Page 178 - Geochemistry of Oil Field Waters

IODINE 165

tration in the interstitial waters increases with depth (Shishkina and Pavolva,
1965).
The iodide in bottom water layers and in the interstitial water of muds in
some Japanese lakes was found to be selectively captured by flocculated
iron, manganese, and aluminum hydroxides which sank to anaerobic layers
(Sugawara et al., 1956). Reduction of the hydroxides releases iodide to the
bottom waters. However, the release of iodide is incomplete, and the
flocculates reach the bottom muds where the Eh is even more negative,
resulting in high accumulation of iodide in interstitial water of muds.
The primary source of organic matter in marine and oceanic basins is
photosynthesis by plankton algae. Algae are directly or indirectly the food
resource of all the remaining life in the basins, and the proliferation rate
differential and the types of feeding organisms influence the sediment deposi-
tion rate as well as the amount of iodide and bromide in the sediment
(Bordovskii, 1965).
Shales, sandstones, and carbonates contain about 2.2, 1.7, and 1.2 ppm,
respectively, of iodide (Mason, 1966). Sea water contains about 0.05 mg/l,
and most subsurface petroleum-associated brines contain less than 10 mg/l;
however, some have been found to contain up to 1,400 mg/l.
Fig. 5.18 is a plot of the chloride concentrations versus the iodide concen-
trations for some brines taken from some Pennsylvanian and Mississippian
age sediments. Iodide is tremendously enriched in all of these brines com-
pared to the normal evaporite-associated brine. Some mechanisms such as
leaching or solubilization of iodine, iodate, or iodide compounds, ion fil-
tration, anion exchange, and desorption had to occur, to account for this
enrichment of iodide in the aqueous phase. A similar plot for some waters
taken from Tertiary, Cretaceous, and Jurassic age sediments gave similar
results except that these particular brines were not as heavily enriched in
iodide.
The iodide concentration of some subsurface waters is dependent on the
proximity of argillaceous deposits containing organic matter, rather than on
dissolved mineralization. Gas may play an important part in the accumula-
tion of iodide in subsurface waters. Some gas structures are bounded by
iodide-rich waters, and the iodide content is depleted at a distance from the
gas structure (Ovchinnikov, 1960).
Studies of some reservoirs, Holocene to Miocene in age, in lagoonal sedi-
mentary basins of thick sediments with wide areal extent, indicate that a
genetic relation exists between iodide in the formation waters and the
accompanying natural gas (Marsden and Kawai, 1965). Possibly the high
concentrations of iodide are the result of concentration by algae and other
marine organisms from ancient sea waters; their remains became part of the
sediments, and later the iodide was solubilized. However, because the iodide
usually is strongly incorporated in the sediment, such sediments must con-
tain large quantities of iodide, and other mechanisms must operate to solu-
bike the iodide in associated waters.