Fault-Controlled Fluid Flow Along a Basin-Bounding Fault System: the Helmsdale Fault (Scotland)

Abstract

Summary Within syn-rift basins in the Norwegian Continental Shelf, the juxtaposition of rift-related clastic deposits in the hanging wall of basin-bounding normal faults against crystalline basement is a recurrent structural setting where hydrocarbon reservoirs can be found. Whereas fault sealing due to smearing and development of low-permeability fault rocks has been largely studies, the effect of mineralization and fault-controlled diagenesis has been less investigated. Here, we report detailed field and microstructural observations along the Helmsdale Fault, a normal fault zone bounding the north-western side of the Inner Moray Firth Basin in NE Scotland that was mostly active during rifting in the Late Jurassic. The hanging wall of the Helmsdale Fault consists of the Late Jurassic (Kimmeridgian-Tithonian) Helmsdale Boulder Beds that are made of alternating debris flow to fault scarp deposits, whereas the footwall is composed of the Helmsdale Granite (Silurian-Devonian in age). Significant fluid flow along the Helmsdale Fault is testified by widespread calcite veining, locally making up to 5 m thick fault cores of stacked veins. A complete suite of microanalysis including optical microscopy, clumped isotope, geochronological analysis as well as fluid inclusion analysis have been performed to characterise the fluid paragenesis along the Helmsdale Fault.