Petroleum exploration geologists have observed a close association between natural gas zones and abnormally pressured compartments. The potential that this association has for petroleum exploration has motivated the development of many theories explaining the nature and origin of these compartments and whatever it is that isolates them from the rest of the aquifer. These theories are too numerous and too involved to be discussed adequately in this paper. However, Table 2 provides a summary of the literature pertaining to abnormally pressured reservoirs reviewed for this thesis.
1. DIAGENETIC TRAP A. Quartz overgrowthes *Law and Dickinson, 1985. B. Crushing of sedimentary rock fragments *Cant, 1983, SW Alberta. C. Cementation by carbonates and clay *Powley, II. HYDRODYNAMIC TRAP A. Berg, 1985, NE Powder River Basin, WY. B. Moore, 1984, SW Powder River Basin, WY. C. Lin, 1981, Powder River Basin, WY. III. WATER TRAP (water on top of gas) A. Gies, 1984, SW alberta, Canada. B. Davis, 1984. C. Masters, 1979, Deep Basin, Western Canada. IV. EXPULSION OF INSITU NATURAL GAS A. Silver, 1968, San Juan Basin, NM and CO. V. STRATIGRAPHIC TRAP A. Stone and Hoeger, 1973, Big Muddy S. Glenrock area.
Under pressurized compartments existing along the axis of the Denver-Julesburg basin are documented by Belitz (1984) and Matuszczak (1973). The Wattenberg field, which produces natural gas from a large under pressured reservoir in the Muddy Sandstone, is located along the synclinal axis of the Denver-Julesburg basin northeast of Denver. There is a large area in the Wattenberg field which continues to produce gas with out producing water. This demonstrates the lack of hydraulic communication between the under pressured and the normally pressured parts of the Muddy Sandstone (Matuszczak,1973).
The mechanism which traps the gas in these compartments is, presumably, the same mechanism which keeps the water out. According to Matuszczak (1973), the trap on the south and west sides of the Wattenberg field is formed by the pinchout of the reservoir sandstone into a thin, tight siltstone and silty sandstone. On the northeast and east sides of the field the gas is trapped by a loss of permeability resulting from an increase in the presence of siliceous cement and clay (Matuszczak, 1973).
Petrographic work reported by Dickinson and Guatier (1983) indicates that the loss of permeability at the abnormally pressurized compartment boundary is caused by one or more of the following: (1) precipitation of calcite and/or silica cements early in the burial history, (2) grain deformation and compaction, and (3) filling and coating of primary and secondary pores with illite, chlorite, microcrystalline quartz or ferroan carbonates.
Figure 4 indicates the presence of one or more under pressured compartments in the Muddy Sandstone which extend the full length of the project area from north to south. Evidence for compartmentalization is revealed on the potentiometric map as an abnormally low potentiometric surface throughout the study area, especially on either side of the synclinal axis. Potentiometric contours were drawn through the low pressure compartments even though they are hydraulically isolated from the rest of the aquifer because there was not enough data to define their boundaries.
Evidence supporting the presence of under pressured compartments in the Paleozoic aquifer is limited by the complete lack of potentiometric data available for the Paleozoic aquifer in the interior of the basin. Hoeger (1968) reports, in a general discussion of the hydrostratigraphic units of the Denver-Julesburg basin, that the formations beneath the Permian Lyons Sandstone are under pressured, although to a lesser extent than the Lyons Sandstone, which is a stratigraphic equivalent to the Satanka Shale of southeast Wyoming. No potentiometric data could be found to either support or to disclaim this statement for my area.
If the Paleozoic aquifer does have under pressured compartments like the Muddy Sandstone then recharge is probably blocked from reaching the synclinal axis of the basin by these compartments. Without more conclusive evidence, however, it is impossible to say what influence hydrologically isolated compartments have on the circulation patterns in the Paleozoic aquifer.
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