Possible Cause for Texture Variation in Low-Magnesium Calcite Microcrystals

Abstract

Low-magnesium calcite (LMC) microcrystals are ubiquitous in carbonate reservoirs and constitute the framework that houses micropores. Oil column height in giant oilfields is often sufficient to overcome the entry pressure needed to charge these micron-sized pores. Charged micropores then create a production conundrum of whether to quickly produce the reservoir (and bypass these micropores) or effectively produce the reservoir (which would require slower production rates). LMC microcrystals can be classified into three different textural classes, granular (subhedral or euhedral), clustered (loose or fused), and fitted (partial or fused) which have specific petrophysical characteristics. Microcrystal textures will further impact production because textures dictate petrophysical properties such as porosity, permeability, and pore throat radius. What remains in question is the genetic relationship between LMC microcrystal texture and the diagenetic environment in which they form because heterogeneity occurs laterally and vertically through facies. Calcite precipitation is thought to be a surface-controlled, therefore surface chemistry could hold the key to linking LMC microcrystal texture to physiochemical parameters. This study utilized high resolution scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) to investigate the Mg/Ca ratio across the diameter of LMC microcrystals. LMC microcrystals from a depositional chalk (Tor Formation, Offshore Norway) that have a diameter of greater than five microns show distinct Mg/Ca zonation: Mg/Ca was 1-7 mmol/mol at the microcrystal core and 12-28 mmol/mol at the rim. Zonation suggests precipitation of the overgrowth occurs at a different stage in the diagenetic evolution of the microcrystals, and likely represents precipitation from fluid in a shallow burial environment. LMC microcrystals less than five microns in diameter did not exhibit zonation but did exhibit elevated Mg/Ca. In fact, Mg/Ca in these smaller microcrystals was 12-28 mmol/mol, a range identical to the rim of the larger microcrystals suggesting that these small crystals formed at the same time as the overgrowths of larger microcrystals. Temperature increase associated with burial also increases the activity of the Mg ion in solution. Mg is then more likely to be incorporated into the growing microcrystal lattice. Mg is smaller than Ca and therefore causes lattice strain and decreases the negative enthalpy of calcite precipitation (i.e., reduces the drive for precipitation). With Mg inhibiting crystal growth in specific growth hillocks, “rounded” textures can develop. Calcite precipitation is a surface reaction and elevated Mg/Ca at the rim of larger LMC microcrystals and in smaller LMC microcrystals suggests that the Mg ion strongly contributes to texture development.

AAPG Datapages/Search and Discovery Article #90370 ©2020 AAPG Middle East Region Geoscience Technology Workshop, 3rd Edition Carbonate Reservoirs of the Middle East, Abu Dhabi, UAE, January 28-29, 2020