CONJUGATE FAULTING AND THE COMPLICATIONS AT THE CROSSOVER OF AN X-FAULT SYSTEM

Accurate mapping of oil field segmentation in extensional systems becomes difficult near the crossover point with conjugate faults. Conjugate faults (also known as X-faults) must truncate and offset at least one of the strands to operate kinematically. One scenario would involve full displacement on the left-dipping strand, then truncation and offset by the right-dipping strand. Its converse would be a second scenario, but frequently, both strands appear to have moved intermittently (~simultaneously). This third scenario causes numerous abandoned strands and develops a complicated region at the nominal crossover point of the two (now several) faults. Should a reservoir layer lie across the crossover point, then a difficult to map and difficult to image (via seismic reflections) region commonly exists. A quite useful example of X-fault complication, developed at small scale (cm offset faults), was mapped in profile by Watterson et al. (1998). Their constant area restoration of the observed deformed state profile geometry has been repeated using LithoTect software that allows for shear strain flattening of tilted elements. Restoration implies a small stratigraphic clarification to their published figure and suggests that a layer at the crossover point can become isolated laterally from its offset HW or FW, almost akin to an isolated boudin. This extreme form of reservoir layer segmentation has been observed in an offshore oil field. Lower Miocene sands form a structural-stratigraphic trap across a Late Miocene (contractional) anticline. In the Early and Middle Miocene during and following the deposition of the reservoir, the regional strain was extensional. A north-trending zone of conjugate normal faulting disrupts the structural trap on its west-dipping flank. This X-faulting has isolated a down-dip portion of the reservoir and separates it from its up-dip neighbor even though both reservoirs align to a common, dipping plane. A sequential restoration illustrates how this deformed state geometry has evolved.