IAMG 2001--Cancún Technical Program--Session J

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Interpretation of the Fault System in a Hydrocarbon Source Rock--The Kimmeridge Clay Formation in the North Sea, United Kingdom by Miguel Guerrero Muñoz, Instituto Mexicano del Petroleo/University of Manchester, UK The Kimmeridge Clay Formation is widely regarded as the predominant source rock of the North Sea oil. However, in the Wessex Basin it is not the major source to known hydrocarbons, not because of its lithology and organic matter content, but because of its low maturity, a function of its maximum burial depth. The organic matter in the Kimmeridge Clay Formation of the Wessex Basin is dominantly marine type II kerogen with only a minor contribution of terrestrial material. The Kimmeridge Bay Field (discovered in 1959) produces from fractures in the middle Jurassic Cornbrash limestones and Oxford Clay, being sourced from the Lias. Oil in the field was being generated by and migrating during the Early Cretaceous, with peak generation in the Late Cretaceous. The migration of oil from the area of generation to the Kimmeridge Bay Field reservoir must involve faults as well as a component of lateral migration through carrier bed formations. The continued replenishment of the Kimmeridge Bay Field suggests that some faults are still open. Over 25 years this field has produced 2.5 x 106 bbl without decline. This seems more than the fracture system in the closure can contain and suggests that the field is being constantly replenished from a deeper reservoir. Small N-S faults in the Kimmeridge Clay Formation east of Kimmeridge Bay, Dorset, southern England, are all normal. Conjugate faults formed during overpressure generation during burial in the late Jurassic and early Cretaceous, by a process comparable to fluid pressure hydrofracturing. They are not related to compressional tectonics associated with the formation of the Kimmeridge Bay anticline. Faults at Clavellls Hard were examined in detail, these faults have arrays of smaller displacement faults associated with them that cluster either side of the main fault, defining a damage zone in the mudstones. The extent and symmetry of the damage zone was investigated by measuring the abundance of small faults in a series of traverses across these faults. The faults and associated calcite-filled veins are considered from outcrop evidence to be entirely enclosed within the Kimmeridge Clay Formation and were the conduits for intraformational fluids that were expelled upwards. As such they are ideal for investigating the nature and composition of fluids generated in this mudstone sequence during early burial. Paper in PDF format Munoz, Acrobat PDF, 1 M.