3D Outcrop Modeling of a Carbonate Shoal Complex Using Cellular Automata, Jabal al Akhdar, Oman

Hasler, Claude-Alain¹; Adams, Erwin W.²
¹Carbonate Research, Shell Global Solutions, Rijswijk, Netherlands.
²Sarawak Shell Berhad, Lutong, Malaysia.

Over the past decades, reservoir models kept evolving, and especially the resolution and complexity has grown in parallel with computing power, the advance of modeling tools, but equally important the need for more robust models as hydrocarbon fields mature. Carbonate reservoirs, and in particular those comprising shoal deposits, are illustrative. The traditional "layer cake" models are no longer appropriate today, when the deployment of IOR/EOR techniques is the norm in numerous fields in the Middle East. Clearly, there is a need for building more robust static reservoir models of carbonate shoal deposits that incorporate key heterogeneities. Here, an interwell facies modeling method is presented based on cellular automata forward modeling. Observations on modern carbonates in the Bahamas have been used to constrain the key processes responsible for shaping shoals. Additionally, cellular automata rules have been developed that mimic early diagenesis processes. The final outcome of the cellular automata are a high-resolution decameter-scale 3D grid, reproducing the geological stacking recorded through time of depositional and diagenetic facies for a given accommodation cycle. Before using the cellular automata on a real subsurface case, where hard data are limited to cored wells and uncertainties are ample, it is crucial to validate the method with outcrop analog data where vertical and lateral facies distributions can be observed and measured directly.

Permian-Triassic outcrops exposed on Jabal al Akhdar, Oman, were chosen because, (1) their age and lithologies are similar to those of major hydrocarbon reservoirs in the Middle East and especially the Khuff Formation, (2) the quality of the outcrops and their accessibility, and (3) the outcrops are well studied and digital outcrop models and lidar data were available. This study focused on two marker levels representing the sedimentary record of oolitic shoals complexes. Based on a compiled accommodation curve derived from logged sections, the model was able to mimic key heterogeneities mapped in outcrop. Field observations and 3D model realizations indicate that the lateral variations recorded in the sedimentary deposits of carbonate shoals are more important than commonly accepted. This study demonstrates that interwell facies modeling using cellular automata is able to model both depositional and early diagenetic facies environments that are typically associated with shoal depositional systems.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012