Vertical lithostatic stress

The vertical lithostatic stress at the level of the GTS test galleries is around 9-l2MPa. However, the horizontal stresses are substantially higher than the depth-related overburden pressure. Significant horizontal forces have been measured in the main compression direction NW-SE. Maximum and minimum horizontal stresses are in the range 18/45 MPa and 15/32MPa. More details may be found in Briiuer et al (1989) and Pahl et al (1989)... 

In most extensional basins it can be assumed that minimum total stress is horizontal, and maximum total stress (S,) is equivalent to vertical lithostatic load. From Eq. (3) it can also be predicted that minimum total stress plus tensile rock strength is equivalent to the maximum formation fracture pressures that are measured from leak-off tests performed after drilling out casing shoes (LOT), or more approximately from formation interval tests (FIT). As suggested by Gaarenstroom et al. (1993), the lower bound envelope of LOT values may correspond to the... 

Fig. 14. Relationships between pore-pressures, the hydrostatic gradient, the fracture pressure gradient (approximation to the minimal horizontal stress, Sf,) and the lithostatic pressure gradient (approximation to the vertical stress, S ). Pore-pressures from sea floor to base Pliocene equals hydrostatic. The yellow, dark blue and red pore-pressure trend-lines represent the pore-pressure versus depth gradients for the Paleocene-Eocene, Mid-late Cretaceous and Upper Jurassic-lowermost Cretaceous, respectively. The portion of the red trend-line below approximately 2550 m MSL equals the maximum reservoir pore-pressure trend-line of Fig. 13 and reflects the counter-pressure of the topseal controlling the pore-pressure distribution of hydraulic compartments II, III and (probably) IV.

The initial stresses and boundary stress conditions were introduced in accordance with the in situ data described in Section 2.3. Vertical stress varies between 8.04 MPa and 13.4 MPa, depending on the lithostatic pressure (with the overburden varying from 300 to 500 m). The maximum horizontal stress is constant, equal to 34.5 MPa (mean of the measured values) and oriented N 130° (i.e. perpendicular to S2 schistosity), whereas the minimum horizontal stress is 23.5 MPa (mean of the measured values) and oriented N 40° (i.e. parallel to S2 schistosity). 

Initial condition for the resaturation calculations correspond to mechanical equilibrium under gravity and a lithostatic vertical stress at the top of the considered shear zone of 3 MPa. 

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