Impact of Diagenesis on the Reservoir Properties of the Cretaceous Kharaib Formation, Sajaa Field, Sharjah, United Arab Emirates

Abstract

The Early Cretaceous Kharaib Formation is one of the most prolific hydrocarbon bearing reservoirs in the Middle East. Based on sedimentary characteristics, the Kharaib Formation is divided into three main intervals/zones: the lower, middle and upper Kharaib. Interpretation of the depositional fabrics and prediction of reservoir parameters is often difficult due to the Kharaib Formation's diverse array of sedimentary characteristics. To address the considerable heterogeneity of the Kharaib Formation, this study investigates depositional characteristics and the diagenetic impact on reservoir quality within the Sajaa Field. The Kharaib Formation sediments were characterized using a Dunham/Embry & Klovan (1971) defined classification scheme. Analytical techniques via thin section analysis, scanning electron microscopy (SEM) and capillary pressure data were used to qualitatively and semi-quantitatively assess porosity and the diagenetic processes involved in its evolution. Key reservoir quality parameters for Kharaib intervals were defined by combining petrographic analysis and conventional core methods. The sedimentological characterization of the Kharaib sediments in the Sajaa Field showed a range of Orbitolina-bearing carbonate textures, which were interpreted as inner-ramp deposits. The stacking pattern of the depositional facies showed intermediate-order trends. The conventional core analysis measurements showed poor to good reservoir properties with porosity values of 0.7 – 23.2% and permeability values from negligible to 2 – 3 md. The pore system is dominated by grains and matrix-hosted micropores, which are primarily generated via diagenesis. Qualitative thin section analysis identified grain-hosted micropores as the most significant microporosity type, highlighting that grain-rich textures are relatively more porous compared to mud-rich textures. Diagenesis transformed both the matrix and various grain types (e.g., micritized grains, foraminifera etc.), generating considerable micropore volume. High resolution SEM imaging, along with semi-quantitative assessment, highlighted that diagenetic modification of the micritic sediments led to an abundance of sub-rounded to polyhedral crystals exhibiting primarily sub-punctic to punctic intercrystalline contacts that generated a porous fabric. Mercury injected capillary pressure (MICP) data, confirmed the abundance of micropores within the pore size distributions of studied samples. This study demonstrates the importance of considering textural variation within the Kharaib Formation when assessing the reservoir coupled with the fractured nature of the unit. The established microporosity-depositional relationships when combined with the updated core descriptions now allow for the integration of textural trend information in the field that can be applied to refine reservoir property and abundance distributions.