Page 265 - Origin and Prediction of Abnormal Formation Pressures

PORE WATER COMPACTION CHEMISTRY AS RELATED TO OVERPRESSURES 237

Sea Level

,-':.:!~!:-!:i!". ;'!"--':-').':.''.~:."..".~::."::.)).~i...:. o a "'-"?'-':-."::.
::-'.;.:.::..:.-.'..:: ::...:.;::;:..:.........:.....'..-...:.:.:-:.;... 9 0 0 ".';?:~

'.-'-?'-~":..':.. "'"-"';::."-..... "" #:."::..-"-".'i!i:!'i'~':':.:"-):'."." :., '~o~

Fig. 10-4. An idealized cross-section of some sands and shales in the offshore subsurface of southwest
Louisiana illustrating salinity relationships. (Modified after Timm and Maricelli, 1953, pp. 396, 397 and
408; in Rieke and Chilingarian, 1974, fig. 146, p. 271.)

samples from many gas wells, on the other hand, yield much lower salinities. Kharaka et
al. (1977) believed that these samples are not representative of the true formation waters'
salinity owing to the dilution of the condensed water produced with the natural gas.
Their study shows that the salinity in the geopressured zone is generally, but not always
lower, than salinities in the normally pressured zone. Dickey et al. (1972) reported that
formation waters from four wells in southwestern Louisiana showed almost normal con-
centrations of total dissolved solids in the geopressured zone for those depths of burial.
The findings of Fowler (1968) suggest that the salinity of water in undercompacted
shales in the Chocolate Bayou Field, Brazoria County, Texas, is higher than in the
well-compacted ones. A definite correlation between the high salinity of interstitial
fluids and abnormally high pressures was found to exist. This is possibly due to the
fact that undercompacted shales did not have a chance to contribute their fresher water
to the associated sandstones. Fowler (1968) also studied the variation in salinity of
produced water with time. The typical pattern is one of decreasing salinity with time,
and the freshest water is found in sands receiving most of this water from associated
shales. Plummer and Sargent (1931) and Elliott (1953) also studied the variability of
pore waters from specific zones in wells taken over a 1- to 48-month time period. They
found that there was no definite trend of ion concentration with time.
Chave (1960, p. 359) reported that the percentage difference in the concentration
of a given ion could vary from less than 1% to as much as 168%. Chave commented
that the reasons for the differences between resampling by Plummer and Sargent (1931)
and Elliott (1953) were not clear. He suggested that they could represent the natural