Page 116 - Hydrocarbon Exploration and Production Second Edition
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Reservoir Description                                                 103


                                                    Axis of
                                                    rollover
                                      Fault         anticline
                                      plane















             Figure 6.6  Geometry of growth faulting and resulting anticline (rollover) (after Petroleum
             Handbook,1983).


                Four mechanisms have been suggested to explain how faults provide seals. The
             most frequent case is that of clay smear and juxtaposition (Figure 6.7):

               Clay smear: soft clay, often of marine origin, is smeared into the fault plane during
                movement and provides an effective seal.
               Juxtaposition: faulting has resulted in an impermeable rock ‘juxtaposed’ against a
                reservoir rock.

                Other, less frequent fault seals are created by

               Diagenetic healing: late precipitation of minerals on or near the fault plane has
                created a sealing surface (see Section 6.1.3 for more detail).
               Cataclasis: the fault movement has destroyed the rock matrix close to the fault
                plane. Individual quartz grains have been ‘ground up’ creating a seal comprising
                of ‘rock flour’.
                In many cases, faults will only restrict fluid flow, or they may be ‘open’, that is
             non-sealing. Despite considerable efforts to predict the probability of fault sealing
             potential, a wholly reliable method to do so has not yet emerged. Fault seal modelling
             is further complicated by the fact that some faults may leak fluids or pressures at a very
             small rate, thus effectively acting as seal on a production time scale of only a couple of
             years. As a result, the simulation of reservoir behaviour in densely faulted fields is
             difficult and predictions should be regarded as crude approximations only.
                Fault seals are known to have been ruptured by excessive differential pressures
             created by production operations, for example if the hydrocarbons of one block are
             produced whilst the next block is kept at original pressure. Uncontrolled cross-flow
             and inter-reservoir communication may be the result.
                Whereas faults displace formerly connected lithologic units, fractures do not show
             appreciable displacement. They also represent planes of brittle failure and affect hard
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