Sedimentological and petrophysical heterogeneities controls on reservoir characterization of the Upper Triassic shallow marine carbonate Kurra Chine Formation, Northern Iraq: Integration of outcrop and subsurface data

Abstract

The lack of knowledge about the relationship between carbonate sedimentary facies and pore system modifiers in northwestern and northern Iraq severely restricts carbonate oil exploration in these areas. This research aims to highlight the significance and influence of sedimentological and petrophysical heterogeneity and their impact on reservoir characterization of the Kurra Chine Formation, which have been studied in oil wells at Ain Zalah (AZ-29), Butmah (Bm-15), and Kand (K-1), in addition to the Sararu outcrop in northern Iraq. In this study, integrated methods that incorporate thin section petrography, well logs, statistical techniques and core analysis were used for the investigation. The study revealed that the Kurra Chine Formation in the studied sections mainly consists of limestone, dolomite, shale, and anhydrite, the last of which is absent in the Sararu outcrop. Its thickness in previous outcrop and wells was 848 m, 1136 m, 985 m, and 1149 m for Sararu, AZ-29, Bm-15, and K-1, respectively. Petrographically, five main microfacies types and nine submicrofacies were identified in the Kurra Chine carbonates of the studied sections. The percentages and distribution of these submicrofacies in the study area are diverse and show their heterogeneity. Diagenetic processes like micritization, dolomitization, cementation, neomorphism, compaction, stylolitization, solution, pyritization, and silicification resulted in increasing heterogeneity in the formation. They played a significant role in improving or reducing the reservoir characterizations of the formation. Dolomitization and dissolution processes enhance the petrophysical properties of the studied formation, especially the medium-to coarse-grained crystalline idiotopic dolomite. On the other hand, cementation, compaction, and neomorphism reduce the pore volume of the Kurra Chine Formation rocks. Qualitatively, porosity in the Kurra Chine Formation was divided into two main groups: interparticle (intergrain and intercrystal) and vuggy pores. The interparticle pore space is subdivided into three petrophysical classes: Class 1, Class 2, and Class 3. Class 2 represents the most common interparticle porosity type and is very important because it contains bituminous material. Moldic and intragrain pores are the most abundant separate vug pores within formation successions. The touching vug pores are represented by fractures, microfractures, and fenestrals. Quantitatively, three types of porosity have been calculated for the formation using conventional well logs: effective, interparticle, and separated vuggy porosity. The Butmah-15 well reservoir property outperformed the other two wells in terms of reservoir property indicators, particularly in the middle of the studied formation at depths ranging from 3089 to 3399 m. Porosity in this depth interval is interparticle, with values ranging between 0 and 0.35, while permeability values range from zero mD in beds cemented by anhydrite cement to 6196 mD within MF6 in limestone beds that are rich in interparticle and fracture porosity. The AZ-29 and K-1 wells have poor reservoir properties, especially effective porosity and permeability. This research provides a better understanding of the relationship between carbonate sedimentary facies and pore system modifiers of shallow marine environment carbonates, and it can aid in reservoir evaluation for better exploitation of these valuable resources in Iraq and elsewhere in carbonate sedimentary basins.