جامعة ليفربول2024-12-252024-12-25https://dspace.academy.edu.ly/handle/123456789/947The salinity measurements of the aqueous fluid inclusions were classified into low salinity group (≤3.5 wt % NaCl) and moderate to high salinity group (>3.5 - 25.0 wt % NaCl). Fluid inclusion salinities indicate that the elevated amount of dissolved NaCl, in the water that led to vein growth, is related to proximity to halite-bearing evaporites (although doubtless other factors may have had an additional control such as full-geometry of fault systems). The vein salinity integrated with the interpreted values of water 18O revealed the origin of the water and hence revealed fluid movement patterns in the basin. The indication of high salinity and relatively low 18O-water in veins in the Blue Lias, Beacon Limestone and Frome Clay Formations, in the area west of Portland, reveals that meteoric water penetrated down to Mercia halite-rich evaporites probably during basin structural inversion and then into the overlying Lower and Middle Jurassic. The similarity of geochemical signatures of these fluids with early Cenozoic oil-field formation water in Lower Triassic reservoirs, found in the eastern part in the basin, emphasises there was a large scale fluid movement (open fluid flow system). In contrast, low salinity and relatively high 18O-water in veins in the Lulworth, Durlston and Chalk Formations, in the area east of Portland, were probably derived from marine connate water (suggesting a relatively closed diagenetic system with little fluid flow). This connate water was diluted by very low salinity-high 18O water, possibly from various diagenetic dehydration reactions during burial and diagenesis. The association of relatively high salinity and high 18O-water from the Kimmeridge and Upper Greensand veins suggest mixing of both previously mentioned water sources (mixed open and closed diagenetic systems). It seems that there is no simple relationship between fault systems and the vein microstructures and geochemistry. However, the presence of oil inclusions in late Cretaceous E-W reactivated fault, at a considerable distance from the sourceABSTRACT The development of the Wessex Basin started in the Permian and lasted through to the late Cenozoic. The Wessex Basin experienced a complex structural history. During the Permian, the basin was initiated by episodes of lithospheric extension preferentially along pre-existing major E-W normal faults, accumulating sediment in subsequent E-W trending graben and half graben in the western part of the current region. The locus of basinal rifting and subsidence migrated eastward progressively into the Mesozoic and merged with additional E-W fault-bounded depocentres such as the Weald Basin (in the east). The middle to late Cretaceous saw the end of active lithospheric extension and the beginning of regional aseismic thermal subsidence. In the latest Cretaceous and early Cenozoic, parts of the Wessex Basin were uplifted and eroded. At this time, major faults underwent reverse movement and former depocenters became anticlines. These anticlines created neighbouring depocenters to the north of the axis of inversion as a result of approximate N-S compressive tectonic stresses. This antithetical change from extensional to contractional pattern influenced the prevailing basinal structures and generated the Cenozoic basin. Basin subsidence and inversion each led to localised patterns of petroleum and water migration. Neither the timing nor routes of fluid migration are yet fully resolved. Therefore, the Wessex Basin provides an opportunity to study linkages between the structural evolution and the fluid movement and to construct the timing and migration pathways of the petroleum and solute-bearing water through veins and cemented fracturekitchen, implies that these faults have acted as a conduit for oil migration probably during the late Cretaceous-early Cenozoic inversion event. The considerable flux