Seismic Indications of Deep-Seated Faults and Their Impact on Trapping System

Agounizera, S., Sonatrach, Algeria

The presence of thick Triassic and Jurassic evaporites influences seismic data quality of Cambrian horizons in the region between the giant Hassi-Messaoud oil field and the Dorban trough and poses an important problem for prospect evaluation. It is difficult to extrapolate folds and faults in the post salt section down to the prospective Paleozoic section and weak reflectivity within the Cambrian clastic section makes it difficult to pick horizons and faults independently. In the past, Paleozoic structure maps have been constructed by adding isopachs to the Mesozoic horizons. We think that this approach is the primary reason for drilling dry holes on invalid Cambrian traps.

A new approach has been used to map the deep Cambrian seismic horizons based on the deep horizon correlation, identification and extension of deep-seated faults upward in the Cambrian section and their relation to the induced post-salt structures. This effort has been supported by new seismic acquisition and processing work focused low frequency, long off­set data. This data has permitted consistent correlations of seismic sequences in the Paleozoic section and has provided a good match with VSP data.

The approach was used to map the Ordovician Hamra Quartzites at the new Hassi Terfa (HTF-1) discovery. The HTF-1 is a very significant discovery that confirmed the prospectiv­ity and petroleum potential of the Hassi-Dzabat area. New mapping demonstrated the pres­ence of fault-bounded, horst-graben traps and identified the presence of a new sub-uncon-formity exploration play between Hassi-Messaoud and the Dorban Trough.