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Title: Diagenesis of a fractured chalk reservoir : Machar oilfield, Central North Sea
Author: Doran, Helen
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2004
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The Machar Cretaceous chalk oilfield, Central North Sea, is a structural trap of chalk folded above a diaper of Zechstein salt, and formed a regional leakoff point for overpressure from Jurassic sandstones deeper in the basin. Chalk was deposited as pelagic sediment, but re-deposition by gravity flows improves reservoir quality. Diagenesis of the Machar chalk matrix occurs during burial to 1Km. Stylolites form in the chalk at depths below 600m and cause cementation of the reservoir through pressure dissolution and precipitation in a closed system. During diagenesis of the chalk matrix the underlying salt diaper evolves and as a result four fracture sets begin to form in the chalk reservoir. Hydrocarbon charge at mid-Miocene halted matrix cementation resulting in an exceptionally porous (30%mD) reservoir. The growth and evolution of the diaper is marked by the formation of healed fractures within the chalk reservoir. Fracture filling calcite records the evolution of the Tor Formation reservoir from a closed system, rock-dominated environment to an open diagenetic system. Four different fracture types have been discovered within the Tor formation on the Machar field. Of these, the first to form was Fracture Type 1, relating to bedding stylolites formed through minor extension at the crest of the reservoir as the salt attempts to remain buoyant. This fracture is filled with Calcite 1. δ13C values within this calcite (+ 2.0 to + 2.3‰ PDB) are similar to the values measured in the matrix chalk (+ 1.5 to + 3‰ PDB), suggesting that like the matrix cement this formed in a rock-dominated system. 87Sr/86Sr (0.7078 to 70787) values within Calcite 1,also similar to the matrix values (0.70770-0.70791) supports this theory. Negative δ18O (-5 to -7‰ PDB) values measured in Calcite 1 are explained by increased burial temperature during precipitation. Fracture type 2, the second fracture to form, is filled with Calcite 2; δ13C values (+ 3.6 to + 5.9‰ PDB), are more positive than the matrix values (+ 1.5 to + 3.0‰ PDB), and mark the opening of the reservoir to an external fluid. Fracture type 3 is filled with Calcite 3, and forms as Palaeogene sediments begin to down build around the evolving salt diaper. Calcite 3 precipitated from an external fluid that is restricted in its migration between tectonic stylolites. Calcite 3 is replaced by saddle dolomite, celestite, barite and fluorite.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available