Rollover anticlines

A secondary feature is the development of rollover anticlines which form as a result of the downward movement close to the fault plane which decreases with increasing distance from the plane. Rollover anticlines may trap considerable amounts of hydrocarbons. 

Fig. 3. NE-SW cross-section of the Jeanne d Arc basin, showing the lystric Murre Fault which bounds the basin in the west. (1) Stable shelf (Bonavista Platform) (2) rollover anticline resulting from antithetic movement of the hanging-wall block along the Murre Fault (3) central Jeanne d Arc basin (4) Central Ridge. TR, interpreted Triassic continental red beds at the base of the Mesozoic sedimentary succession overlain by Lower Jurassic evaporites involved in diapirism (squared pattern). Insert map shows hydrocarbon fields of the Jeanne d Arc basin mentioned in text and location of section. From Tankard Welsink (1988). A, T, movement of fault block away from viewer and towards viewer, respectively.

Fault traps are of several varieties, but in all eases it is essential for the fault to be impervious and sealing. This is by no means always the case, and maity faults are open conduits to fluid movement. It is seldom possible to estab-hsh whether a fault is open or closed without drilling to test. Tension of the cmst generates normaF faults across which part of the sedimentary section is absent (Fig. 6B). The Fahud field of Oman is a good example of a normal fault petroleum trap. When a normal fault moves, the strata on the downthrown side often flop down into the space created by the fault movement. These are called rollover anticlines. It is often noted that strata thicken when traced from the upthrown to the downthrown side of the fault, and that the throw of the fault increases when measured downward from sediment increment to increment (Fig. 6C). 

FIGURE 6 Cross sections to illustrate the different types of fault traps. (A) Reverse fault caused by compression. (B) Normal feult caused by tension. (C) Growth fault with petroleum trapped in adjacent rollover anticline. 

This demonstrates that the fault is removed repeatedly through time. Such faults are thus termed growth faults. Growth faults host petroleum traps, both where reservoir beds are truncated and sealed by the fault, and in rollover anticlines on the downthrown side. Examples are common in the Tertiary deltas of Nigeria and the Gulf Coast of the United States. 

Figure 5.7 Geometry of growth faulting and resulting anticline (rollover) (after Petroleum Flandbook, 1983)...

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