Reservoir characterization of ramp carbonates: Lessons from the Lower Miocene Euphrates and Jeribe Formations, Kurdistan, N. Iraq.

Sedimentological and diagenetic factors determine porosities and permeabilities within carbonate ramp deposits, and hence their potential reservoir properties. But information on variability of these characteristics on length scales of 102 km, the dimensions of some shallow-dipping, homoclinal ramps, is rare. This study, combining field and subsurface samples of carbonates of the Lower Miocene Euphrates and Jeribe Formations in the Zagros Basin, northern Iraq, demonstrates consistent microfacies but with diagenetic variations from proximal locations in the north-east to more basinal locations >100km to the south west. Microfacies analysis of the studied sections determines that the Lower Miocene carbonates were deposited in the inner ramp setting of the carbonate ramp and were affected by two diagenetic evolution stages. Dolomitization and dissolution processes have greatly affected the reservoir properties. The heterogeneity of porosity and pore connectivity in the studied units developed from the diagenetic overprint, where early dolomitization and dissolution have been shown to have significantly affected porosity and permeability values in the basinal locations, whereas early and burial cementation have reduced porosity and permeability in the north-east locations. Pore throat size, which controls fluid flow in reservoir rocks, is rarely established in heterogeneous carbonates, and in carbonate reservoirs complex pore networks occur. Thus it is crucial to investigate the factors governing reservoir quality in the carbonate rocks of the Euphrates and Jeribe Formations, potential carbonate reservoirs in Kurdistan. This study considers the factors affecting the distribution of porosity, permeability and reservoir quality using regional stratigraphy, outcrop sections, well logs and petrophysical analyses of core material from 10 wells, as well as a large suite of laboratory petrophysical measurements. Four main rock types have been identified based on the petrophysical properties, geological and diagenetic processes for the Euphrates and Jeribe Formations. Rock types I and IV were indicated as non-reservoir units and rock types II and III as reservoir rocks. This study reveals a lack of correlation between carbonate microfacies and reservoir properties, due to the variation in diagenetic process over distances of 10’s to 100 km. This study therefore demonstrates the need for following a rock-typing methodology when dealing with heterogeneous carbonates with varied diagenetic overprints, to create a static model framework appropriate for dynamic reservoir modelling.