Movement of native fluids during scanning electron microscopy imaging of petroliferous siltstones: Evidence from the Montney Formation, western Canada

The nature and distribution of native fluids, both hydrocarbons and water, exert fundamental control on the petrophysical properties and fluid dynamics of unconventional, low-permeability reservoir rocks. Here, we report dynamic native fluid phenomena that occurred serendipitously during scanning electron microscopy (SEM) imaging of petroliferous siltstones from the Triassic Montney Formation of western Canada. These observations of fluid movement provide rare in-situ contextual information about the nm-to um-scale distribution of fluids in tight unconventional reservoirs. Dynamic oil behaviour is indicated by expulsion structures that formed in pore-filling solid bitumen when trapped oil erupted from the sample while under vacuum pressure conditions. These structures strikingly show that oil and solid bitumen intimately coexist within the pore network of tight rocks in the oil reservoir-fluid window. Dynamic brine behaviour is evident from strings of halite crystals overlying pores between minerals. The halite crystals precipitated from hypersaline brine that migrated from the interior to the surface of the sample. This behaviour indicates well-connected brine exists in Montney siltstone pores, even those substantially filled with hydrophobic solid bitumen. These SEM observations support previous petmphysical studies that suggest the electrical conductivity of Montney rocks is controlled primarily by their bulk volume of well-connected hypersaline water.

Automated summary

The study observed dynamic oil and brine movement phenomena within unconventional rocks at the scale of nanometers to micrometers, providing important in-situ information about fluid distribution. These phenomena demonstrate the intimate coexistence of oil and solid bitumen within the pore network of oil and gas reservoir rocks, as well as the presence of well-connected brine.