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. / text

Lower Cambrian Eriboll Formation sandstones of the Ardvreck Group that crop out in the Hebridean foreland west of the Paleozoic Moine Thrust Zone (MTZ) in the Northwest Highlands of Scotland contain five sets of opening-mode fractures with varying degrees of quartz deposits (cement) and topographically prominent but small displacement (mostly less than 10 m) northeast-striking faults. The faults crosscut and in some places displace the MTZ. I interpret these faults to post-date the MTZ and consider them to be late structures (kinematically unrelated to MTZ emplacement). Sparse slip lineations on fault surfaces and offset patterns are evidence for strike-slip to oblique slip. Using geologic mapping I show that relative to their lateral and vertical extents, the faults display small amounts of offset (less than 5 to 10 m). My research documented the patterns and petrology of fractures in a well exposed section of the foreland, documented for the first time fracture patterns adjacent to and within the post-MTZ fault zones, and proposes an account of how fault and fracture patterns developed and their probable effects on fluid flow. Fractures are barren (joints), partially filled (quartz lined), or completely filled (veins). Older fracture sets are typically completely filled, whereas younger sets may be lined with a thin veneer of quartz cement or are barren. Listed in order from oldest to youngest fractures containing quartz strike north, NW to WNW, NE, west, and north (sets A through E respectively). Previously proposed relative ages of the sets were confirmed using crosscutting relationships and preferred orientations of macro- and microfractures (Laubach and Diaz-Tushman, 2009). This study focuses on late northeast-striking fractures (set C) which I interpret to be related to the formation of the small-offset faults. Many of the attributes of late fractures and faults in the Eriboll Formation resemble those found in core from highly quartz cemented sandstone natural gas reservoirs ("tight gas sandstones"). I demonstrate that the well exposed fracture patterns I documented are good analogs for tight gas sandstones, by investigating fracture characteristics such as network configurations and connectivity, fracture intensity (abundance), fracture scaling, fracture length and spacing, and the degree of quartz cement deposits in fractures and cataclastic fault rock. Many of the narrow macroscopic fractures and microfractures I documented using CL methods contain varying amounts of quartz deposits. The excellent preservation of Eriboll outcrops is probably a manifestation of little or no fracture pore space preservation in many of the numerous fractures that are apparent in outcrop. Set C fracture abundance is not distributed in a uniform envelope (or "halo") around the late faults. Using scanlines, I show that set C fracture distribution is heterogeneous and highly variable over short lateral distances (tens of centimeters to meters). I also investigate wing crack assemblages (secondary opening-mode fractures) that are locally associated with set C fractures. The assemblages accommodate small amounts of the distributed displacement (a few millimeters) adjacent to fault zones and are locally responsible for increased amounts of fracture connectivity by linking neighboring fractures. Variations in fracture pattern complexity appears to be related to the presence (or absence) of wing crack assemblages. Localized wing crack development on closely spaced, en echelon set C fractures also leads to precursory development of fragmented lozenges of highly deformed volumes of rock (damage zones) that resemble geometries similar to those seen in preserved Eriboll fault cores. Fault-related deformation in the Eriboll Formation is markedly different than that in the underlying Late Proterozoic Torridonian Applecross Formation (subarkose fluvial sandstone), which is characterized by simple halos fault-related fracture arrays surrounding the same late (post-MTZ) faults. In addition to composition, the Eriboll and Applecross differ in mechanical layer thickness (centimeters versus > tens of meters), mechanical properties (high versus low brittleness), and greater propensity for fractures to be filled with quartz cement in Eriboll sandstones owing to quartz cement growth being impaired by the abundance of non-quartz substrate (feldspar and clay minerals) along fracture walls in the Applecross Formation. Although the Eriboll sandstones are more highly fractured than the older Applecross sandstones, Eriboll fractures are more prone to be filled by quartz cement. In this thesis I also report previously unrecognized early (set A; pre-MTZ) minor normal faults, sandstone petrography and rock mechanical properties of selected Eriboll sandstone samples, and the influence of fractures on the glacial geomorphology of the area. I also describe a previously unmapped igneous dyke. I describe previously unrecognized vugs that are partly strata bound and partly localized along fractures. The attributes of these vugs and a review of the literature suggests that these features could represent evidence of pre-glacial silici-karst in Eriboll quartzites. / text