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.

中文翻译：

---

伊拉克西北部三个侏罗系和白垩系碳酸盐岩储层的白云石化、胶结作用和储层质量

摘要 白云石化是碳酸盐岩储层质量改善的关键成岩过程。它在伊拉克北部和西北部的侏罗纪和白垩纪碳酸盐岩储层中很重要，在许多油田（基尔库克、艾因扎拉和布特马）中创造了初级和次级油层。本文讨论了白云石化和胶结作用对三个白云石化碳酸盐岩地层（Butmah、Mauddud 和 Wajnah）储层质量的影响。确认了三种不同的白云石化类型：回流白云石化、混合带和埋藏白云石化。进行了广泛的岩石物理测量，结果表明，白云岩晶体尺寸和胶结作用的差异控制了孔隙度、孔隙和孔喉尺寸分布、孔隙网络连通性和渗透率的复杂性。与其他两种白云石化模型相比，回流白云石化表现出广泛分布的孔隙度和渗透率，表明在不存在酸酐胶结物或随后溶解的地方（如 Butmah 地层）具有更好的储层特性。在 Mauddud 和 Wajnah 地层中遇到了混合带白云石化，提供了中等至良好的储层特性，而在所有三个研究地层中，埋藏白云石化通常提供差到中等的储层特性。注意到白云岩胶结在 Butmah 和 Mauddud 地层中都提供了较差的储层特性。Mauddud 和 Wajnah 地层都显示出与散射程度的 poroperm 关系，这取决于晶体大小和孔隙连通性的差异，表明地层异质性的程度。综合起来，定量和定性观察表明，白云石化不能孤立地考虑，而是与溶解、胶结和压裂共同作用和共同依赖。所有这些过程都可以引起和扩大白云石化作用，但它们本身可以被白云石化作用引起或扩大。我们结合所有观察结果提出了一个新模型，该模型将不同的储层质量结果与不同成岩过程的发生、强度和历史联系起来，作为成岩过程对储层质量影响的岩石物理量化的第一步。所有这些过程都可以引起和扩大白云石化作用，但它们本身可以被白云石化作用引起或扩大。我们结合所有观察结果提出了一个新模型，该模型将不同的储层质量结果与不同成岩过程的发生、强度和历史联系起来，作为成岩过程对储层质量影响的岩石物理量化的第一步。所有这些过程都可以引起和扩大白云石化作用，但它们本身可以被白云石化作用引起或扩大。我们结合所有观察结果提出了一个新模型，该模型将不同的储层质量结果与不同成岩过程的发生、强度和历史联系起来，作为成岩过程对储层质量影响的岩石物理量化的第一步。