Characterization and impact on reservoir quality of fractures in the Cretaceous Qamchuqa Formation, Zagros folded belt

Abstract

Reservoir quality in tight fractured carbonate rocks is controlled by the fracture properties, the tectonic origin of fractures, the relationship of fractures to other sedimentary features within the rockmass and diagenesis. In this study fracture analysis, porosity and permeability of the Qamchuqa Formation in the Miran West block of the Zagros folded belt in Kurdistan has been studied. This was undertaken using core analysis, micro-resistivity image logs, drill stem tests (DST), mud logging data, Repeat Formation Test (RFT), drilled cutting samples and wireline log data. The integrated analysis shows that the Qamchuqa Formation is characterized by heterogeneous sedimentary and tectonic features. The sedimentary features include burial stylolites, dissolution seams and some fractures which formed at early stages of burial and diagenesis (Phase 1). Open fractures, partially open mineralized fractures, veins and tectonic stylolites were formed by later tectonic activity (Phase 2). The finding of this study highlight that the fracture properties including aperture, length, frequency and origin, play an important role in controlling the reservoir's quality. The open fractures and partially open mineralized fractures enhance permeability providing an inter-connected fracture network, leading to a high fracture permeability that controls fluid flow in this tight carbonate reservoir rock. The fracture network had little impact on the volume of hydrocarbon storage which is controlled by the pore matrix. In contrast, structures including stylolites, dissolution seams and veins generally have a negative impact on the reservoir quality by blocking the fluid flow. This work emphasizes the importance of understanding the regional geological history and the role of sedimentary and tectonic features when investigating in tight carbonate reservoirs.