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Geological Society, London, Special Publications; 2004; v. 237; p. 283-303;
DOI: 10.1144/GSL.SP.2004.237.01.17
© 2004 Geological Society of London

Case Studies - Water and Petroleum Fluid Compositional Variations

Spatial variations in the composition of formation waters from the central North Sea: implications for fluid flow in the deep high-pressure high-temperature hydrocarbon play

R. A. McCartney1, P. Winefield2, P. Webb2 & O. Kuhn3

1 GeoScience Limited, Falmouth Business Park, Bickland Water Road, Falmouth TR11 4SZ, UK mccartney{at}geoscience.co.uk
2 Shell U.K. Exploration and Production Ltd, 1 Altens Farm Road, Nigg, Aberdeen AB12 3FY, UK
3 Shell China Exploration and Production Co. Ltd, China World Trade Centre, No. 1 Jian Guo Men Wai Daije, Beijing 100004, P.R. China

Formation water composition data and formation pressure data from oil, gas and gas condensate fields covering an area of approximately 15000 km2 in the Central Graben Area of the UK North Sea have been analysed. The purpose was to determine whether large-scale barriers to flow influence the compositional distribution of formation waters and how this aids understanding of overpressure distribution and potential hydrocarbon migration pathways. Analyses (Na, K, Mg, Ca, Sr, Ba and Cl) of water and Elemental Residual Salt Analysis (ERSA) samples from both the water and hydrocarbon legs of wells were analysed statistically to reveal eight different water types in the study area. These were located in distinct geographical areas. Aquifer overpressure from formation pressure data for numerous wells in the area were plotted on large-scale regional maps to reveal the distribution of pressure ‘cell’ or compartments. An integrated interpretation of the location of pressure cells, salinity variations and water types provided information on the current and past fluid flow across the pressure cell boundaries, the operational longevity of these boundaries and the nature of the Mesozoic aquifer. The implications of the work are important for understanding: (a) the usefulness of the new ERSA technique for estimating formation water compositions; (b) the effectiveness of integrating fluid composition and pressure data to understand compartmentalization; (c) fluid flow dynamics in the HPHT sector of the Central North Sea; and (d) likely hydrocarbon migration routes and the character and distribution of overpressured basin compartments.