What are Rollover Anticlines ?

Rollover anticlines are common types of folds in extensional basins. In the North Sea for example, they are a common structural style for hydrocarbon exploration. Basically, during the extension, the beds will rollover into a listric fault to prevent a hole from forming (Figure 1). Clair Field (West of Shetland, UK), Phase 1, is a good example of a highly productive rollover structure (against the seismic scale Clair Ridge Fault; Witt et al. 2010).

The rollover and the listric fault fault may be well imaged, making for a relatively straightforward seismic interpretation. The underlying fault, pre-growth and growth packages are relatively easily picked out in the Moroccan example in Fig. 2. 

Here’s how to construct the fault

As the shape of the fault is related to the shape of rollover (e.g., White et al. 1986), we can construct the fault at depth - using a Chevron construction. You can use structural softwares but I show how to do this by hand in Fig. 3 for a simple vertical shear case. You can do this on a seismic section, imported into a drawing package - I simply use powerpoint in the example. 

Step (a): Ensure that your seismic image (1:1 scale) has top rollover interpretation (green line) and upper fault segment (black line). You can also include fault interpretation (if there is one) below the top rollover (dashed red line). We can then compare this to the construction. Worth doing even if the structure is clear. 

Step (b):  Draw the original bed (regional) - flat blue line. Create a vertical grid (grey lines) at intervals equal to the fault heave (H). Use a graph paper background. 

Step (c): Draw vectors (of heave and throw) as shown. Project the vectors downwards to create the fault surface - 1st one done here - move the vector down along route shown by vertical dashed lines along the grid. 

Step (d): Project remaining vectors down to create the fault surface. 

Although I've removed the seismic background, the seismic quality (in this example) is poor below the top rollover and there is large uncertainty in fault interpretation - although an attempt at an interpretation is shown by the dashed red line. This is where the construction is very helpful - note the good match (d) with the upper half of the dashed red (fault) line but then the 2 diverge. The construction is structurally more realistic as the fault shape is related to the rollover anticline. 

It is therefore worthwhile attempting a construction, to reduce seismic interpretation uncertainty and/or to structurally validate the interpretation. You can also experiment with different shear angles (see White et al. 1986). 

References

White, N.J, Jackson, J.A, McKenzie, D.P. 1986. The relationship between the geometry of normal faults and that of the sedimentary layers in their hanging walls. Journal of Structural Geology, Vol 8, No. 8, 897 - 909.

Witt, A.J., Fowler, S.R., Kjelstadli, R.M., Draper, L.F., Barr, D. & McGarrity, J.P. 2010. Managing the start-up of a fractured oil reservoir: development of the Clair field, West of Shetland. In: Vining, B.A. & Pickering, S.C (eds) Petroleum Geology: From Mature Basins to New Frontiers. Proceedings of the 7th Pet Geol Conf, 299–313.