Fluid Release Valve

Shosa seals can open and shut like valves. When open, very large volumes of gas can be released very quickly. Figure 9 gives a calculated example for a series of three 1cm-thick partings with 0.1 md permeability. The caption explains the valve-like operation.

**Figure 8.** Observed (black points) and modeled (yellow band) porosity-depth profile from Gulf of Mexico. Profile records initiation and migration of a seal. Figure from [57].

**Figure 9.** Illustration of Shosa seal operating as a pressure release valve. (**Top**) The capillary-blocked pressure drop across the 3-parting seal is 22.5 bars. (**Middle**) If gas were to penetrate the seal, as shown in the second panel, resistance to flow would be only the permeability of the partings. An expulsion Darcy flux of 10 m y<sup>−</sup><sup>1</sup> would produce a pressure drop across all three partings of less than 0.2 bars. Gas in the compartment could be expelled rapidly and completely. (**Bottom**) Once pressure in the compartment had been dissipated, water would be imbibed into the seal strata until the seal was restored, as shown in the bottom panel. Pressure in the compartment might then build up again if gas generation continued, until a second rupture and discharge occurred. Figure modified from [41].

### Gas Pulsar Formation of Mississippi Valley-Type (MVT) Deposits

Figure 10 illustrates how failure of the seal between the gas-filled Arkoma and the darcy permeability aquifer system underlying the interior of the North American continent could have delivered pulses of hot brine to the sites of metal deposition. Weathering of Precambrian rocks in North America produced the extensive mid-continent darcy permeability Lamotte and Mt. Simon sandstones. These were subsequently overlain by karstic (and therefore very permeable) carbonates. The bines that produced the mid-continent MVT deposits discharged from these aquifers where the basal sands or carbonates approached the surface or were intersected by numerous or major faults. The pattern of hematite reduction in the Lamotte and the decrease northward from the Hicks Dome source of anomalous flourine in the St. Peter sandstone, sugges<sup>t</sup> the mineralizing brines came from the Arkoma (Figure 10).

**Figure 10.** Periodic massive expulsion of gas from the Arkoma into the Lamotte and karstic Cambrian carbonates could displace sufficient volumes of brine rapidly enough to form the MVT deposits found in the North American interior (grey stippled areas). Brine flow from the Arkoma is suggested by the pattern of hematite reduction in the Lamotte, and the pattern of F carried north from the Hicks Dome and associated intrusions. Figure from [41] is a composite of figures from [58–60].

How could the presently gas-filled and under-pressured Arkoma have propelled the brines? The hypothesis is that back in the Permian when the Arkoma was actively generating gas it was overpressured. Periodically the capillary barrier between the Arkoma and the Lamotte and overlying carbonate was invaded by gas, rendered permeable, and perhaps as much as 3240 km<sup>3</sup> of gas was rapidly injected into the aquifers. The brine displaced by this gas exited the aquifer system rapidly at all the most permeable excape locations, e.g., at exactly the sites of MVT deposition. After decompression, imbibition of water resealed the barrier seal, the Arkoma repressured and then delivered a second pulse of gas which caused a second pulse of brine and mineralization, etc. The expulsion could be rapid enough to warm the sites of MVT deposition as observed [41]. The underpressured state of the gas in the Arkoma indicates that the gas pressure after the last expulsive pulse was, and is presently, controlled by capillary seals at the top of the gas zone (e.g., [55]).

Late Paleozoic Remagnetization of the North American Mid-Continent

A plethora of paleomagnetic studies have confirmed that the entire mid-continent of the U.S. was re-magnetized in the Late Paleozoic. The area re-magnetized, by magnetite deposition, is that shown in Figure 10, but extends into Indiana, Kentucky and Ohio as well. It is considered a consequence of tectonically-driven brine migration [61], and occurred at the time the MVT deposits formed [62–64]. It is logical that the pulses of massive brine expulsion discussed in the previous section were also responsible for this continent-scale re-magnitization.
