Page 252 - Origin and Prediction of Abnormal Formation Pressures
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224 H.H. RIEKE, G.V. CHILINGAR AND J.O. ROBERTSON JR.
after sedimentation. Kryukov and Zhuchkova (1963) and Sayles and Manheim (1975)
reported on the chemistry of fluids during burial. Ancillary experimental compaction
studies of peat and the mobilization of major inorganic ions were reported by Bailey
et al. (2000) on three different naturally occurring peats, i.e., Cladium, Rhizophora,
and Cyrilla. Berry (1959, 1973), Stephenson et al. (1994), Kouznetsov et al. (1994),
Chilingar et al. (1996), and Khilyuk et al. (2000) relate pore-fluid chemistry and the
movement of fluids to tectonic activity.
Cannon and Craze (1938) stated that uplift and erosion are not the probable causes of
abnormally high pressures in the U.S. Gulf Coast Basin. Rowaik (1975), Magara (1978),
and Luo and Vasseur (1995), however, pointed out that uplift and erosion are important
factors in the evolution of pore pressures in such environments. Subnormal pressures
can be created by uplift and erosion through the reduction of temperature, which causes
shrinkage of the fluids and porosity rebound. This can create a differential pressure in
the overall hydrodynamics of the uplifted sediments. Dobrynin and Serebryakov (1989)
attributed the occurrence of abnormally low pressures located in the Nepsko-Botuobin
anticline in eastern Siberia to changes in the surface temperatures of the Earth during
geologic times.
There are three commonly accepted practical classifications of pore waters based on
their chemistry and the geochemical interpretation of the results:
9 Palmer (1911) m American
9 Sulin (1946) m Russian
9 Schoeller (1955) m French
Collins (1975) gave an excellent review of the water analysis procedures and
resulting interpretations from the above classification schemes for the Gulf Coast and
mid-continent reservoir pore waters.
The present-day understanding on how these pore waters originate, what affects their
chemical compositions, and how fluids migrate are presented and debated in this chapter.
Arguments and perspectives, both pro and con, are presented to facilitate examining the
role of pore-water chemistry in excessive-pressured zones.
OVERVIEW AND CONSTRAINTS
The chemical composition of pore waters in abnormally high pressure zones in
sedimentary basins often differs from the composition of the pore waters in associated
normally pressured formations. This is especially true for subsiding immature Tertiary
and young Quaternary sedimentary basins with high sedimentation rates and thick
shale components. Chilingarian et al. (1994) pointed out that overburden pressures
on sediments in these basins may reach as high as 45,000 psi (about 300 MPa).
This results in a strong driving force for the migration of excess pore waters during
gravitational compaction when large amounts of interstitial water are squeezed out of
argillaceous sediments and are expelled into the associated permeable beds. Since the
mid-1950s, there has been a steady progress toward comprehending how subsurface
pore fluids have evolved under gravitational compaction conditions, and the differences
in pore-water chemistry between normally compacted and under-compacted sediments.