COMBINED FLUID INCLUSION--SEM CATHODOLUMINESCENCE ANALYSIS OF MICROFRACTURE OPENING IN A TIGHT-GAS SANDSTONE OUTCROP ANALOG: ERIBOLL FORMATION, NW SCOTLAND

This ongoing project investigates the structural and diagenetic history of quartz-lined fractures in the Cambrian Eriboll Formation, NW Scotland, as an outcrop analog of naturally fractured tight-gas sandstone reservoirs. Field observations of crosscutting fractures and SEM-cathodoluminescence imaging of crack-seal cement textures indicate multiple stages of fracture opening and sealing. In contrast to previous fluid inclusion and cathodoluminescence studies on cemented macrofractures in tight-gas sandstone reservoirs, we determined fluid inclusion homogenization temperatures on cemented transgranular microfractures. Homogenization temperatures of fluid inclusions in microfractures are in the range of 175-199°C which is comparable to maximum burial temperatures of some producing tight-gas sandstone reservoirs. These temperatures are up to 30°C higher than previous homogenization temperatures measured for macrofractures in the same outcrop.

Previous fluid inclusion measurements on macrofractures were obtained before SEM-cathodoluminescence imaging to avoid possible damage of inclusions during imaging. To aid the textural interpretation of microfractures during fluid inclusion microthermometry, we systematically tested the effect of SEM beam damage by measuring homogenization temperatures of fluid inclusions of varying depth within multiple samples before and after cathodoluminescence imaging. We found that beam damage is noticeable only within the uppermost 5-10 µm of the sample, resulting in leaked inclusions. Below this zone, repeat measurements of fluid inclusion homogenization temperatures after CL imaging are within 1°C , demonstrating that beam damage in our samples is negligible below the topmost layer of the sample, thus allowing SEM-cathodoluminescence imaging prior to fluid inclusion microthermometry.