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Calcite veins at outcrop in the Mesozoic, oil-bearing Wessex Basin, UK, have been studied using field characterization, petrography, fluid inclusions and stable isotopes to help address the extent, timing and spatial and stratigraphic variability of basin-scale fluid flow. The absence of quartz shows that veins formed at low temperature without an influence of hydrothermal fluids. Carbon isotopes suggest that the majority of vein calcite was derived locally from the host rock but up to one quarter of the carbon in the vein calcite came from CO2 from petroleum source rocks. Veins become progressively enriched in source-rock-derived CO2 from the outer margin towards the middle, indicating a growing influence of external CO2. The carbon isotope data suggest large-scale migration of substantial amounts of CO2 around the whole basin. Fluid inclusion salinity data and interpreted water-δ18O data show that meteoric water penetrated deep into the western part of the basin after interacting with halite-rich evaporites in the Triassic section before entering fractured Lower and Middle Jurassic rocks. This large-scale meteoric invasion of the basin probably happened during early Cenozoic uplift. A similar approach was used to reveal that, in the eastern part of the basin close to the area that underwent most uplift, uppermost Jurassic and Cretaceous rocks underwent vein formation in the presence of marine connate water suggesting a closed system. Stratigraphically underlying Upper Jurassic mudstone and Lower Cretaceous sandstone, in the most uplifted part of the basin, contain veins that resulted from intermediate behaviour with input from saline meteoric water and marine connate waters. Thus, while source-rock-derived CO2 seems to have permeated the entire section, water movement has been more restricted. Oil-filled inclusions in vein calcite have been found within dominant E-W trending normal faults, suggesting that these may have facilitated oil migration. [ABSTRACT FROM AUTHOR]

We present a structural, microstructural, and stable isotope study of a calcite vein mesh within the Cretaceous Natih Formation in the Oman Mountains to explore changes in fluid pathways during vein formation. Stage 1 veins form a mesh of steeply dipping crack-seal extension veins confined to a 3.5-m-thick stratigraphic interval. Different strike orientations of Stage 1 veins show mutually crosscutting relationships. Stage 2 veins occur in the dilatant parts of a younger normal fault interpreted to penetrate the stratigraphy below. The δ18O composition of the host rock ranges from 21.8‰ to 23.7‰. The δ13C composition ranges from 1.5‰ to 2.3‰. This range is consistent with regionally developed diagenetic alteration at top of the Natih Formation. The δ18O composition of vein calcite varies from 22.5‰ to 26.2‰, whereas δ13C composition ranges from −0.8‰ to 2.1‰. A first trend observed in Stage 1 veins involves a decrease of δ13C to compositions nearly 1.3‰ lower than the host rock, whereas δ18O remains constant. A second trend observed in Stage 2 calcite has δ18O values up to 3.3‰ higher than the host rock, whereas the δ13C composition is similar. Stable isotope data and microstructures indicate an episodic flow regime for both stages. During Stage 1, formation of a stratabound vein mesh involved bedding-parallel flow, under near-lithostatic fluid pressures. The 18O fluid composition was host rock-buffered, whereas 13C composition was relatively depleted. This may reflect reaction of low 13C CO2 derived by fluid interaction with organic matter in the limestones. Stage 2 vein formation is associated with fault-controlled fluid flow accessing fluids in equilibrium with limestones about 50 m beneath. We highlight how evolution of effective stress states and the growth of faults influence the hydraulic connectivity in fracture networks and we demonstrate the value of stable isotopes in tracking changes in fluid pathways. [ABSTRACT FROM AUTHOR]