Integrated study of water Sr isotopes and carbonate Sr-C-O isotopes reveals long-lived fluid compartments in the Langfjellet oil discovery, Norwegian North Sea

Routine measurements of reservoir pressure variation with depth can detect pressure discontinuities indicative of barriers to vertical fluid movement. This study investigates how pressure data can be augmented by detailed profiles of formation water 87Sr/86Sr ratio to determine the precise location and cause of such barriers, and by C–O–Sr isotope analysis of carbonate cements to determine the duration over which the barrier has persisted. The study focuses on the clastic Hugin Formation reservoir in the Langfjellet Oil Discovery (Norwegian North Sea). Here, pressure data indicated a barrier somewhere within a 25 m depth interval. Formation water 87Sr/86Sr was measured with high spatial resolution by extraction from core samples using the residual salt analysis (RSA) method. This revealed three homogeneous populations of water separated by a small step in 87Sr/86Sr over a 7 m interval containing coal and shale layers, and a very large step in 87Sr/86Sr over a 1.2 m interval corresponding to a thin coal and shale layer situated below a major flooding surface. The latter is the main candidate for the pressure barrier. Modelling confirmed that this inferred pressure barrier also greatly retards Sr diffusion. Carbonate cements occur disseminated throughout the reservoir and in several heavily-cemented zones. Oxygen isotope-derived temperatures indicate that these formed in two episodes: (1) Pre-compactional, precipitated shortly after deposition in the zone of bacterial methanogenesis (~30 °C, ~200 m depth, ~162 Ma); (2) Post-compactional incorporating thermal decarboxylation-derived carbon (~90 °C, ~2500 m depth, ~46 Ma). Carbonate 87Sr/86Sr data reveal the same compositional populations present in the current formation water to be present in both cement generations. The water compositional stratification must thus have been present when the early and late cements precipitated, down till today. The persistence of a compositional step for most of the geological history of the rocks confirms the presence of a major fluid communication barrier. The Sr RSA data show invariant water composition across the heavily carbonate cemented intervals, implying no barrier effect. The combination of pressure data (to identify pressure barriers), Sr RSA (to add spatial resolution) and Sr–C–O isotopes of carbonates of different ages (to add a time dimension) is useful for identifying major long-term fluid communication barriers and differentiating them from smaller, less effective or shorter-term features. The method has applications for identifying seals in exploitation of petroleum and water resources, and underground storage of CO2 and radioactive waste