Pore pressure variation within the Tuscaloosa trend: Morganza and Moore‐Sams Fields, Louisiana Gulf Coast

Abstract

Repeat formation tester (RFT, mark of Schlumberger) pore pressure data from the lower Tuscaloosa Formation (Upper Cretaceous) are analyzed across a pressure transition zone in two gas fields of the Tuscaloosa trend, Louisiana, documenting a pressure seal within a sandstone‐dominated interval that maintains a pressure anomaly of ∼20 MPa. Pressure measurements within the formation span a depth interval of 5500–6060 m and are limited to the crests of rollover structures associated with growth faults; the onset of overpressure occurs at a depth between 5620 m and 5690 m in two fault blocks, while the strata are displaced across the fault by 100 to 120 m, suggesting that the pressure seal may be near‐horizontal, crosscutting stratigraphic boundaries. The pressure seal zone is as thin as 38 m and occurs within interbedded sandstones and shales. Overpressures in all wells of the Moore‐Sams Field and in two wells of the Morganza Field follow local hydrostatic gradients, indicating that portions of the overpressured zone are in fluid communication though isolated from shallower, normally pressured fluids. In other wells, overpressures increase with depth in a stair‐step manner to 117 MPa at depths of 5.9 km. The occurrence of the pressure seal in interbedded sandstones and shales where higher permeability is expected suggests that the sandstones of the seal zone are unusually tight. A companion petrographic study of sandstones near the pressure seal suggests that extreme compaction of the sandstones after dissolution of grain‐supporting cements may have contributed to the low permeability of the seal zone.