Abstract: Geologic Evolution and Hydrocarbon Prospects in the Chotts Basin, Tunisia

Chris Johns, Ali Chine

The Chotts basin, running east-west across central Tunisia, is a complex Late Paleozoic to Mesozoic graben system with a sediment thickness reaching over 5000m. It covers an area of over 15,000 km² but is underexplored. It has been drilled by only a few wells; several of which gave oil and gas shows. A number of oil and condensate fields lie immediately south. ETAP, the national oil company, has undertaken a detailed investigation of the basin, greatly improving our understanding of its evolution and prospectivity.

The basin is floored by Lower Paleozoic sediments. These occur at shallow depth on the southern flank where they were affected by periodic contemporary tectonism. The succession includes Ordovician clastics with good reservoir potential and both Ordovician and uppermost Silurian source rocks. Locally, the latter unconformably overlie Ordovician reservoir sections.

The basin developed into a major, east-west trending, intracratonic wrench basin during the late Permian. Carbonate facies dominate the southern shelf area and, although lithofacies distributions are poorly constrained, the existence of quality source rocks is a strong possibility. The graben complex was inverted and partly eroded prior to deposition of Upper Triassic volcanics and sandstones. The sandstones are a proven reservoir and several leads are identified.

Substantial subsidence occurred in the northern part of the basin from the Middle Jurassic to Early Cretaceous. Jurassic carbonates provide numerous reservoir sections, while Callovian shales constitute a proven, mature source rock. Large fault- and fold-related traps were formed during latest Cretaceous to Paleocene and Mio-Pliocene orogeny; they provide promising objectives.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France