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