Magnitudes and orientations of present-day in-situ stresses in the Kurdistan region of Iraq: Insights into combined strike-slip and reverse faulting stress regimes

Abstract

The study area is in the Kurdistan Region of Iraq, including the northwestern extension of the Zagros suture zone, where present-day stress data derived from actual measurements is rare. The magnitudes and orientations of principal in-situ stresses were determined using well logging data. The deepest drilled formation in this study was the Albian upper Qamchuqa Formation in northeastern Iraq, which is a carbonate hydrocarbon reservoir. In the present study, several wireline logs including borehole image logs, conventional wireline logs, and six-arm caliper logs for two vertical wells provided detailed borehole breakout and drilling-induced tensile fracture (DITF) data between depths of 1600 and 2240 m. Two oil wells (wells A and B) were evaluated, and in well A, several breakouts occurred in eight distinct zones. Below the depth of 1600 m, 26 pairs of breakouts with considerable length were observed. The standard deviation of breakout azimuths was 11°, and the mean azimuth of the breakouts was N162°E. Using breakout and DITF data, the orientations of the maximum and minimum horizontal stresses (SH and Sh) were determined, and the results were validated by six-arm caliper measurements and lithological evaluation from wireline log data. The mean azimuth of maximum horizontal in-situ stress was N72°E, which showed relative consistency with the NE–SW to E–W direction of tectonic movement and previous studies in the nearby Zagros suture zone. Ultimately, the magnitudes of the three principal in-situ stresses were determined by two methods: poroelastic strain theory and breakout analysis. The stress regime below the depth of 1600 m was a reverse faulting stress regime, and above this depth, it likely changed to a strike-slip faulting stress regime. The determined stress regime was consistent with the nature and dynamics of the tectonic plate movement of the Arabian and Eurasian (Iranian) plates. Consequently, results suggest that the pattern of present-day tectonic stress is controlled mainly by the collision between the Arabian and Eurasian plates.