Anomalous Hydraulic Pressures within the Michigan Basin StrataWe see the presence of anomalous pressures within the area around and beneath the Saginaw Bay as possible indicators of the proposed cosmic impact. The fracturing and excision of significant bedrock would generate many of the structural irregularities discussed in the paper we have summarized by excerpts, below. The full paper has been purchased and is available from us on request.
Anomalous Pressures in the Deep Michigan Basin
Jean M. Bahr, Gerilynn R. Moline, Gregory C. Nadon
University of Wisconsin—Madison
Madison, Wisconsin, U.S.A.
Published : in Ortoleva, P., ed., Basin compartments and seals: AAPG Memoir 61
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 within the 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 the regional 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.
In a hydrodynamic sense, overpressures and underpressures resulting from topographically driven flow are not truly anomalous. However, there are other mechanisms besides regional flow that could give rise to “anomalous” overpressures and underpressures. Such anomalous pressures could be the consequence of currently active pressure-generating mechanisms, such as compaction or hydrocarbon maturation. They could also be manifestations of relatively slow equilibration to a steady-state system following a change in boundary conditions or the elimination of a pressure source. In cases of slow equilibration, the anomalous pressure zones could be considered to be in limited hydraulic communication with the surrounding system and would thus at least partially fulfill the definition of a compartment as proposed by Powley (1990). However, it should be recognized that because no formation is completely impermeable, such nonequilibrium conditions and the zones of anomalous pressures themselves are inherently transient. Nevertheless, as demonstrated by Toth and Millar (1983) for a one-dimensional example, significant permeability contrasts are required to allow anomalous pressures to persist over geologic time scales.
In this chapter we examine evidence for anomalous pressures in the deep Michigan basin. The primary formation of interest is the Middle Ordovician St. Peter Sandstone, which became an active gas play in the 1980s with production extending to depths of over 3500 m. Related chapters in this volume (Drzewiecki et al.; Shepherd et al.; and Wang et al.) describe the sedimentologic, diagenetic, and thermal processes that have affected this formation. Figure 1 shows structure contours on the top of the St. Peter, in meters relative to sea level (masl), along with locations of major gas fields. Over most of the Lower Peninsula of Michigan, the top of the St. Peter occurs at elevations of less than –1700 masl. Producing zones are concentrated in the deep areas of the basin to the west and north of Saginaw Bay.
Interpretation of Regional-Scale Pressure Anomalies
The areas of excess heads in the St. Peter and Glenwood encompass most of the deep producing fields in these formations. The geometry of the zone of excess heads in the St. Peter, which appears to par- Figure 7. (A) Heads and surface elevation along section B–B’; (B) structure section and zones of overpressure along B–B’ showing top of the Utica, Trenton, Glenwood, St. Peter, and Prairie du Chien. allel the major glacial moraines surrounding Saginaw Bay as shown in the map of Farrand and Bell (1982), suggests that glacial loading during the advance of ice through the Saginaw Bay lowland may have generated the current overpressures. Given the absence of current conditions promoting hydrocarbon maturation or active compaction, the existing overpressures must have persisted at least since the end of the last glaciation. The region extending west and north of Saginaw Bay may have at one time formed a relatively continuous zone of anomalous pressures in the St. Peter, the Glenwood, and possibly the Trenton and Black River formations. In the terminology of Al-Shaieb et al. (1991), this broad region could be considered a megacompartment. The upper seal probably consists of lowpermeability zones in the Glenwood that in some areas extend into the St. Peter or the Trenton. The basal seal appears to be located in the lower St. Peter or in the Prairie du Chien.
Overpressures appear to be dissipating by leakage through high-permeability zones located in anticlinal structures and possibly related to basement faults. In some areas of this region, maximum overpressures are found in the Glenwood, while in others the St. Peter contains the highest overpressures. Thus, both upward and downward drainage appear to be occurring. Over much of the region, low-permeability zones within the Glenwood and within the St. Peter must be present to inhibit equilibration with normal pressures in the underlying Prairie du Chien and overlying Black River.
