Use of Seismic Attributes and Acoustic Impedance in 3D Reservoir Modelling: An Example from a Mature GOS Carbonate Field (Ras Fanar)

By

 Richard D. Vaughan¹, Alaa Atef¹, Nader El-Outefi¹

(1) German Oil & Gas Egypt - GEOGE, Cairo, Egypt

 Discovered in April 1978, Ras Fanar Field in the Gulf of Suez of Egypt has produced approximately 91 MM STB oil, from Middle Miocene coralline algal carbonates which are informally known as the “Nullipore Facies” and to a lesser extent the South Gharib Formation. The field lies some 2 Km offshore east of Ras Gharib and produces from a NW-SE trending structural trap bounded by a major fault system to the SW and tilted to the NE. The depositional setting of the reservoir comprises a narrow 4-5 Km wide carbonate ramp which extends eastwards from Ras Gharib Field to Ras Fanar Field. In previous interpretations the carbonate Nullipore Facies was interpreted as eroded and subsequently on-lapped by South Gharib sediments along a single unconformity. The current geological model allows for both lateral and vertical facies transitions between the Nullipore carbonates and South Gharib evaporitic-siliciclastic-carbonate units. Several phases of upward-shoaling deposition and erosion are evident. Periods of erosion coincide with high porosity layers within the reservoir resulting from karstification and solution collapse brecciation. Diagenetic overprinting of the original facies was intense therefore a petrophysical-based modelling procedure was adopted.

This paper presents a simple but effective 3D reservoir modelling workflow which used absolute acoustic impedance (AI) data and its relationship to effective porosity as a deterministic 3D modelling parameter (“porosity-facies”) rather than the more traditional facies based approach. Implicit in the modelling workflow was the assumption that the 3D distribution of the resulting “porosity-facies” could be interpreted using a geological model which conformed to a “modern” sequenced-based approach to reservoir zonation and the model-derived vertical and horizontal “porosity-facies” trends were reflected in the actual evaporitic-carbonate facies transitions seen in the Gulf of Suez today, as well as nearby outcrop analogues.