Papers Timing diagenesis in the Tartan Reservoir (UK North Sea): constraints from combined cathodoluminescence microscopy and fluid inclusion studies S. D. Burley* Geologisches Institut, Universit~it Bern, Baltzerstrasse 1,3012 Bern, Switzerland and J. Mullis Mineralogisch-Petrographisches Institut, Universit~t Basel, Bernoullistrasse 32, 4056 Basel, Switzerland and A. Matter Geologisches Institut, Universit~it Bern, Baltzerstrasse 1,3012, Bern, Switzerland Received 16 March 1988; revised 20 October 1988 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 so'~Jrces 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 prlor 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 conduits 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. Keywords: diagenesis; fluid inclusions; fault-related cementation; fluid migration Introduction There are now many descriptive models of diagenesis in the literature. However, to make such models predictive depends not only upon an understanding of the diagenetic processes involved, but also upon an ability to quantify the importance of each process, Present address: Departmentof Geology, University of Manchester, Oxford Road, ManchesterM13 9PL, UK 0264-8172/89/020098-23 + 4 colour plates $03.00 ©1989 Butterworth & Co. (Publishers) Ltd 98 Marine and Petroleum Geology, 1989, Vol 6, May identify the sources, sinks and transport mechanisms of reactants, and define the timing of each diagenetic event. The development of actualistic models is further compounded by the complexity and variability of diagenesis in the sub-surface, and by the inability of standard petrographic techniques to distinguish or resolve multiphase or zoned cements in many reservoir sequences. We describe here the application of hot-cathode luminescence (hot-CL) microscopy