Page 260 - Origin and Prediction of Abnormal Formation Pressures

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

from oil and gas wells is probably not representative of the interstitial water owing
to contamination, dilution by condensed water vapor, questionable fluid sampling,
preservation, and laboratory analytical procedures.
Water produced with the oil can represent a mixture from different production
horizons in the case of commingled production or as a result of poor cement job or
leaky casings. During the cementing of casing into place in a well, the filtrate from
the cement slurry can penetrate the permeable sandy zones and contaminate the pore
water. Hydraulic fracturing fluids can also contaminate reservoir waters. In some cases
it takes more than three months for the fracturing fluids to be produced back after
fracture stimulation treatment. It is prudent to view with caution pore-water chemistry
results reported in the literature that are based entirely on produced water sampled at the
wellhead.
These are just some of the problems encountered by investigators looking at
pore-water chemistry. With respect to unconsolidated sediment samples, the handling
of the samples can be critical in obtaining accurate analytical results. Problems include
changes in temperature and pressure, contamination by bottom water and seawater as
the cores are retrieved. Further changes can take place owing to evaporation, oxidation,
and the type of equipment and supplies used in extracting and storing the sample, and
the magnitude of the pressure at which the pore water is squeezed out of the sediment
sample. Mangelsdorf et al. (1969) first demonstrated that temperature changes alter
ion-exchange equilibria and bring about changes in pore-water chemistry. Bischoff et al.

Fig. 10-3. It is important to have a sense for the relationships among pore-water chemistry, petroleum
basin types and the origin of the abnormal fluid pressures. Klemme's (1984) petroleum basin classification
with cross-sections showing idealized basin profiles, basin parameters, stratigraphy and structures are used
to proffer some generalizations about what kind of water chemistry might occur in these basin types.
Basin examples given for Kiemme's (1984) basin types are the following. Basin Type I (U.S./Canada
Williston) has abnormal pressures due to hydrocarbon generation rather than compaction. Basin flushing by
water has influenced the water chemistry. Basin Type IIA (U.S. Wind River) has basin-centered abnormal
fluid pressure zones. Well-logs show that resistivity increases in the more thermally mature rocks. Water
chemistry is modified by coalbeds and artesian flow into the basin. Most of these reservoirs in this type
of basin are gas and have water-free production. Basin Type IIC (U.S. Gulf Coast) and IV (Mississippi
Delta) exhibit compaction water chemistry associated with regressive sedimentary sequences, growth faults,
mud volcanoes, and smectite to illite clay mineral transformation. Rift basins offer a more complex picture.
Basin Type IliA (North Sea Viking Graben) basins illustrate that two distinct abnormal pressure zones
can exist. One pressure zone is above another below the characteristic (unconformity/disconformity) zone
which tends to occur in this type of basin. The abnormal pressure zones are basin-centered making the
water chemistry profile complex. Type IIIB (South Sumatra, U.S. Ventura, Maracaibo) basins have a broad
range of imposed stress conditions that commingle the effects of local and major tectonic forces and
gravitational compaction on the water chemistry. Usually the water chemistry shows a salinity decrease with
depth but can change areally and vertically over the section and in individual structures. Basin Type IIIC
(Australia's North West Shelf) having abnormal formation pressures which depend upon the rate and type
of sedimentation. Water chemistry is a mixed bag. Basin Type IV are forearc basins (Sacramento, U.S.)
which has a mixed history, of tectonism, compaction and basin flushing. Major deviations from compaction
water chemistry expectations can be attributed to dehydration in the change from gypsum to anhydrite in
evaporitic sequences, coalification, and lack of argillaceous sediments. (After Klemme, 1983, fig. 3, p. 170;
reprinted with the permission of the Oil and Gas Journal.)