3D Reservoir Characterization and Modelling in Carbonates – Quo Vadis? An example from the Upper Jurassic Arab Formation, UAE

Jürgen Grötsch
Syria Shell Petroleum Development BV, Technology and Studies Center, Dummar, Damascus, Syria
e-mail: [email protected]

Over the last few years, advances in 3D reservoir modeling technology and computing power have significantly altered workflows, approaches and possibilities to integrate data in a more realistic fashion into subsurface models. One of the prime reasons is the ability to use high-resolution dynamic models and to reduce up-scaling requirements significantly. This has resulted, however, also in new requirements for geoscientists w.r.t. how we describe reservoir rocks in carbonate fields and obviously w.r.t. how we populate and attribute petrophysical and SCAL data in 3D reservoir models. Such a new approach has been tested successfully in static and dynamic models on Cretaceous (Grötsch et. al, 1999, Melville et. al, in press), and Jurassic giant fields, both, onshore Abu Dhabi (Grötsch et. al, 2003). The latter example is used to lead through this workflow.

A 3-D geological model of the Kimmeridgian-Tithonian Manifa, Hith, Arab, and Upper Diyab formations in the area of the onshore Central Abu Dhabi Ridge was established based on a new sequence stratigraphic, sedimentologic, and diagenetic model. It was part of an inter-disciplinary study of the large sour-gas reserves in Abu Dhabi that are mainlv in the Arab Formation. The model formed a basis for dynamic evaluations and recommendations for further appraisal and development planning in the studied field.

Fourth-order aggradational and progradational cycles are composed of small-scale fifth-order shallowing-upward cycles, mostly capped by anhydrite within the Arab-ABC. The study area is characterized by a shoreline progradation of the Arab Formation toward the east-northeast marked by high-energy oolitic/bioclastic grainstones of the Upper Arab-D and the Asab Oolite. The Arab-ABC, Hith, and Manifa pinch out toward the northeast. The strongly bioturbated Lower Arab-D is an intrashelf basinal carbonate ramp deposit, largely time-equivalent to the Arab-ABC. The deposition of the Manifa Formation over the Arab Formation was a major back-stepping event of the shallow water platform before the onset of renewed progradation in the Early Cretaceous.

Appraisal drilling results have confirmed that well productivity in the Arab-ABC is controlled mainly by thin, permeable dolomitic streaks in the fifth-order cycles at the base of the fourth-order cycles. This has major implications for reservoir management, well completion and stimulation, and development planning. Good reservoir properties have been preserved in the early diagenetic dolomitic streaks. In contrast, the reservoir properties of the Upper Arab-D oolitic/bioclastic grainstones deteriorate with depth due to burial diagenesis.

Diagenetic modification of the sediments, by processes such as syndepositional dolomitization, dissplacive anhydrite formation, non-ferroan calcite burial cementation, and leaching, prohibited the establishment of a direct relationship between primary depositional facies and rock properties, therefore a reservoir rock-type (RRT) scheme was established to encompass all lithostratigraphic units. Standard core-analysis data acquisition and attribution of saturation functions to the static and dynamic model were performed on a cell-by-cell basis using these established rock types. Upscaling was performed with an emphasis on preserving the cyclicity and thin increased-permeability streaks in the dynamic model. The series of extracted dynamic simulation models allowed definition of the development requirements of the reservoir.

The results demonstrate the importance of integrating sedimentological analysis and diagenesis with reservoir rock typing and static and dynamic modeling in order to enhance the predictive capabilities of subsurface models. It also shows the new requirements for subsurface modeling workflows and software tools.

References

Grötsch, J., Al-Jeelani, O. & Al-Mehairi, Y. (1998): Integrated Reservoir Characterisation of a giant Lower Cretaceous oil field, Abu Dhabi, U.A.E., SPE 49454, 77- 86, ADIPEC 1998, Abu Dhabi.

Grötsch, J., Suwaina, O., Ajlani, G., Taher, A., El-Khassawneh, R., Lokier, S, Coy, G, van der Weerd, E., Masalmeh, S. & van Dorp, J. (2003): The Arab Formation in central Abu Dhabi: 3-D reservoir architecture and static and dynamic modeling, GeoArabia, Vol. 8, No. 1, Gulf PetroLink, Bahrain. 

Melville, P, Al-Jeelani, O., Al-Menhali, S. & Grötsch, J. (in press): Comparison of seismic character and geological model of the giant Asab carboante field, Abu Dhabi, UAE,.- in G. Eberli & J.-L. Massafero, Carbonate Systems and Reservoirs, AAPG Memoir, Tulsa.