Abstract: Overpressures Assessment from a Minimum Principal Stress Approach
GRAULS, DOMINIQUE
Abnormal fluid pressure
regimes are commonly encountered at depth in most of sedimentary basins.
Relationships between vertical effective stress and porosity have been
applied, since 1970 in Gulf
Coast
area, to assess the magnitude of overpressures.
Positive results have been obtained in sand-shale; vertical stress dominated
Tertiary basins, whenever compaction disequilibrium and "no lateral strain"
conditions applied. However, overpressures resulting from other and/or
additional causes (tectonic stresses,
hydrocarbon
generation, thermal stresses,
fault related transfers, hydrofracturing...) could not quantitatively be
assessed using this approach.
An hydromechanical approach
is then proposed: at any depth, the fluid pressure upper bound is controlled
by in-situ conditions of hydrofracturing or fault reactivation. Fluid driven
fracturing implies episodically open system, under slightly negative minimum
effective stress regime. A good knowledge of present day tectonic stress
regime allows a direct estimate of minimum stress evolution and then a
quantitative fluid pressure assessment at depth, as pressure regimes, whatever
their origins, tend to reach rapidly, in closed or undrained geological
systems, a value close to, or very slightly in excess of minimum stress.
Therefore, a significant improvement in overpressure
assessment can be
expected, as this methodology, not only restricted to the undercompaction
phenomena, can be applied in various geological contexts.
In addition of overpressure
assessment, the minimum stress concept allows a better understanding of
petroleum system, as fault related
hydrocarbon
dynamical transfers, hydrofractured
domains and caprocks sealing efficiency are depending upon the subtle interaction
between the
overpressure
and minimum principal stress regimes through time.
AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria