Sedimentology Genetic Modeling of an Offshore Abu Dhabi Carbonate Reservoir

Bourdarot, Gilles (ADMA-OPCO) | Khemissa, Hocine (ADMA-OPCO) | Al Shemsi, Abdullah (ADMA-OPCO) | Murat, Bruno (Beicip-Franlab) | Richet, Remy (Beicip-Franlab) | Games, Federico (Beicip-Franlab) | Porcher, Florent (Beicip-Franlab)



During this innovative work a 3D Stratigraphic forward Modeler initially designed for geohistorical basin modelling has been used to produce multi-realization of 3D facies distribution of a giant offshore carbonate reservoir in Abu-Dhabi.

The Stratigraphic 3D forward Modeler simulates carbonate production and transport by solving a diffusion equation. The modeling started from a given age, through a sequence time steps. At each time step, three main parameters controlling deposits are modelled: 1) Accommodation space which reflecting the total available water depth for sediment deposits, 2) Sediment Supply by setting carbonates production laws inside the model, 3) Transport and Wave Reworking using a diffusion law, function of wave energy, slope and a diffusion coefficient depending on carbonate nature and grain size.

The model is calibrated to facies thickness and texture described at 17 cored wells by a previous 4th order Sequence Stratigraphic Sedimentology study.

The model has a grid of 200 X 200m, similar to the reservoir simulation grid size, a 50,000 years time step which corresponds to 136 layers, ensuring a vertical resolution in the range of 1 to 2 meters. The total size of the model is 2.7 Million cells.  The resulting model reproduces the spatial facies distribution in the reservoir and with more than 80% success the facies texture and thickness at cored wells for each 4th order sequences.

By varying the parameters controlling sediment deposits, using an Experimental Design approach, several realizations of the geological model have been produced to capture uncertainties on facies distribution between wells, each of them honoring the facies description and thickness at cored wells.

This new approach increases considerably the consistency, resolution and reliability of the geological model between wells.

1. Introduction

3D stratigraphic forward modeling is used since 1992 in order to help petroleum geologists to quantify the sedimentary architecture of a basin, in 3D, both in siliciclastic and carbonate environments (Grangeon1,2, Doligez3).

The main principle of 3D stratigraphic forward modeling is to simulate the paleostratigraphic evolution of the basin geography through time, using long-term (T=100's kyr - 10's Myr) and large-scale (L=10 - 100's km) sediment transport equations. Application of mass balance principle allows to define sedimentation or erosion rate at each point of the basin and at each time and thus, to simulate progressively the basin deformation and filling, and to quantify in 3D the sedimentary architecture.

At exploration and appraisal scales, petroleum geologists are usually using such a tool to define facies distribution, reservoir thickness, connectivity in an unexplored area or along a future appraisal well, to build a 3D geometrical and facies model of a basin, in order to locate possible fields .