toledo pH 4.01, 7.00 and 10.01 standard buffer solutions. The uncertainty of the pH measurements was ±0.02 based on replicate analyses of the standard buffer solutions. Measured on-site pH values are reported in Fig. 2. Samples for alkalinity measurement were collected in cleaned 500-ml polyethylene terephthalate (PET) bottles. Further samples were immediately acidified, after filtering using a 0.22-μm syringe filter, by adding 2–3 drops, or approximately 0.01 ml of double-distilled nitric acid, containing 67–69% by weight HNO 3 , to 50-ml production well samples in cleaned PET bottles. The PET bottles originally contained drinking water but were rinsed several times first with the production well water before sampling. Both sets of samples were stored in an insulated cooler until transported for chemical analysis. The alkalinity of the first fluid sample was measured by acid titration using the Gran function plot method35. The measured alkalinity was then used to calculate the DIC concentration using PHREEQC36 together with the measured pH and fluid compositions of the major elements at the 45.5 °C subsurface temperature. PHREEQC is a geochemical modelling code designed to perform a variety of aqueous geochemical calculations, including calculations of saturation indices. The second fluid sample was analysed for major cation and Si concentrations by inductively coupled plasma optical emission (ICP-OES) spectroscopy using an Agilent 5110 ICP-OES at KAUST. This spectrometer analysed the compositions of Fe, K, Mg, Si, Ca and Na using wavelengths of 239.563, 769.879, 279.800, 251.611, 315.887 and 568.821 nm, respectively. This instrument was calibrated using Sigma-Aldrich ICP standard solutions of 0.1, 1, 10 and 100 ppm concentration. The limits of detection for Ca and Si were 0.01 and 0.02 ppm, respectively. The analytical uncertainty was <0.1 ppm for all measured elements. To assess the potential contamination from the use of the PET bottles used in fluid sampling, three blanks were prepared using distilled demineralized water. These blanks were prepared and analysed identically to those used for the production well sampling. In each case, the concentration of all measured elements in the prepared blanks was below the respective analytical detection limits. Measured concentrations over time of all sampled fluids are illustrated in Extended Data Fig. 2 and tabulated in Supplementary Table 1.
On the basis of the ICP-OES, pH and alkalinity measurements, the saturation state of the sampled fluids with respect to selected minerals was calculated using PHREEQC36 with its Kinec_v3 database23. The Kinec_v3 database is the most recently updated database for use with PHREEQC. As the concentration of Al was below the analytical detection limit of our measurements, the concentration of this element was set to be in equilibrium with diaspore in the calculations. This choice was made as diaspore is readily observed to precipitate during experimental studies of basalt dissolution and that chlorite and zeolite minerals, such as clinochlore and laumontite, are more soluble than diaspore at our field conditions. The resulting saturation indices are provided in Supplementary Tables 2 and 3 and Extended Data Fig. 3.
Carbon isotope measurements
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