Pressure and permeability data for the Kielpinski well in Bay County are plotted on Figure 9. This well is located near the middle of section B–B’, in the region where gradients between the Glenwood and St. Peter reverse. The pressure data show that the St. Peter is consistently overpressured relative to the line corresponding to a density of 1.16 g/cm3 projected from the surface. Also plotted for reference on Figure 9A are two DST measurements from other wells in Bay County. One of these shows an overpressure of magnitude similar to that of the majority of the RFT measurements. Several of the RFT pressures are significantly higher than the others. Such abrupt vertical variations in pressure within a single well could not be the result of local fluid density variations but, instead, require variations in permeability to maintain a large pressure gradient. The permeability data in Figure 9B show variations of about three orders of magnitude within the St. Peter, with high-permeability contrasts occurring in the depth intervals corresponding to the greatest overpressures. The correspondence of zones of permeability contrasts to large vertical variations in pressure indicates that permeability variations within the St. Peter have led to a hierarchical system of stacked subcompartments. These subcompartments are intervals that are adjusting even more slowly than the St. Peter as a whole to normal pressures in the regional hydrodynamic system. Similar patterns of variable permeability and vertical variations in pressure are found in the State Foster 1-19 well (Figure 10). This well is located in the Rose City field in Ogemaw County.
Figure 9. Pressure and permeability data determined from repeat formation tests in the Kielpinski well, Bay County. Pressures versus elevation; open symbols are RFT pressures; solid symbols are DST pressures from other wells in the region; the solid line is the theoretical pressure profile for a column of fluid with a density of 1.16 g/cm3 extending to the land surface.
SUMMARY AND CONCLUSIONS
Anomalous pressures within the deep Michigan basin, although subtle compared to overpressures observed in regions of active subsidence such as the Gulf Coast, are nevertheless identifiable through a careful evaluation of heads and gradients between units. A large area of overpressures, which can be considered a megacompartment, exists within the St. Peter Sandstone and the Glenwood Formation to the west and north of Saginaw Bay. The overpressures must be considered inconsistent with a steady-state, topographically driven flow system because heads are highest in the regional discharge area and because vertical gradients are reversed compared to those expected for the current regional flow system. The megacompartment includes most of the producing fields in the deep areas of the St. Peter and Glenwood. Abrupt vertical variations in pressure within individual wells suggest the presence of a series of smaller, stacked compartments within the megacompartment.
It seems likely that the existing overpressures have persisted at least since the last glaciation. Overpressures within the megacompartment appear to be slowly equilibrating with normal pressures within the surrounding stratigraphic units, primarily by leakage through anticlinal structures that likely contain high-permeability fault and fracture zones. The large vertical variations in pressure revealed by the RFT tests require low-permeability zones within the St. Peter to inhibit flow and adjustment to the surrounding pressures.
The primary significance of the pressure patterns revealed by this study lies not in the existence of the overpressures themselves, but in what these overpressures reveal about the controlling permeability distribution within the basin. When permeability contrasts are the result of lithologic variations such as those between carbonate rocks and sandstone, they are relatively easy to infer from standard wireline log signatures. However, in a sedimentologically homogeneous sandstone such as the St. Peter, significant permeability contrasts can only be explained as the result of diagenetic features, which may be difficult to identify using conventional log analysis. The pattern of overpressures observed in the St. Peter, particularly the vertical variations that suggest stacked compartments, has led to a closer examination of diagenetic banding within the formation, described in the chapters by Drzewiecki et al. and Shepherd et al. in this volume. It has also prompted development of statistical techniques suitable for detecting subtle changes in wireline log signatures that can be correlated to porosity and permeability, as Moline et al. describe in this volume. Both the diagenetic and log signature studies should enhance understanding of the distribution of low-permeability zones within the basin and provide tools for improved identification of potential reservoirs and traps.