Fracture Analysis in the Outcrops of the Qara Chauq Anticlines, Zagros Fold and Thrust Belt of Kurdistan Region in Northern Iraq

Abstract

The Zagros Fold and Thrust belt is one of the world's most prolific petroleum provinces. Most hydrocarbon reserves are stored in naturally fractured reservoirs and such fracture systems can therefore have a significant impact on reservoir performance. Fractures are one of the most important paths for fluid flow in carbonate reservoirs. Fracture data were collected in the outcrops of the Kirkuk Group of Oligocene age around Qara Chauq South and Qara Chauq North anticlines located near the Kirkuk Oil Field. The studied formations outcropping in the Qara Chauq are the main reservoir units in the Kirkuk and Bai Hassan fields. In Kirkuk and surrounding fields, hydrocarbon production comes mainly from primary porosity with assistance from secondary porosity created by dolomitization, karstification, dissolution, vugs and fractures. Fracture attributes collected from outcrops are fracture orientation, density and length. The results show that fractures in the studied reservoir formation are not uniformly distributed due to massive lithologic nature and lack of well bedding. Furthermore, fracture orientations show a clear relationship to the local fold axis in the outcrops. NW-SE fracture set is perpendicular to the NW-SE fold axis. However, some fractures do not show any relation to the local folding. These fractures may have formed in a pre-folding or post folding stage. Other fracture orientations exhibit a symmetrical relation to the maximum horizontal stress direction. The comparative analysis of outcrop data underlines the importance of representative analogue data for reservoir modelling and production strategies.