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Case Studies |
1 Fina Exploration Ltd, Fina House, Ashley Avenue, Epsom, Surrey, UK
2 Fina Exploration and Production, Fina Research SA, Zone Industrielle C, 7181 Seneffe (Feluy), Belgium
Post-Chalk, particularly Tertiary, reservoired oils of the North Sea show a wide range of gross composition and frequent indications of biodegradative alteration. The latter has been associated with shallower reservoir depths and complex emplacement and accumulation histories. Comprising a detailed case study, lateral and vertical variations in petroleum composition within the Eocene-reservoired Chestnut and Alba Fields in the Outer Witch Ground Graben have been evaluated using bulk properties gas chromatography (GC), GC-mass spectrometry and isotopic techniques. The oils have experienced varying degrees of in-reservoir biodegradation. They were generated at an early main phase of source-rock maturity. The migration pathway to the Eocene reservoir is thought to be controlled by selective reactivation of Jurassic faults at times of compressional movement during the Tertiary. Using this model in conjunction with subsidence-history profiles, the timing of generation has been investigated and the temperature of the Eocene reservoir at petroleum emplacement has been estimated. The dominant control on biodegradative alteration is believed to be associateded with the distance from meteoric water influx and the available oxygen budget. According to distance from the hydrostatic head, an apparent decline in biodegradation, albeit showing contradictory levels of alteration, was observed along the trend. Various in-reservoir mechanisms to account for this apparent decline, including topping-up, intermediate transformation products and differential oil-column attack, have been evaluated. The study continues with the objectives of investigating the compositional variations of petroleums reservoired within Lower Cretaceous sandstones and the charging relationships between these and the Eocene reservoirs.
