Page 251 - Origin and Prediction of Abnormal Formation Pressures

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Chapter 10

PORE WATER COMPACTION CHEMISTRY AS RELATED TO
OVERPRESSURES

H.H. RIEKE, G.V. CHILINGAR and J.O. ROBERTSON JR.

INTRODUCTION
Much has been written in the petroleum geology literature on the geochemical
evolution of pore liquids and gases associated with fluid flow systems in recent
and ancient sedimentary basins. The dialogues include observations about the origin
of interstitial fluids, measurements of the active chemical diagenetic processes, and
resulting mass-transport properties, which arise during the development of sedimentary
basins. Effects of thermal and chemical factors and the dynamic transfer of fluids within
the basins leave imprints on the pore-fluid chemistry and generation of abnormally high
(AHFP) or abnormally low formation pressures (ALFP).
It is the purpose of this chapter to present and validate a hypothetical model that
explains the differences between the salinities of pore water in sandstones and shales
in the gravitationally compacted sedimentary basins of Tertiary age. The explanation
presented here is based on two diverse, relative scales of resolution- microscopic (10 -2
to 10 -4 m) and gigascopic (> 105 m). The gigascopic scale presents evidence from field
observations, whereas the microscopic scale focuses on laboratory experiments that
dealt with the chemistry of fluids in the pore space. Mathematical and conceptual
models are presented and discussed, which support these observations. Additionally, the
relevance of the isotopic character of shale pore water is evaluated for this environment.
The all-inclusive premise is that the pore-water salinities in shales are lower than those
in associated sandstones in compacting sedimentary basins. A corollary to our premise
is that the salinities of solutions 'squeezed out' during compaction are a function of the
overburden pressure, temperature, and rate of compaction. The salinities of solutions
expelled out of pelitic sediments decrease with increasing depth in young basins having
a high sedimentation rate and, therefore, a high sediment compaction rate. In basins
with low sedimentation rates, the compaction rates are low and the pore-water salinities
increase with increasing depth. AHFP zones are normally absent in these basins.
A majority of previous investigations focused on placing the observed single-phase
fluid flow systems and their geological outcomes into four general categories: after
sedimentation, during burial, during tectonic deformation, and during uplift and erosion.
These four categories define the accepted, customary steps in sedimentary basin filling
and evolution.
The following references on pore-fluid chemistry and fluid flow provide a background
to the above four categories. Goldberg et al. (1971), Manheim (1976), Sayles (1979),
and Gieskes et al. (1990) presented chemical analysis of sea, bottom, and pore waters