Stress Superposition Explains Fracture Patterns in some Middle East Oil Fields

Abstract

AbstractTwo main structural types can be distinguished in the oil fields of the Middle East. (i) Fields transected by regional strike slip faults (type I structures). (ii) Fields with local faults that are associated with and confined to anticlinal structures (type II structures). Type II structures are either (i) growth fold, mostly bound by deep seated faults which were reactivated under transpression or (ii) product of structural inversion, during Late Cretaceous and Late Tertiary major tectonic events. The reactivated faults have usually very small lateral displacement and the structures have very gentle dips.The characteristic fault fracture patterns of type II anticlines in the Middle East have two diagnostic aspects: (i) faults and fracture corridors have uniform strike oblique to anticlinal axis and (ii) they disappear away from the structural highs towards franks and away. These two characteristic aspects can be explained only by superposition of regional stress and local tension related to structural bending.Stress superposition not only explains the diagnostic faults and fracture patterns of type II structures, but also account for several details such as higher degree of fracturing in doubly plunging structures and fracture density as a function of the angle between fold axis and maximum horizontal stress at the time of fracturing.Fault reactivation and structural growth in discrete pulses seem to be prerequisite for the typical fault fracture corridor patterns of type II structures. In many of type II anticlines, the paleo-stress that generated fold-bound faults and fracture corridors is oblique to fold axes. This suggests folding and fracturing took place under transpression and is related to fold reactivation or structural inversion. In either case, existence of a pre-existing structural control is essential, such as regional wrench faults or deep seated basement weakness zones.