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Abstract:

Jurassic plays in the North Sea are sandstones deposited in fluvial, shallow marine or submarine-fan environments located in the footwalls or hanging walls of rotated fault blocks. After two decades of exploration, a large data base is available on the diagenesis of Middle Jurassic Brent and Upper Jurassic Piper-Claymore-Brae-Fulmar plays. There is still no consensus, however, on whether the diagenesis of these sandstones is dominated by meteoric-water flushing or by the influence of thermobaric waters released during burial.

Diagenetic assemblages in mudstones and sandstones of the basin-margin sequences currently exposed on the mainland UK can be used as modern analogs for the inferred Mesozoic subaerial exposure of rotated fault-block crests in the North Sea. In this respect, mudstones are particularly sensitive indicators of subaerial exposure and rapidly develop soil profiles. Back-scatter electron microscopic (BSEM) observation of mudstones from outcrops indicates that kaolinite is largely authigenic. Basin-margin sandstones commonly preserve the early diagenetic clay-mineral assemblage (kaolinite, smectite and chlorites) but are generally dominated by vermiform authigenic kaolinite. Analytical transmission electron microscopy (ATEM) analyses of these clay minerals indicate large variations in their chemistries.

Each Jurassic reservoir in the North Sea is characterized by a particular diagenetic-mineral assemblage, of which authigenic clays constitute an important component. Early clays in sandstones may coexist with later authigenic clays, although the earlier formed clays are often replaced. In general, authigenic smectites are present only in the shallower reservoirs; kaolinites typically occur at the crest of structures above 4 km depth and close to faults; illite is most abundant in the deeper reservoirs exceeding 4 km depth; authigenic chlorite is rare. Detailed petrographic observation, supported with SEM and TEM investigation, indicates that clay-mineral authigenesis was typically multiphase. K-Ar dating of illites supports this interpretation with youngest ages in the Tertiary. Oxygen isotope analyses of illites and kaolinites remain problematical. ATEM analysis of authigenic clays in mudstones and sandstones documents considerable uniformity of chemical composition at depth. The authigenic mineralogy evolves toward a widespread and uniform phengitic illitequartz-albite-ankerite assemblage at depths below 4 km.

Authigenic clay-mineral assemblages support the argument for Mesozoic flushing with meteoric waters in some reservoirs but not in others. No single 'diagenetic model' can thus be applied to Jurassic North Sea fault-block plays; there is probably a spectrum of scenarios from reservoirs that have been extensively flushed with fresh water to those that have not experienced meteoric-water ingress. The potential for freshwater flushing depends upon a combination of depositional environment (fluvial and barrier-beach sandstones having greater potential than shallow and deep marine sandstones) and structural location (footwall crests have a greater potential for flushing than the hanging walls). Meteoric-water flushing is most likely where footwall erosion is greatest at the site of maximum displacement along a fault. Not all footwall crests, however, need have experienced subaerial exposure; the width of an individual fault block controls height of the scarp, whereas rate of erosion dictates fault-scarp persistence.

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