Structural Insights on the Offshore Northern Tunisia Margin and the Petroleum System Implications

Abstract

The northern offshore Tunisia shows structural features of an accreationary margin, as a part of the Maghrebide-Apennine fold-thrust belt. The Petroleum exploration in the northern Tunisian margin has been encouraged by both the widespread occurrence of oil seeps onshore and the gas/oil discoveries in the nearby geological provinces.

The two identified plays, the Numidian Flysch Fm. (Oligo-Miocene clastics) confirmed offshore, and the underlying carbonate targets of the Tellian units (i.e., Cretaceous Abiod Fm. and Ypressian Bou Dabbous Fm.), are proven to be fractured reservoirs in nearby provinces. The shallow coring acquisition campaign and the onshore oil seeps/outcropping analysis, exhibits a possible evidence of a petroleum system, being the Timing and Trap, the highest risk components.

To reduce this uncertainty, a field geology, Gravimetric-Magnetic and seismic data survey, and reprocessed vintage seismic campaigns, helped to identify the main structural styles (NE-SW thrusting, NW-SE right strike slip motion and the NW-SE normal faulting).

The seismic interpretation confirmed the presence of thrust anticlinal structures associated. The presence of residual noise, diffracted energy and salt bodies are the main factors of deteriorating the seismic imaging. Also, the presence of different directions steeply dipping beds, generally poorly imaged by the 2D seismic reflection, yields to such poor seismic quality.

The data acquisition allowed to differentiate the active margin structures during the Middle-late Miocene Alpine orogeny, where the main asymmetric structural features, generally associated with a major Triassic evaporate activity (vertical and lateral flow).

It was identified the main NW-SE orientation during the Alpine orogeny period and generates a NE-SW south verging thrusting and back-thrusting, a NW-SE right motion faults and an associated synthetic normal faulting. As a result, it was located the two main zones boundary between a northern Kabylie terrain zone, and a southern African plate zone which drives the main structural styles (NW-SE trend). The latter represents the main interest zone, where the petroleum system elements are an onshore Tunisian analogy.

The Oligocene-Miocene Numidian flysh emplacement, was define as the onset of the trap formation and the northern offshore critical moment. There is a timing uncertainty due to the main expulsion peak estimation from the main source rocks. During Pliocene times, the NE arc migration generates extensional shallow structures, supported by the Tyrrhenian Sea opening. This shallow structure generates a difference between main structural styles between the east and west of the mentioned southern African basement (southern Ras Korane and Ras Rihane blocks). The western African basement has structures associated mainly to a NW-SE collision. However, the eastern African basement (Nadhour block) has a Pliocene-Recent times extensive faulting structures (NE arc migration of Sicily, Calabria blocks and NW Ionian sea subduction). The magmatic activity associated with this NW-SE collision, has been reflected on the 2D density model and the magnetic features at the western African plate. The main generation and expulsion model depends mainly on the gravimetric anomalies, where the magmatic influences the heat flow increase, and therefore the source maturity. The Grav-Mag analysis at the Northern Tunisia offshore identify the main structural features, in order to discriminates the eastern African plate (Nadhour block) as the main prospective zone.

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015