The Clyde field in the southern North Sea is a Jurassic (Fulmar Sand) reservoir developed on a fault-bounded terrace on the margin of the Central graben. The structural style of this region was formerly thought to comprise simple tilted fault terraces that were active through the Jurassic. Seismic mapping of the Clyde reservoir and deeper Zechstein, Rotliegendes, and Devonian events shows that Jurassic and Triassic rocks form a characteristic thick structural high, lensoid both in plan and cross section, (see figure) above deeper events dipping toward the basin.

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Palinspastic sections were constructed to honor the seismic and well data, and were tested iteratively using new techniques for balancing structural sections in extensional regimes. Cross-sectional area and bed length balance clearly demonstrate the presence of both shallow and deep fault detachments and eliminate models involving tectonic inversion or halokinesis. The deep basement faults form the main regional terrace system and sole-out in the crust at about 15 km (9 mi). A second shallower set of faults defines a listric slide and detachment at the top Permian. The listric shape of the detachment accounts for the lower boundary of the Triassic-Jurassic high. Seismic remapping has substantiated this model and demonstrates the decoupling of the basement and shallow listric fault set . In plan, the shallower faults are arcuate with pull-apart lows on the downthrown side that show synsedimentary growth of the Upper Jurassic clays. Compaction buckling and uplift of the Jurassic section toward the toe of the slide form positive structural features that are oil-productive. The faults along the margins of the structures have a significant strike slip component where they parallel the structural dip.

The development of this growth fault model for the Clyde field has assisted in understanding the seismic mapping and in establishing a predictive model for the field geology. In particular, the recognition of Jurassic growth and strike slip components on the faults has significance in terms of variations in reservoir quality and possible reservoir discontinuities resulting from fault seals.

Figure

Of regional interest is the possibility of further growth fault plays within the North Sea basin. This contrasts with the classical development of growth faults on a continental margin. The distinctive geometry of large growth faults can generate structural highs that are offset from the basement and overlying base Cretaceous structure. This model, along with the lensoid cross section above a simpler basement and distinctive seismic expression of shallow dipping faults, is being used to identify other potential plays that may be analogous to the Clyde field.

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