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A fault zone model is presented which is capable of explaining the migration, di stribution and trapping of hydrocarbons in Nigerian rollover structures. The main factor accounting for the occurrence of hydrocarbons in a rollover structure appears to be the presence of a large wedge of overpressured marine source-rock shale on the upthrown side of the structure bounding growth fault. The driving force for the migration is considered to be the flUid-pressure differential resulting from the juxtaposition of overpressured shale against initially hydropressured paralic sands across the fault.ThiS mechanism can explain the presence of hydrocarbons in the lowermost series of paralic sands opposite overpressured shale but not that of shallower accumulations. Model experiments and outcrop studies in Germany have indicated that shear zones of normal faults in sand/shale sequences-usually consist of smeared-in laminae of shale and wedges of sand. In Nigeria the spillpoints of accumulations in rollover structures often coincide with the highest point of contact of bounding growth fault and reservoir. These observations suggest that migration of hydrocarbons along major faults may oocur along sandy stringers and wedges in shear zones. In this way, superimposed reservoirs can be successively filled with hydrocarbons when connections between hydrocarbon-bearing sand in the shear zone and the reservoir sand are available.

A geomechanical delta model is presented that explains and permits quantitative reproduction of the main features associated with growth faulting. The model is based on a soil‐plasticity analysis of gravity sliding of overpressured clays and silts on very gentle delta slopes. In this analysis, the packets involved in gravity sliding —called ‘units’—are well quantified. The delta body may behave as a stack of such units (‘multi‐unit’ delta model), which can behave differently depending on such parameters as sedimentation rates, changes in lithology and compactional behaviour. Two prominent and essentially different structural expressions of the model are discussed: (a) the regularly spaced growth‐fault pattern, without recognisable toe regions; (b) the complete slide structure with a well‐developed toe region. The shapes of the roll‐over structures (particularly the positions of the crests), the thickening of the layers near the growth faults, antithetic faulting and horsetailing of growth faults can be derived from the model without the need to invoke any deeper seated mechanism in the substratum. The basic assumptions of this geomechanical model are supported by observations in Ireland and examples from literature. It may contribute to the reconstruction of the hydrocarbon migration history in a delta by accounting for the synsedimentary development of stresses and fault structures that may control hydrocarbon migration. Recently, geomechanical aspects came in focus again for the assessment of seismic amplitude anomalies in the search for hydrocarbons (Crans and Berkhout, 1979).