Facies, Deposition And Diagenesis Characteristics Of The Upper Kharaib Member (Late Barremian), Onshore Abu Dhabi, UAE

Abstract

Abstract The Upper Kharaib Member of the Lower Barremian in Abu Dhabi onshore is represented by the Formation-B, which is separated from the overlying Shuaiba formation by 45 to 50 feet of dense Limestone of the Hawar formation. It is classified into B-Upper and B-Lower, comprising seven reservoir quality sub-zones (BI, BII, BIIIU, BIIIL, BIV, BV and BVI) separated by six stylolitic intervals. The average thickness for B-Lower is about 100 ft as compared to 60 ft in the upper zone. This study provides integrated sedimentological and diagenetical characteristics - in a field-scale - of the Formation-B in Abu Dhabi, UAE. A unified lithofacies scheme has been used to describe the Formation-B. These litho-facies have been grouped into eight genetically related lithofacies associations that reflect their corresponding deposition environments. The stacking of the lithofacies associations (inner ramp, mid-ramp and mid to outer ramp) define the broad 3rd order trends observed across the study field within the Formation-B, which have been compared to the regional sequence stratigraphic framework of Sharland et al. (2001). The lateral lithological variations occur at a higher order (i.e. 4th/5th order), most likely driven by autocyclic topographic/hydrodynamic variations, in addition to sea level changes; these will impart lateral reservoir heterogeneity. Within subzones BIV to BI, variations in lithofacies associations are observed due to the patchy nature of the higher energy inner ramp facies. The study concluded that the main controllers on reservoir quality distribution are texture and primary composition, as the matrix and grain ratio, together with allochems type, abundance, and size, define the genetically distinct characteristics of the lithofacies associations and provide the precursor fabric for subsequent diagenetic processes to occur. Cementation affects all the lithofacies; the morphology and pervasiveness of the crystals are dependent on primary and secondary pore space available for cementation. The key pore enhancing phases are dissolution events, mainly early and late dissolution enhancing micropores and enhancing/creating macropores, commonly partially or completely negating the effect of cementing phases. As the effects of the diagenetic overprint are linked to primary texture and composition, the distribution of the lithofacies and lithofacies associations will influence reservoir quality. The grainier lithofacies are more commonly seen to be influenced by pore-enhancing late dissolution phases, thus these facies host better reservoir quality. This is best developed in the upper portion of the Formation-B. The combined understanding of the sedimentological framework and the diagenetic overprint provides a robust tool for predicting the reservoir architecture.