Clastic diagenesis and porefluid evolution, an isotopic study, Magnus oilfield, North Sea

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Awarding institution
  • University of Strathclyde
Date of award
  • 1990
Thesis identifier
  • T6743
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  • The Upper Jurassic Magnus Sandstone is a medium-grained feldspathic submarine fan sandstone, which was derived from the northwest and deposited in the East Shetland Basin as four stacked lobes. These formed a reservoir up to 200m thick within the Kimmeridge Clay oil source rocks.The oilfield was formed by uplift and subaerial erosion of a fault block from early to mid Cretaceous, followed by burial beneath Late Cretaceous mudstones and marls which form the seal. Major diagenetic events within the reservoir sands comprise dissolution of feldspar and garnet and resultant precipitation of quartz overgrowths and kaolinite, followed by growth of magnesian siderite, ankerite and illite. The cement sequence is very similar in each of three wells studied along a 4km traverse from the crest to the flank of the field. The present day reservoir is buried to 3200m and 120°C, with a formation fluid salinity half that of seawater and a 8180 +2%0. Calcite cement is virtually absent, probably due to a lack of detrital shell carbonate. Siderite is distributed throughout the sandstone but localised around degrading detrital biotite grains and mud clasts. Rhombic crystals show distinct compositional evolution through three growth zones, and have more magnesian compositions downdip. This suggests different porefluid compositions across the field. Ankerite is only found close to mudstones, from where Fe and Mg were released during burial. These carbonates have 813C values which suggest a supply from decarboxylation of organic material. Within each well, diagenetic mineral 8180 values consisent with increasing temperature follow the mineral diagenetic sequence. Geothermal gradients of 45°Ckm-1 are inferred during rapid burial. The 8180 values for each mineral are persistently 3-4%0 more negative for samples from the crest of the field than from the flank. This is interpreted to indicate persistently stratified porewaters during burial. The influence of isotopically negative meteoric-derived water in the crest contrasts with more marine-derived porewaters downdip. This surprising deduction suggests that diagenesis was dominated by local flow or diffusion and not by regional fluid movement. This oilfield perhaps represents an end member of the diagenetic spectrum.
Resource Type
Date Created
  • 1990
Former identifier
  • 991393033402996