Reservoir Characterisation and Rock Type Classification in the Lower Arab Formation (Offshore Abu Dhabi): How to Successfully Integrate Detailed Core Descriptions into Field-Scale Geological Models
Authors: Lettéron A. , Pacheco Bell F. , Galluccio L. , Breislin C. , Mishra A. K. , Tendil A. 
Published: November 2024

Geological static models are constructed using 3-D grid cells which are populated with quantitative rock properties that are generally constrained by the Reservoir Rock Types (RRTs). These input data are of key importance for hydrocarbon exploration and production. The classification of carbonate RRTs, deriving from the complex interplay between sedimentological processes and diagenetic modifications, is challenging and suffers from limitations. This study presents the results of a combined sedimentological, diagenetic, and reservoir quality (RQ) characterisation of the Arab Formation (offshore Abu Dhabi). The work underlines the importance of well-defined RRTs for reservoir geomodelling.

Detailed core sedimentological characterisation of three wells capturing the lower Arab Formation (730ft) has been carried out. Through core-to-log calibration, these wells were correlated with three uncored wells to better constrain the stratigraphic architecture of the reservoir zones. An in-depth petrographic evaluation was performed on 528 thin-sections associated with porosity/permeability measurements. The key sedimentological and diagenetic parameters were semi-quantified by ascribing numbers reflecting their respective abundance. These detailed descriptions were linked to the petrophysical properties derived from the conventional core analysis (CCA) and the pore classes based on mercury injection capillary pressure (MICP), developing a new RRT classification that incorporates sedimentological and diagenetic factors. The depositional maps and diagenetic trends were used to enhance the characterisation of the reservoir in the interwell areas by improving the spatial prediction of petrophysical properties based on the rock types.

Five genetic elements were defined and propagated laterally in one correlation panel and spatially in ten maps. Maps were also produced to capture the spatial variability of diagenetic parameters affecting rock properties. This work reveals an overall ‘layer cake’ organisation for the reservoir zones across the field. Two vertical shallowing-upward trends evolve from subtidal inner ramp into intertidal (IT) and supratidal (ST) deposits, respectively associated with the lower Arab Formation. A broad increase in RQ is evident passing from proximal ST and IT environments to more open and higher energy inner ramp (IHE) settings, characterised by skeletal/peloidal and oolitic grainstones forming shoal complexes. Beyond these depositional controls, the importance of fabric-destructive dolomitisation is revealed as the key diagenetic process enhancing RQ. The best RQ is hosted within fully dolomitised IHE samples, which are associated with macropore-dominated, dolomite-specific facies. Nine RRTs have been identified, each imprinted by a unique porosity/permeability relationship, capillary pressure profile and pore-throat size distribution. This RRT classification differentiates an interval of rock with consistent and predictable geological and petrophysical properties that will impact the fluid flow and fluid distribution in the reservoir and the geological model.

Through the integration of petrophysical properties with the geological understanding, a major step forward has been made in this study to constrain RRTs in vertically compartmentalised reservoirs.