Deep Pressure in the Lower Tuscaloosa Formation, Louisiana Gulf Coast
Repeat formation tester (RFT) pore pressure measurements spanning a depth range of 5500-6060 m in the lower Tuscaloosa Formation (Upper Cretaceous) document a pressure discontinuity of >20 MPa at -5680 m forming a pressure seal in two natural gas fields in the Tuscaloosa trend, Louisiana. In the Morganza field the depth to the top of overpressure varies by less than 30 m across two adjacent fault blocks, though equivalent strata are downthrown by 100 to 120 m. In contrast, the depth to the top of overpressure in the nearby Moore-Sams field rises slightly across the same fault. Therefore, the nearly horizontal top of overpressure does not appear to coincide with time- or lithostratigraphic boundaries.
The overpressures in all of the Moore-Sams and some of the Morganza fields wells follow a local hydrostatic gradient with increasing depth indicating that pore fluids below the pressure seal are in communication, and demonstrating that sandstone connectivity occurs below the pressure seal as well as above. In the remaining Morganza wells, overpressure increases with depth in a stair-step manner that may comprise offset local hydrostatic gradients, to magnitudes of 117 MPa at depths of 5.9 km. The occurrence of the pressure seal within interbedded sandstones and shales, where high sandstone connectivity is expected, suggests that the sandstones of the seal zone are unusually tight.
The above observations coupled with a petrographic study of sandstones from the vicinity of the pressure seal suggest that extreme compaction of the sandstones after dissolution of carbonate cements may have contributed to the low permeability indicated by the pressure data, and that the seal formed a kilometer or more shallower than it is today.
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Basins worldwide exhibit an unexpected degree of hydrologic segregation. There can be regions of a sedimentary basin that are isolated from their surroundings by a relatively thin envelope of low-permeability rock with an interior of sufficiently high permeability to maintain a consistent internal hydrostatic fluid pressure gradient. These have been named pressure compartments. Presure compartments have several remarkable features, just one of which is that internal fluid pressures can greatly exceed or be significantly less than any regional topographically controlled hydrologic head or drain. This publication contains 30 chapters that take detailed looks at pressure compartments in general, and detail case studies of these compartments in specific basins, such as the Anadarko and Gulf of Mexico. The volume also looks at other considerations in sedimentary basins such as hydrodynamic and thermal characteristics, and mechanical properties of rock.