Reservoir Quality Heterogeneity of the Asab Formation: A Detailed Sedimentological and Reservoir Quality Evaluation
Authors: Galluccio L. , Breislin C. , Tendil A. J.-B. , Lehmann Christoph , Al Hosani Aysha , Al Mannaee Alya , Alawadhi Salman , Arianto Muhammad Ali , Aziz Lalu Abdul , Fournillon Arnaud 
Published: October 2023

The oolitic grainstones characterising the Upper Jurassic Asab Oolite Formation represent some of the best performing reservoir facies in onshore Abu Dhabi. Yet the understanding of the factors controlling their heterogeneous reservoir quality are not well-understood and the available literature is both limited and fragmented. This study focuses on an integrated sedimentological, diagenetic and reservoir quality evaluation of the Asab Formation with the aim of increasing its regional understanding.

The sedimentology of both new and legacy cored wells was reviewed to characterise the formation, whilst diagenesis and reservoir quality evaluations were based on thin-section, SEM and cathodoluminescence analyses. The collected data were used to interrogate RCA and MICP measurements to unravel the controlling factors on the reservoir performance. These were subsequently upscaled vertically and laterally to constrain reservoir architecture via correlation panels and maps.

The main results highlight that the Asab Oolites Formation comprises a very vertically consistent depositional make-up, with high-energy inner ramp deposits locally evolving into shoals, composed of oolitic grainstones. These are laterally continuous and only locally show thinning, as indicated by the sequence stratigraphic framework. The latter consists of one large scale regressive trend bounded at the top by the Hith Formation, within which are nested three, smaller scale regressive hemicycles, bounded by hardground or firmground surfaces capping variably laminated grainstones. The pore systems of these facies are dominantly characterised by micropores within the oolites and, to a lesser extent, by interparticle macropores, which are only preserved when cementation and/or compaction do not pervasively occlude them. Local dolomitised horizons are also observed, which generally are the best performing layers. As a result, diagenesis is considered the main control on the reservoir performance of the Asab Oolites Formation, with localised minor impact of depositional controls, such as allochems’ size dictating pore size (e.g. rudstones vs grainstones). The highest permeability values for a given porosity are recorded for dolomitised fabrics (deposited below the Asab Oolites), whilst within the grainstones, porosity and permeability trends are controlled by cementation of macropores, compaction, and degree of microporosity of the ooids (main component of the oolitic grainstones). When mapped, these porosity and reservoir quality trends constitute a powerful tool for reservoir characterisation and prediction.

This study represents an important step forward for the understanding of the Asab Oolites Reservoir. The main findings support rock-typing evaluation in a context where logs by themselves are almost completely non-diagnostic. The innovative data collection and interrogation techniques brought to bear on the assessment of reservoir quality and sweet spots allowed for a detailed understanding of a formation that has long been neglected in literature.