The characteristics of fracture networks in the Shiranish Formation of the Bina Bawi Anticline; comparison with the Taq Taq Field, Zagros, Kurdistan, NE Iraq

Abstract

The Zagros Fold and Thrust Belt of NE Iraq hosts a prolific hydrocarbon system. Reservoirs are commonly found in fractured Cretaceous carbonates (Shiranish Formation) such as in the Taq Taq Field located in the Kirkuk Embayment of the Zagros foothills. Data providing information on fractures in the Taq Taq Field are core, image logs and flowmeters from wells, and surface observations. For comparison, an outcrop study has been undertaken around the Bina Bawi Anticline (10 km from Taq Taq Field), where the same stratigraphical unit is exposed in a continuous, lenticular-shaped belt. Fracture data have been collected using scanlines on bedding surfaces in the limbs and hinge of the anticlines. Both the Bina Bawi Anticline and Taq Taq Field show a systematic relationship between fracture sets and fracture lineaments, with a dominance of NE–SW-oriented structures. This orientation is perpendicular to the major folds and parallel to the maximum horizontal in situ stress. There are three fracture populations in the Bina Bawi Anticline, classified according to their relationship with the fold axis and bedding: (i) NW–SE-striking fractures normal to bedding: (ii) NE–SW-striking fractures normal to bedding; and (iii) conjugate oblique fracture sets subnormal to bedding. Both fracture intensity and fracture terminations are controlled by the location within the anticline; the hinge zone displays the highest intensity and the most fracture-abutting terminations. Cross-cutting relationships suggest that a prefolding stage of NE–SW tensional fractures predates folding-related tensional and shear fractures. Few uplift fractures can be indicated. We propose that the former fracture set (joints) formed in a foreland setting and was controlled by the far-field stresses, whereas later fracturing occurred due to outer arc extension during flexing of the Bina Bawi and Taq Taq anticlines. Our comparative analysis of outcrop and well data underline the importance of representative analogue data for reservoir modelling and production strategies.