A repeat 4D seismic survey was acquired in 2012 over the Halfdan oil field in the Danish North Sea Central Graben. The main objectives were to understand the lateral and vertical waterflood sweep in the reservoir, to identify unswept areas, to guide future well interventions, and to improve the reservoir model. The main oil reservoir is the Cretaceous chalk Tor Formation, and the field is developed via a line-drive waterflood. The reservoir saturation changes caused by water injection lead to large changes in acoustic impedance which can be imaged with 4D seismic. There were two main 4D processing projects for the Halfdan field. The 2012–2005 4D data were processed as a stand-alone project because of the high repeatability of the surveys, and a separate three-vintage processing project for all time steps also was carried out. When the three-vintage processing was not available, the original 4D processing was used for the 2005–1993 time step. Seismic inversion was performed because production effects have a large impact on the acoustic impedance in the chalk reservoir. The rock-physics model was also used to model 4D acoustic-impedance changes based on pressure and saturation changes in the reservoir. The 4D acoustic-impedance changes from the seismic and reservoir model were analyzed in conjunction with the production and geologic data on a well-by-well basis to better understand sweep efficiency.
New coring data have been acquired along the western Gulf of Lions showing anomalous concentrations of methane (up to 95720 ppm) off the Rhône prodelta and the head of the southern canyons Lacaze-Duthiers and Cap de Creus. Sediment cores were acquired with box and kasten cores during 2004-2005 on several EuroSTRATAFORM cruises. Anomalous methane concentrations are discussed and integrated with organic carbon data. Sampled sites include locations where previous surveys identified acoustic anomalies in high-resolution seismic profiles, which may be related to the presence of gas. Interpretation of the collected data has enabled us to discuss the nature of shallow gas along the Gulf of Lions and its association with recent sedimentary dynamics.The Rhône prodelta flood deposits deliver significant amounts of terrigenous organic matter that can be rapidly buried, effectively removing this organic matter from aerobic oxidation and biological uptake, and leading to the potential for methanogenesis with burial. Away from the flood-related sediments off the Rhône delta, the organic matter is being reworked and remineralized on its way along the western coast of the Gulf of Lions, with the result that the recent deposits in the canyon contain little reactive carbon.In the southernmost canyons, Lacaze-Duthiers and Cap de Creus, the gas analyses show relatively little shallow gas in the core samples. Samples with anomalous gas (up to 4982 ppm of methane) are limited to local areas where the samples also show higher amounts of organic matter. The anomalous samples on the head of the southern canyons may be related to methanogenesis of recent drape or of older sidewall canyon infills.
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