The offshore Nile Delta basin is considered as one of the most promising hydrocarbon provinces in Egypt, with an excellent potential for gas and condensate reserves following future exploration. Most of the discoveries in this basin, such as the reservoirs of the Upper Miocene and the Middle to Upper Pliocene, have been enabled by the use of a direct hydrocarbon indicator (DHI), based upon a class III seismic amplitude versus offset (AVO) anomaly. However, there are gas-bearing formations in the lower Pliocene that have been successfully tested, where the sand did not show any seismic amplitude anomaly in full stacks or in near- and far-offset sub-stacks. The AVO analysis of this sand reservoir is referred to as AVO class II-p. Another case of a subtle AVO class I anomaly in a lower Pliocene gas reservoir has also been tested by three wells.
These variations in AVO types push us to find a new methodology to reduce the risk of unsuccessful exploration wells, mainly using the seismic data. The enhanced AVO pseudo-gradient attribute (EAP) has previously been used in other studies, mainly to highlight AVO class III anomalies. However, in the present paper we demonstrate a workflow to identify all the principal AVO classes observed in this province. Computing the EAP attribute from our data, we find that AVO class I has negative EAP values, while the other classes have positive values. Class III and classes II and II-p may be distinguished from each other, as the former yields a strong positive EAP value whereas the latter two classes yield weak EAP responses.
After determining the AVO class, we define and use a new model attribute, herein termed NM, to differentiate between gas- and water-bearing formations for each class of AVO anomaly found in this province. This new method was successfully tested in many areas in the Nile Delta basin, where it has helped to identify subtle anomalies and thereby to open the gate for further exploration activities in the area.