Genesis and morphology of intracrystalline nanopores and mineral micro inclusions hosted in burial dolomite crystals: Application of Broad Ion Beam-Scanning Electron Microscope (BIB-SEM)

Abstract

Dolomites are important hydrocarbon reservoirs but the origin of porosity associated with dolomitization however remains, to some extent, a matter of debate. The study of porosity in dolomites is commonly focused on macro- and micro-meter (greater than tens of micrometer) scale intercrystalline pores that are visible in thin sections and at times in hand specimens. Meanwhile, comprehensive formation evaluation of such reservoirs should also incorporate properties of pores at the nanometer scale.Burial dolomites of the lower Ordovician (Tremadocian) Boat Harbour Formation of the St. George Group carbonates in western Newfoundland, Canada was subjected to Broad Ion Beam (BIB) argon milling. Thereafter, Scanning Electron Microscope (SEM) was used to examine, at high resolution, micrometer to nanometer scale pores hosted in the dolomite crystals. The ion milling is a novel approach, which provides flat surfaces that lack topography caused by differential hardness and it also reduces the probability of creating artifact induced pores that may be caused by plucking during manual sample polishing.The study shows micro- to nano-pores (≪500 nm–∼3 μm) that occur in the dolomite crystals. The pores are indiscriminately distributed within the core sections of dolomite crystals, which are rimmed by non-porous cortices. Rather than dissolution pits, the morphology of these intracrystalline pores indicates that they most likely originate from coalescence of anhedral-subhedral crystallites nucleated in close proximity to one another during the formation of a major dolomite crystal face. Some of the nano- to micro-pores may have remained open while others trapped fluid or ‘accidental’ mineral (mainly calcite) inclusions as the crystals grew to form euhedral rhombs.Calcite (the most abundant) and silicate inclusions were formed as ‘accidental’ minerals rather than as daughter minerals and are petrogenetically associated with the iterative mechanism of dolomitization.