3 years ago

Thin-film dawsonite in Jurassic coal measure strata of the Yaojie coalfield, Minhe Basin, China: A natural analogue for mineral carbon storage in wet supercritical CO2

A large amount of thin-film dawsonite has developed in coal cleats and also in fractures within the roof sandstone in Jurassic CO2-rich coal measure strata of the Yaojie coalfield, Minhe Basin, China. The CO2 in these strata has been in a supercritical state since natural emplacement. In this study, the Yaojie coalfield is considered a natural analogue site for CO2 capture, utilization, and storage (CCUS). The thin-film dawsonite in coal measure strata was analyzed to investigate the evolution of carbon in supercritical CO2 after CCUS, as it provides a possible example of the formation of carbonate minerals from a wet supercritical CO2 fluid. The cleat- and fracture-filling dawsonite takes the form of white or silver-white radial thin-film crystals (generally <3.2mm in diameter and <20μm in thickness) and appears in a variety of shapes, with the mineral fibers growing divergently within the plane of the film. The thin-film dawsonite within cleats is readily separated from the coal, and its crystal morphology is controlled by the cleat space. The thin-film dawsonite in the roof sandstone does not completely fill fractures, and the morphological characteristics of the surfaces of the mineral crystals indicate that crystal growth was not constrained by the surrounding rock. The sources of the carbon for the cleat-filling dawsonite were magmatic CO2 and CO2 derived from the decomposition of basement marble, which resulted from dynamic–thermal metamorphism of the F19 fault zone. The carbon isotopic compositions of the cleat-filling dawsonite from different carbon sources are transitional, and the carbon source for all of the sandstone fracture-filling dawsonite is the CO2 released from the decomposed basement marble. The morphological and mineralogical characteristics of the thin-film dawsonite show that it most likely nucleated and grew directly in the liquid-like water film formed by wet supercritical CO2 on solid phase surfaces, rather than by epitaxial growth on the rocks surrounding the cleats and fractures, probably via layer-by-layer crystallization. The sodium and aluminum in the dawsonite were derived from formation water. The formation of dawsonite in the coal measure strata of the Yaojie coalfield indicates that some of the free-state CO2, which increases in content due to desorption of the coal matrix, can ultimately be converted into carbonate minerals during CCUS.

Publisher URL: www.sciencedirect.com/science

DOI: S0166516217302124

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