QUANTITATIVE FAULT SEAL PREDICTION

Fault seal can arise from reservoir/nonreservoir juxtaposition or by d evelopment of fault rock having high entry pressure. The methodology f or evaluating these possibilities uses detailed seismic mapping and we ll analysis. A first-order seal analysis involves identifying reservoi r juxtaposition areas over the fault surface by using the mapped horiz ons and a refined reservoir stratigraphy defined by isochores at the f ault surface. The second-order phase of the analysis assesses whether the sand/sand contacts are likely to support a pressure difference. We define two types of lithology-dependent attributes: gouge ratio and s mear factor. Gouge ratio is an estimate of the proportion of fine-grai ned material entrained into the fault gouge from the wall rocks. Smear factor methods (including clay smear potential and shale smear factor ) estimate the profile thickness of a shale drawn along the fault zone during faulting. All of these parameters vary over the fault surface, implying that faults cannot simply be designated sealing or nonsealin g. An important step in using these parameters is to calibrate them in areas where across-fault pressure differences are explicitly known fr om wells on both sides of a fault. Our calibration for a number of dat a sets shows remarkably consistent results, despite their diverse sett ings (e.g., Brent province, Niger Delta, Columbus basin). For example, a shale gouge ratio of about 20% (volume of shale in the slipped inte rval) is a typical threshold between minimal across-fault pressure dif ference and significant seal.