Influence of fractures on the reservoir quality of Lower Miocene carbonates in Northern Iraq

The Zagros Fold and Thrust Belt of the Kurdistan region of northern Iraq host major volumes of hydrocarbons in multiple fractured carbonate reservoirs throughout the Jurassic, Cretaceous, Palaeogene and Neogene. Limited research has been undertaken to characterize these recently discovered carbonate reservoir rocks. Here, the reservoir quality of the Neogene Euphrates and Jeribe Formations is investigated. Samples from five outcrops along the Azhdagh and Mamlaha anticlines and from four nearby wells (North Oil Company wells JM37, KM3, SAR1 and KU1) have been analysed to understand better their reservoir properties. In both formations very low matrix porosities (< 12 ± 0.5%) were measured and no visible pore connectivity was observed in thin sections. Matrix permeability was less than 0.10 ± 0.008 mD. The low porosity of the Euphrates and Jeribe Formation carbonates is related to diagenetic modifications including intense cementation and compaction. Given the low matrix porosity and permeability, the effect of fractures on fluid flow was investigated as a possible mechanism for enhanced reservoir quality. Naturally fractured core samples and highly fractured outcrops for both formations demonstrated enhanced flow of more than three orders of magnitude compared to matrix permeability. Lateral heterogeneities were observed in the area under investigation with lower fracture porosity and permeability in the North East compared to the South West of the region. Outcrop fractures were investigated near the wells on the flanks of the basin. The fracture network was found to be composed of two sets of sub-vertical, perpendicular fracture sets and sub-horizontal bedding. The fractures were closely spaced, persistent, with wide apertures rough surfaces and very well interconnected. The permeability data combined with field fracture survey information provides evidence that the reservoir quality in the drilled wells must in part be a result of a permeable well-interconnected fracture network.