Impact of Jurassic Volcanic Rocks on Hydrocarbon Exploration, North Western Desert, Egypt

While exploring for Jurassic hydrocarbons in Shushan Basin, North Western Desert, Egypt, Partial or complete reservoir substitutions, alteration by volcanic, volcaniclastic deposits have led to a number of unsuccessful wells. It is therefore essential to understand the distribution and geometry of volcanics in the vicinity of the Jurassic reservoirs. Many wells have been drilled and the presence of volcanic rocks were not recognized for more than a decade. The Qasr NE -2X well was the first alert that volcanic rocks occur in the Jurassic section. This paper shows detailed integration between spectral GR, triple combo, spectroscopy and image logs were necessary to define and differentiate between volcanic rocks of similar composition or alteration from similar host rocks. The lithostratigraphic cross sections and thickness variations of Jurassic volcanics are discussed in order to distinguish the shape and the extent of the volcanics, but the key is to be able to recognize the volcanics away from well control. The results showed that the emplacement of Jurassic volcanic rocks were from extrusive volcanic events. Thin section and log response of olivine basalt and tuffaceous caps strongly suggest sub-aerial effusive eruption. The identification of volcanic rocks using 3D seismic data is critical from a prospecting point of view. If we are unable to see the volcanic seismic signature we cannot predict their presence. If the volcanic rocks as described above vary from tuffs, altered basalts to un-altered basalts then the resultant reflection coefficients will also vary and in some cases will be similar to the sedimentary country rocks. Other factors with regard to seismic imaging of variable volcanic rocks are thickness, spatial distribution, and seismic data quality. We are studying the response of volcanic rock types using VSP data and well based synthetics and some results are described in this paper. A number of seismic attributes were evaluated including AVO modeling.