Dolomitisation, cementation and reservoir quality in three Jurassic and Cretaceous carbonate reservoirs in north-western Iraq

Abstract

Abstract Dolomitisation is a key diagenetic process, commonly improving reservoir quality in carbonate rocks. It is important in the Jurassic and Cretaceous carbonate reservoirs of north and north-western Iraq, creating primary and secondary pays in many oilfields (Kirkuk, Ain Zalah and Butmah). This paper addresses the impact of dolomitisation and cementation on the reservoir quality of three dolomitised carbonate formations (Butmah, Mauddud and Wajnah). Three different dolomitisation types were recognised: reflux dolomitisation, mixing zone, and burial dolomitisation. Extensive petrophysical measurements were carried out and have shown that differences in dolomite crystal size and cementation control the complexity of porosity, pore and pore throat size distributions, pore network connectivity, and permeability. Reflux dolomitisation exhibits widely spread porosities and permeabilities compared to the other two characterised dolomitisation models, indicating better reservoir properties wherever anhydride cement is either absent or subsequently dissolved (as in the Butmah Formation). Mixing zone dolomitisation was encountered in the Mauddud and Wajnah formations, providing moderate to good reservoir properties, whereas burial dolomitisation generally provides poor to moderate reservoir properties in all three studied formations. It was noted that dolomite cementation provides poor reservoir properties in both the Butmah and Mauddud formations. Both the Mauddud and Wajnah formations show poroperm relationships with the degree of scatter dependent on both differences in crystal size and pore connectivity, indicating the degree of formation heterogeneity. Combined, both quantitative and qualitative observations indicate that dolomitisation cannot be considered in isolation, but is co-active and co-dependent upon dissolution, cementation and fracturing. All of these processes can both cause and amplify dolomitisation, but can themselves be caused or amplified by dolomitisation. We have combined all of our observations to propose a new model which relates different reservoir quality outcomes to the occurrence, intensity and history of different diagenetic processes as a first step in the petrophysical quantification of the effect of diagenetic processes on reservoir quality.