Present day In-situ stress magnitude and orientation of horizontal stress components in the eastern Illizi basin, Algeria: A geomechanical modeling

摘要

We analyzed drilling induced tensile fractures from resistivity image log data to ascertain the orientation of maximum horizontal stress (S-H) from the eastern Illizi basin, Algeria. An average S-H azimuth of 150 degrees N (+/- 10 degrees) has been interpreted from B-quality induced fractures, as per world stress map guidelines. The overall NW-SE orientation of S-H translates to the relative plate motion of the African and Eurasian plates. Vertical stress (S-v) gradient of 1.07 PSI/ft has been derived from density log. Pore pressure estimated from sonic slowness reveals overpressure in Silurian shale, deposited in a transgressive depositional environment, whereas Devonian and Ordovician hydrocarbon reservoirs have been seen to be normally pressured. Poroelastic strain model has been employed to quantify maximum and minimum horizontal stress (S-h) magnitudes. An effective stress ratio of 0.6, interpreted from leak-off test has also been used to model S-h. Using frictional faulting theory, upper limit of S-H has been quantified. S-H/S-v ratio of 1.04 (1.01-1.26) has been seen in the study area. Based on the relative stress magnitudes (S-H > S-v > S-h), a present day strike-slip faulting regime has been inferred in the eastern Illizi basin, Algeria. Fault reactivation potential at reservoir level has been inferred from stress polygon analysis.