This paper discusses the results of an innovative methodology using Dionisos 4D forward stratigraphic modelling of Middle Eastern carbonate reservoirs on a field scale. Traditional stochastic techniques do not sufficiently capture carbonate reservoir heterogeneities, reducing the accuracy of static and dynamic models. The methodology applied to three Lower Cretaceous UAE reservoirs, uses a deterministic approach that aims to define carbonate heterogeneity and provides a structure to develop a more accurate and usable static and dynamic model for field development. Dionisos uses a predefined sequence stratigraphic scheme as a framework. A reference case model is manually calibrated to environmental parameters, followed by automated multi-realisations that generate several other plausible calibrated models. A sensitivity analysis provides an indication of the influencing environmental parameters controlling facies and texture distribution.The calibrated forward stratigraphic models resulted in the generation of 14 carbonate textures for the three reservoirs using a 200x200 m grid size and a 50 kyrs time step. Carbonate lithology production (mud, fine, coarse, bioconstructions) varies between 0 and 350 m/Ma, wave direction is SW (200 -260°); wave action depth 7-18 m while wave energies vary between 0 and 140 kW/m. Sediment diffusion coefficients by wave transport range from 0.1 (mud) to 0.0008 km 2 /kyr (bioconstructions) while gravity driven transports from 0.1 to 0.001 km 2 /kyr. The lower part of Reservoir A is characterised by low angle TST sequences dominated by algal boundstones-floatstones. Deposition continued with the development of a low relief margin with aggradational to progradational architectures comprising rudist shoals. This defined a topographic split into platform, slope and basin with lateral texture heterogeneities showing a northward deepening trend. The successive clinoform top sets (Seq4a, b) are rich in rudist boundstones-floatstones with lower slope dominated by packstones-wackestones.Reservoir B and C are isopach with strong lateral variability in carbonate texture as evidenced by well data. The overall architecture of the sedimentary systems consists of low relief interconnected algal boundstone-floatstone mounds separated by gentle depressions dominated by fine grained sedimentation. The numerical simulation of these systems was driven by a carbonate production law as a function of the substratum energy and bathymetry under dynamic subsidence/uplift conditions. The innovative workflow applied at field scale allowed the modelling of complex carbonate geometries and associated textures honouring lateral and vertical heterogeneities observed at wells. The application of this workflow as alternative/complement to stochastic methodologies brings further insights on the proposed sequence stratigraphic framework allowing the confidence and predictability of static and dynamic facies models to be increased.

Uncertainty analysis using experimental design and response surface techniques has been extensively used in the field of reservoir simulation. This study outlines an innovative workflow to generate multiple realizations of forward stratigraphic modelling of three Lower Cretaceous reservoirs from onshore Abu Dhabi. Forward stratigraphic modelling is a deterministic technique that simulates basin infill providing a better understanding of vertical and lateral facies distribution and connectivity in sedimentary basins. During the course of forward modelling a variety of environmental and stratigraphic parameters are used. Due to the uncertainties of these parameters it is critical to assess their impact on the development of the basin fill. The experimental design and response surface techniques have been innovatively applied at reservoir scale to enhance the understanding of major controlling parameters on carbonate production and to produce alternative facies distribution scenarios in the study reservoirs. The methodology used in this study was based on running multiple simulations, through varying key input parameters. The best stratigraphic models were then selected based on calibration quality and geological consistency. Calibration quality was assessed by two user defined quantitative functions called Thickness Calibration Indicator and Rock Texture Calibration Indicator. The initial step in the workflow identified uncertain environmental parameters (e.g. eustasy, carbonate production versus depth, carbonate production versus time, wave parameters, gravity and wave transport and erosion rates) from a manually calibrated reference case and ranges of values for each parameter defined based on the knowledge of geology over the area. Latin Hypercube experimental design was then used to define a set of simulations to allow an efficient and uniform sampling of the entire uncertain domain. Sensitivity analysis was then performed on simulation responses (texture and thickness calibration indicators) using the technique of nonparametric Response Surface Modelling (RSM). The influence (quantitative and qualitative) of the impacting parameters on responses was studied to identify the most influential parameters as well as the ranges yielding good calibration indicator values. A further set of simulations was then launched that considered the most influential parameters and their precise ranges. Non critical parameters were assigned with the constant values from the reference case model. These simulations generated a series of well calibrated models. A filtering of simulations with high calibration indicator values and good geological consistency was then performed to choose acceptable multi-realizations. Finally, thickness and texture confidence properties were mapped based on the selected multi-realizations and the reference case. Sensitivity study on three Lower Cretaceous reservoirs from onshore Abu Dhabi successfully addressed the uncertainty associated with forward stratigraphic model input parameters. Sensitivity analysis was performed using Experimental Design and RSM. This was applied to enhance the understanding of the major controlling environmental parameters on carbonate production for individual sequence with each of the study reservoirs.

Facies and their distribution in space are key building blocks to determine the depositional architecture of hydrocarbon reservoirs. For this reason, the unified high resolution facies and sequence stratigraphy boundaries are needed to constrain facies architecture and their properties distribution for constructing the 3D static and dynamic model. A comprehensive facies analysis and modeling within the established stratigraphic framework, was conducted to reduce the uncertainty in correlating and building-up the architecture between wells. Over 30,000ft of core data from 155 wells and their log data are used in integration with the seismic interpretation. Knowledge of the facies results in a better correlation, used then to generate a total of 47 spatial facies maps. Some facies are combined to facies associations (FA) maps, representing the FA at each sequence boundary in its sequence package. For each selected interval, the dominant FA observed in individual wells has been correlated; preserving the general evolution of the depositional environment and the sequence stratigraphy framework. These maps are used to constrain the variograms for the petrophysical properties distribution in terms of orientation and ranges. A conceptual facies model was created based on facies distribution following an evolving platform to basin topography during transgressive (Apt1&Apt2), early-highstand (Apt3), late-highstand (Apt4a&Apt4b), and composite lowstand (Apt5) phases of carbonate platform development during the Aptian. The best reservoir quality is dominated by dissolution related mouldic and vuggy macropores. The grain supported textures demonstrate better overall reservoir quality as a result of more abundant interparticle and intraparticle pores and enhanced macro-vuggy porosity created by leaching process. Some poor reservoir quality is observed in grainy facies due to cementation. A detailed understanding of the core-based facies description/analysis is required for identifying the reservoir properties and its relation to rock-texture; leading consequently to the rock-flow-units in the dynamic model.