Changes in sub‐water table fluid flow at the end of the Proterozoic and its implications for gas pulsars and MVT lead–zinc deposits

Abstract

A fundamental change in the nature of sub-water table fluid flow occurred at roughly the Proterozoic-Phanerozoic boundary when organic matter began to be buried in sufficient quantities that nonaqueous fluids could occupy a significant fraction of the pore space. This allowed the formation of remarkably durable capillary seals that could trap gas in large portions (hundreds of kilometers) of a basin for hundreds of millions of years. Gas loss from these gas zones can be highly dynamic, especially during gas generation. Under the right circumstances, hundreds of cubic kilometers of gas can be rapidly discharged into adjacent permeable aquifers. In Pennsylvanian/Permian time, the Arkoma Basin may have repeatedly discharged such volumes of gas into the very permeable Cambrian sandstone and karstic carbonate aquifers of the mid-continent of the United States. This could have displaced brines rapidly enough to form the Mississippi Valley-type (MVT) lead-zinc deposits of this age that are associated with the Arkoma Basin, heating them only briefly as required by maturity indicators. Sea level rise accompanying the melting of Permian continental glaciers may have triggered the gas expulsion events. This radically new mechanism for the formation of MVT deposits is just one example of the nonlinear dynamics of gas accumulations that are possible since Late Proterozoic time.

The physics of gas chimney and pockmark formation, with implications for assessment of seafloor hazards and gas sequestration

“…When the source gas is exhausted and the pressure at the seal again approaches hydrostatic, capillary forces will draw water into the strata above the pocket and the capillary seals at the top of the decompressed gas pocket will re-heal (Cathles, 2007). At this point all gas leakage from the gas pocket will cease, but it will take some time for the gas chimney to drain its gas and it will never drain completely.…”

“…If a single fluid, for example gas, penetrates the seal, the seal completely disappears and offers little resistance to the nearcomplete and rapid release of gas from the pocket (e.g., Cathles, 2007). The process we envision is that gas is trapped below a finegrained layer or set of layers and accumulates.…”

Seismic chimneys in the Southern Viking Graben – Implications for palaeo fluid migration and overpressure evolution

“…Fracture failure corresponds to hydrofracturing, which occurs if the pore (Hubbert and Willis, 1957). Capillary failure occurs, when pore pressure overcomes the capillary resistance to flow (Clayton and Hay, 1994) and gas as a non-wetting phase is introduced to the pore space leading to rapid loss of sealing capability and thereby rapid fluid release (Cathles, 2007;Cathles et al, 2010). Both seal failing mechanism rely on the buildup of high pore overpressure, which requires the presence of mobile fluids and a highly permeable reservoir facilitating fluid migration as well as a cap rock with a low permeability, which inhibits the reduction of pore pressure via diffuse flow.…”

“…Both seal failing mechanism rely on the buildup of high pore overpressure, which requires the presence of mobile fluids and a highly permeable reservoir facilitating fluid migration as well as a cap rock with a low permeability, which inhibits the reduction of pore pressure via diffuse flow. The buoyancy of free gas causes a local pressure increase on the sealing cap rock, which may result in rapid fluid expulsion if the pore pressure in the reservoir overcomes the capillary entry pressure (Cathles, 2007;Cathles et al, 2010). The amount of pressure increase directly depends on the column height of the gas pocket.…”

Formation of a giant Zn–Pb deposit from hot brines injecting into a shallow oil–gas reservoir in sandstones, Jinding, southwestern China

“…The reduced sulphur produced through BSR most likely accumulated in the gas cap of the reservoir (Figure ), and the brines must flow at a high velocity to penetrate the oil zone and reach the gas cap. Such a fluid flow is typical of overpressured systems (Cathles, ; Cathles & Smith, ), as has been proposed for the Gulf Coast basin (Kyle & Agee, ; Posey, Kyle, & Agee, ), the Maritimes basin (Chi, Kontak, & Williams‐Jones, ; Chi & Savard, ), the Northwest Carboniferous basin in Ireland (Middleton, Parnell, Carey, & Xu, ) and the North Sea (Jonk et al., ). An overpressure‐driven fluid flow system has been proposed for the Jinding deposit based on the development of sand injection and liquefaction structures (Chi, Qing, Xue, & Zeng, , ; Chi et al., , ).…”