Page 266 - Origin and Prediction of Abnormal Formation Pressures

238 H.H. RIEKE, G.V. CHILINGAR AND J.O. ROBERTSON JR.

chemical difference of waters in a given local stratigraphic zone or the samples were
contaminated. In the opinion of the writers, any chemical analyses of pore waters should
be checked by charge balance criteria. The total milliequivalents per liter of cations and
anions must be equal to each other within 5 to 10%. In order to perform this check, the
analysis must be made for all the major cations and anions present in the sample.
Morton and Land (1987a) pointed out that abnormally high pressured Oligocene
Frio sandstones in Texas contain waters having salinities ranging from around 8000 to
more than 250,000 mg/1 (total dissolved solids). The high values could be due to the
dissolution of diapiric salt. Low salinities are attributed to pore-water dilution by water
released from the transformation of smectite clay to illite (hydromica), i.e., by mineral
dehydration reactions.
Manheim and Bischoff (1969) were the first to suggest the increase of pore-water
salinity with depth in relationship to diapiric salt structures in the Gulf Coast. Pore
waters were analyzed from six boreholes drilled offshore in the Gulf of Mexico.
Pore-water samples from the drillholes near diapiric structures showed systematic
increases in salinity with depth. The salinity showed little change with depth in
those boreholes drilled away from the diapiric structures. Manheim and Bischoff
(1969) suggested that salt diffusion from underlying salt structures were the cause of
this increase in salinity. The mass transport of highly saline waters in sedimentary
basins will have a strong impact on the transport of hydrocarbons, ore fluids, heat,
and diagenetically reactive dissolved constituents. Hanor (1987b, 1999) discussed the
concept of thermohaline overturns and the resulting mass transfer of pore water in
southeastern Louisiana. He proposed that there are three major types of subsurface
flow regimes in this area. The uppermost (shallowest) consists of topographically
driven, fresh-water systems (ground water). A thermohaline system can exist at an
intermediate depth where salt diapirs are present. The deepest zone is the regional
overpressured regime. The salinity (total dissolved solids) overprint on the intermediate
zone's pore-water chemistry is a direct result of the presence of salt, and the induced
fluid circulation is driven in part by fluid density inversions resulting from spatial
variations in salinity and temperature. This chemical overprint could be expected to
exist in other young basins where salt beds and diapirism are present. Capuano (1990)
simply stated that compaction-driven flow dominates in the abnormally high pressured
sediments, whereas gravity-driven or thermal-density-driven flow dominates in normally
pressured sediments.

Field case studies
The importance of knowing that the pore-water concentrations are lower in shales
than in associated sandstones was pointed out by Chilingar et al. (1969). Erroneous
interpretation of well logs may result if it is assumed that the salinity of pore waters
in sandstones and associated shales are the same. In order to determine the water
saturation, Sw, in well log analyses, it is necessary to know Rw (resistivity of the pore
water in the formation being evaluated). Under favorable conditions, the latter can be
determined on using the SP curve. This approach, however, is not practical in many
cases owing to the properties of some drilling fluids and other variables that can cause