Anomalous Pressures in the Deep Michigan Basin
In this chapter we examine pressures in the St. Peter Sandstone and associated formations of the deep Michigan basin. A comparison of computed brine heads to surface elevations reveals a large area of overpressures withinthe St. Peter Sandstone and the Glenwood Formation to the west and north of Saginaw Bay. Contrary to the patterns expected for a steady-state, topographically driven flow system, heads in these formations are highest in theregional discharge area. Vertical gradients between the Glenwood and the St. Peter and between the St. Peter and the Prairie du Chien Group would generate downward flow in the regional discharge area, which is also inconsistent with a steady-state flow system. Low-permeability zones must exist within the Glenwood and St. Peter to inhibit equilibration with normal pressures in the underlying and overlying units. Overpressures appear to be dissipating by both upward and downward leakage through high-permeability zones located in anticlinal structures and possibly related to basement faults. Within the larger area of anomalous pressures, repeat formation test data from selected wells reveal even greater overpressures locally within the St. Peter. These vertical variations in pressures are associated with vertical variations in permeability, suggesting a stacked system of compartments separated by low-permeability zones of diagenetic origin.
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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.