Fault-seal analysis South Marsh Island 36 field, Gulf of Mexico

SMI 36 is a predominantly gas field on the Gulf of Mexico shelf. The principal trap is updip fault closure in the hanging wall of a large down-to-the-north normal fault. A second normal fault, the intrareservoir fault, separates the reservoir into two compartments. Juxtaposition (Allan) diagrams of the stratigraphy projected onto this intrareservoir fault surface show several intervals of sand-on-sand juxtaposition. In the lowermost part of the section, the MA sand in the footwall is juxtaposed against the LN and LP sands in the hanging wall. Differential fluid contacts and pressures in these sands across this fault suggest that the fault is sealing. We hypothesize shale gouge as the mechanism for fault seal. Estimates of shale percent along the fault plane between the sand intervals range between 40 and 80%. Mud weights and limited reservoir pressure data show that the deep section is overpressured with a water pressure differential as much as 600 psi (4.13 × 10⁶ Pa) across the fault. The cross-fault differences in the aquifer pressure partially seal the hydrocarbons in the hanging wall. The hydrocarbons in the footwall reservoir support a capillary pressure as much as 80 psi (5.51 × 10⁵ Pa) for a shale gouge of 45%. Production pressure profiles record a cross-fault pressure difference of 3000 psi (2.06 × 10⁷ Pa) across the fault before pressure support between the reservoirs suggesting seal “breakdown.”