4D Surface Reconstruction of Micron-Sized Organic Calcite for the Characterization of Chemical Heterogeneity of Chalk Surfaces

Seawater injection into chalk reservoirs is a method for improved oil recovery (IOR) at the Norwegian Continental Shelf (NCS). During the injection of fluids for IOR, complex physicochemical interactions between the injected fluid and the reactive rock surface will take place, such as compaction and alterations of rock properties (e.g., porosity, permeability, wettability). The distribution of noncarbonate mineral phases in the reservoir and on the surface of the chalk will dictate wettability properties. Yet, the identification and quantification of nanometer-sized mineral phases on calcite surfaces have been challenging due to insufficient spatial resolution and sensitivity of the analytical methods used. On-shore chalk was used in this study. Helium ion microscopy (HIM) combined with in situ secondary ion mass spectrometry (SIMS) was used to produce secondary electron images for mapping surface morphology and topography correlated with chemical maps from SIMS. First, a 3D surface model was created from a series of secondary electron images acquired from different perspectives around the coccolith. A chemical image obtained by SIMS was developed on the same region of interest and projected onto the 3D surface model to create a 4D surface reconstruction (3D+1 concept). This includes surface chemistry information on sub-micron-sized noncarbonate phases on calcite grains. The surface distribution of noncarbonate phases added up to a minimum of 6.3%, where no 40Ca had been detected. Moreover, 39.8% of the entire area is characterized by 40Ca and 27Al plus 28Si. Compared with 5 wt % noncarbonate phases identified by whole-rock geochemistry, we identify at least 200-300% higher noncarbonate phase abundance than expected based on bulk geochemistry. This has a significant implication for the modeling of mineral surface charges, the major criteria for wettability calculations. Therefore, HIM-SIMS studies allow nanoscale mapping and mineral phase identification, which will enhance the knowledge of fluid-rock interactions for purposes related to IOR, carbon capture, and storage as well as for hydrogen storage.