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Magnesium-Rich Rims in Calcite Microcrystals: Possible Cause of Water-Wet Conditions in Microporous Tor Formation Depositional Chalk


Microporosity in carbonates is responsible for complicating oil recovery, causing inaccurate recoverable reserve estimates, and producing high-water saturation measurements on wireline logs. The microcrystals that occlude micropores increase surface area and play a critical role in fluid flow dynamics during hydrocarbon recovery. Bulk geochemical and petrophysical analyses imply progressive calcite cementation with burial.

The Kozeny-Carmen relationship models permeability as a function of pore throat diameter, porosity, and surface area. Crystal growth causes an increase in surface area, which will concomitantly increase the rate of chemical reactions between mineral and fluid, altering reservoir properties, like wettability. Most carbonate reservoirs are moderately oil-wet, but the Tor Formation (Ekofisk Field, Norwegian North Sea) has been shown to be more water-wet. However, micropore-scale wettability factors are poorly understood.

Mg is incorporated preferentially into the crystal lattice during calcite crystal growth at higher temperatures and pressures but can also replace precipitated Ca already in the crystal surface. When Mg replaces Ca, the attached carboxyl group is also displaced, allowing the crystal face to become more water-wet.

Scanning Electron Microscopy and Energy Dispersive X-Ray Spectroscopy were used to examine chemical zonation in the calcite microcrystals from Tor Formation (Late Campanian to Maastrichtian deep-water chalk) core samples. Microcrystals ranged in size from 1 to 10 microns and Mg-zonation is evident. Mg/Ca ratios in mmol/mol shift from the core to the rim of the crystal. Large crystals, 5-10 microns in diameter, tend to have low-Mg content core (0-7 mmol/mol). Small crystals, 1-5 microns in diameter, tend to have higher Mg cores (6-34 mmol/mol). Some samples exhibit large core-to-rim variation (22.2 to 5.7 mmol/mol Mg/Ca), while others less so (9.7 to 7.6 mmol/mol Mg/Ca). We find that magnitude and type of zonation is correlated with crystal diameter.

Therefore, the Mg zonation of Tor calcite microcrystals likely leads to the observed variation in wettability. This finding could not have been made by using traditional bulk geochemistry because such measurements average many discrete zones across ~300,000 microcrystals. Characterization of microcrystal chemical zonation therefore may lead to more effective water flood chemical formulations because they can be based on surface chemistry, not bulk chemistry.