Quartz cement is a major culprit of porosity and permeability loss in deeply buried sandstone hydrocarbon reservoirs. A major debate is whether quartz cement is entirely internally derived or if fluid flow and mass transfer can import silica for quartz cementation. Two Late Jurassic normal faults are exposed within the aeolian (U. Permian) Hopeman Sandstone, onshore within the Inner Moray Firth basin, UK North Sea. The Clashach Fault has a throw <50m. The Burghead Fault has a throw of>100m. The faults crop out within the Hopeman Horst on the southern margin of the 50km wide, 5km deep, half-graben basin. Maximum burial depth of the Hopeman Sandstone within this horst block is 1.5-2.4km. Quartz cementation is asymmetric across the fault planes. Moving through the footwall of the Clashach Fault toward the fault plane, quartz cement volume increases from 9% at 31.7m to a maximum of 26.5% at 13.8m from the plane. From 13.8m to 0.5m from the fault plane authigenic quartz volume decreases from 26.5% to 4.2% at 0.5m. In this zone carbonate cement, which later dissolved, reduced the space available for quartz precipitation. The hanging wall contains mean 4% authigenic quartz. Porosity displays an inverse relationship to quartz cement. Footwall porosity increases from 10% at 13.8m from the fault plane to 18.6% at 31.7m. From 13.8m-0.5m porosity varies inversely with the volume of quartz cement in each sample. Hanging wall mean porosity is 24.5%. Footwall intergranular volume decreases from a maximum of 32% at 13.8m to 28% at 31.7m. At the Burghead Fault, footwall authigenic quartz volume increases moderately from 24.5% at 13m from the fault plane to 29% at 0.5m from the fault plane. Porosity displays an increase from 2.3% at 0.5m to 6.1% at 13m. Permeability decreases from 100-1000mD in poorly cemented hanging wall sandstone to <1mD in extensively quartz cemented sandstones in the footwalls of both faults. Footwall intergranular volume is consistently >30% in the Burghead Fault footwall. Fluid inclusions within footwall quartz overgrowths are single phase, aqueous, indicating cement precipitation <60°C. Quartz cemented microfracture fluid inclusion trails, generated during fault movement, contain 2-phase (L+V) aqueous inclusions with mean homogenisation temperature of 166°C, recording the presence of hot fluids in the sandstone at the time of faulting. Ion microprobe analysis of quartz overgrowth oxygen isotope values for footwall cements around the Clashach Fault shows a linear increase of 8180 values with increasing distance from the fault plane from +17.9960 at 4.2m to +20.8%o at 30.8m. In quartz cements in the footwall of the Burghead Fault, 5180 rises from +17.53% at 0.1m from the fault plane to a consistent range of +19.1%o to +19.596o up to 14m distant. Hanging wall cements have a mean 6180 +23.8960 with range of 6180 +20.1ß'w to +25.09'. 0. An isotopic profile across a single quartz cemented deformation band shows 00 is a minimum mean of +20.7%o. Oxygen isotope and fluid inclusion data records quartz cementation from hot basinal fluids which entered the Hopeman Sandstone adjacent to fault planes. This fluid cooled and mixed with Jurassic meteoric porefluids, precipitating quartz cement at a burial depth of <1.25km. Siliceous fluids were sourced from sediment compaction following Late Jurassic extension in the basin and were expelled up-dip toward the basin margin. Fluid flow was locally focused through the Hopeman Horst, which acted as an exit point for regional fluid expulsion. Within the horst block advecting fluid flow was focused into basinward footwalls by the low permeability vertical fault planes. This was aided by an enhanced reduction in fault permeability by quartz cementation of deformation bands at shallow burial (500m
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:543852 |
| Date | January 2008 |
| Creators | Quinn, Oliver F. |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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