Timing diagenesis in the Tartan Reservoir (UK North Sea): constraints from combined cathodoluminescence microscopy and fluid inclusion studies

Abstract

Fluid inclusions occur in authigenic quartz, baryte, ankerite and calcite cements that partially occlude inferred secondary porosity in Piper Formation sandstones of the Tartan field and adjacent Witch Ground Graben. Detailed petrography combined with hot cathodoluminescence studies enables a fluid inclusion stratigraphy to be related to the authigenic minerals and individual cement zones within them. Microthermometric studies indicate that these inclusions formed at elevated temperatures in the range 70–120°C. These temperatures are interpreted to indicate hot migrating fluids which invaded the on-structure sandstones at burial depths exceeding 1.5 km during the late Cretaceous-early Tertiary. Between successive cement generations major fluctuations in salinity are recorded. There is a marked lowering of salinity between subsequent quartz generations. Baryte and calcite cements, associated with metal sulphides, were precipitated from a low salinity brine. Ankerite cement at the present oil-water contact records precipitation from high salinity pore fluids. The salinity variations are thought to reflect large scale sub-surface fluid migration. Sources of acidity for the generation of secondary porosity and sources of silica, sulphate and base metals for the pervasive late cements cannot all be found within the reservoir sequence. This therefore indicates mass transfer within an open system. The complex diagenetic assemblage is unlikely to have been precipitated from the evolution of a single pore fluid. Mixing of at least two sub-surface fluids of very different chemistry and origin is inferred. Sulphate for barytes and sulphide cements is thought to have been derived as a result of cross-formational flow from Zechstein anhydrite juxtaposed against the reservoir prior to oil migration. Homogenization temperatures of fluid inclusions within cements along major fault planes are offset towards significantly higher temperatures than those inclusions distanced from such faults. This, together with the distribution of porosity and cements on the crestal structures and adjacent to major faults, strongly suggests that the faults were the condults for migrating hot fluids which mixed with the sulphate brine on the Tartan structure. Seismic valving provides an elegant mechanism for transporting large volumes of hot fluids intermittently but repeatedly along the major bounding faults. The source of the hot fluid is inferred to be related to the dehydration and illitization of smectite reaction, maturation of organic matter and expulsion of pore waters in the adjacent Witch Ground Graben, all of which took place broadly coincident with the generation of secondary porosity and late on-structure cementation.