Aptian-Lower Albian Serdj carbonate platform of the Tunisian Atlas: Development, demise and petroleum implication

In this paper, we present an updated biostratigraphic evaluation of the Serdj Formation of the Tunisian Atlas. Newly collected ammonites, combined with available biostratigraphic data, allowed the establishment of a new and robust stratigraphic framework for this well-known formation. For the first time the characterization of the Mellegueiceras chihaouiae Zone has permitted the specification of the Aptian-Albian transition. This study reveals an early Albian age for the top of the Serdj Formation that includes the main reservoir flow unit which is producing oil in both onshore and offshore Tunisia. Outcrop data analysis has revealed nine transgressive-regressive cycles within the Aptian - early Albian carbonates. During early Aptian the carbonate deposition occurred in deep-marine settings that gave way to shallow, open shelf carbonates deposition during the late Aptian - early Albian. These successions provide evidences enabling their correlation with the third-order global depositional sequences detected in the sedimentary basins of the Tethyan domains. Regional surface to subsurface stratigraphic correlation, based on seismic and wire-line logs, enabled: (1) a reliable correlation between the stratigraphic elements of the buried successions with those of the Serdj Formation, (2) the characterization of the platform-basin transition domain and (3) the verification of the hypothesis that the Serdj Formation carbonates constitute the surface analogue of the subsurface oil field reservoirs. The detailed conceptual stratigraphic model suggests that the Serdj platform developed diachronous. Its demise occurred in the early Albian and not during the Aptian-Albian transition as previously believed. The hydrocarbon-productive reservoirs developed during the regressive phases especially in the up-dip settings of the platform where dolomitization greatly enhanced the reservoir quality.