About 50 mi south-southwest of the Hassi Messaoud field, a group of three fields, El Agreb, El Agreb Northeast, and El Gassi, is located in separate structural highs on the faulted El Agreb--El Gassi anticline which is on the same major pre-Hercynian anticlinorium as is Hassi Messaoud.

The El Agreb--El Gassi structure is a northeast-southwest-trending anticline divided into separate blocks by northwest-southeast-trending faults. It is on the western side of the major ancient Hassi Messaoud anticlinorium which separates the Algerian Sahara into western and eastern basins. The high was formed at the end of the Paleozoic, during the Hercynian orogeny. The Gotlandian (Silurian), the Ordovician, and the upper part of the Cambrian are eroded over the crest of the high in this area and in Hassi Messaoud. Later local movements, attributed to the Austrian orogeny, have affected the southern part of the El Agreb field, the southernmost field of the group, and have uplifted the area northeast of the northernmost El Gassi field.

The Cambrian quartzitic sandstone, Hassi Messaoud Sandstone, is the reservoir rock of El Agreb--El Gassi fields. The thickness of the reservoir varies, but averages 400 m.