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Micro- and Nanometer Porosity in Dolomites of Tremadocian Boat Harbour Formation Carbonates, Western Newfoundland, Canada


Dolomite rocks are very important hydrocarbon reservoirs. However, besides the fact that the mode of dolomite formation is not fully understood, the origin of porosity associated with dolomitization remains, to some extent, a matter of debate. Study of porosity in dolomites is commonly focused on micrometer (greater than tens of micrometer) scale intercrystalline pores of dolomites that are visible in hand samples and thin sections. Meanwhile, comprehensive formation evaluation process should incorporate the properties of pores at the micrometer to nanometer scale. Diagenetic dolomite of the Lower Ordovician (Tremadocian) Boat Harbour Formation of the St George Group carbonates in western Newfoundland, Canada was subjected to Focused Argon-ion beam milling (FIB). Thereafter, Scanning Electron Microscope (SEM) was used to examine, at high resolution levels, micrometer to nanometer scale pores hosted in the crystals of the dolomites. The FIB is a novel approach which provides flat surfaces that lack topography due to differential hardness and also reduces the probability of creating artifact induced pores that may be caused by plucking during manual sample polishing. The study shows micro- to nanopores (∼500nm- ∼3μm) that occur in the dolomite crystals. These pores are indiscriminately distributed within the crystals cores, which are rimmed by non-porous cortices. Many of the micro-/nanometer pores have etched edges. The pores are mostly irregular in shapes but sometimes polygonal. Nanometer pores (200-600nm) were also found in co-occurring euhedral and framboidal pyrite crystals. It seems that the intracrystalline pores hosted in the dolomite crystals result from preferential dissolution of calcite remnant that hosted within the replacive dolomite crystals, due to the solubility difference between dolomite and calcite minerals. Dissolution may have been caused by local H+ enrichment of the diagenetic fluid associated with pyrite formation. Dolomite crystals from partially dolomitized sections of the same formation contain abundant calcite inclusions as revealed by Energy Dispersive X-ray analyses. On the other hand, the intercrystalline micropores pores within the pyrite crystals aggregates were likely created during the coalescing of framboidal and euhedral pyrite microrhombs (100nm-3μm). Further research will focus on a geometrical 3D reconstruction of the pores to investigate their connectivity.