Fluid inclusion studies of microfractures in Eriboll Formation, NW Scotland : insights into timing of fracture opening
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The Cambrian Eriboll Formation exposed in the footwall of the Moine Thrust, NW Scotland, provides a suitable outcrop analog for naturally fractured tight-gas sandstone reservoirs. Previous studies distinguished five regional sets of quartz-lined or quartz-filled macrofractures (>10 m in opening displacement) that have the following strikes, from oldest to youngest, N, NW to WNE, NE, EW, and NNE (set A through set E), respectively (Laubach and Diaz-Tushman, 2009). Crosscutting relations among microfractures imaged by scanning electron microscope cathodoluminescence (SEM-CL) indicate that microfracture sets follow the same age sequence as macrofractures. Macrofractures >100 m wide are characterized by crack-seal textures interpreted to reflect multiple generations of fracture opening and cemention. In contrast, multiple stages of fracture opening and sealing are not observed in thinner microfractures. Microfractures in the Eriboll Formation are completely to partially filled with quartz cement. Microfractures contain trails of fluid inclusions trapped during fracture cement precipitation. Using microthermometry, I determined that set A microfractures have the highest range in trapping temperature of all sets, ranging from 175°C to 222°C. Fluid inclusion trapping temperatures in set B range between 181°C and 183°C, in set C between 132°C and 143°C, and in set D between 128°C to 188°C. Fluid inclusion assemblages (FIAs) of set E fluid inclusions recorded the lowest temperatures between 79°C and 91°C. Fluid inclusion microthermometric data shows a wide range of up to 46°C in homogenization temperatures for all fluid inclusion assemblages. I attribute this wide range to a combination of (1) partial re-equilibration of inclusions by later thermal events, (2) protracted sealing of microfractures under changing burial temperature conditions, and (3) repeated opening and sealing of microfractures without a recognizable textural record of crack-seal. I interpret the lowest temperature, after pressure correction in each FIA, to record the temperature of initial fracture opening and refer to this as the initial trapping temperature Ti. Initial trapping temperatures (Ti) of 22 fluid inclusion assemblages (FIAs) in different microfracture sets record an overall decrease in temperatures from set A to set E. Based on the fluid inclusion trapping temperatures, I determined the duration of microfracture opening and sealing in comparison with the reconstructed thermal history of the Eriboll Formation. This comparison suggests that microfracture sets A through set E formed between 445 Ma to 205 Ma. Set A formed before the emplacement of the Moine Thrust. Set B and set C formed shortly after the emplacement of the Moine Thrust during Early Silurian times, and set D and set E formed during the subsequent uplift and cooling. The wide range in initial trapping temperature Ti for sets A and D suggests that these fracture sets formed over periods spanning 25 Ma and 30 Ma, respectively. Shorter times are indicated for sets B, C, and E. Long periods of fracture formation are also consistent with a 4°C range in fluid inclusion ice melting temperatures, suggesting fluid inclusion trapping and thus repeated opening and sealing of microfractures as pore fluid composition changed over time. These findings indicate that microfractures could remain open in deep basin settings for geologically long periods of time providing potential pathways for fluids in otherwise poorly conductive sedimentary sequences.