Basin dynamics and petroleum potential of the Eocene series in the gulf of Gabes, Tunisia

Abstract

The sedimentary series in the study area includes marls with limestone interbeds of the Upper Maastrichtian–Paleocene, Ypresian Nummulitid-rich limestones which grew laterally into limestones and black shales, and overlying Upper Eocene marl and fossiliferous limestones. Well correlations and outcrop data point to sedimentary sequences vary considerably in lithofacies and thicknesses, with significant gaps. Seismic stratigraphical modeling basing on petrologic evidence from cores and well-cuttings, as well as wireline log details (sonic and gamma ray) helps recognize three main supersequences SS1 to SS3. The supersequence SS1 (Upper Maastrichtian–Paleocene) has rather oblique seismic configurations testifying to prograding entities during periods of sea-level rise. The supersequence SS2 (Ypresian) is comprised of platform to basin carbonates with transparent seismic facies, local high amplitudes and onlapping configurations relevant to drawdown and aggrading sequences. The third supersequence SS3 (Upper Eocene) shows transparent seismic facies, and also continuous reflections with high amplitudes, which tend to indicate strata deposited under influential subsidence. The geometry of sequences, seismic facies and reactivating faults in the mid-gulf, lend support to a depositional model evolving from tectonically induced slope with bathymetry increasing northeastwards. The Eocene petroleum system associates black shales with high TOC contents (0.15–4%) and elevated HI values (up to 16kg/t HC), and a porous Nummulite-bearing limestone reservoirs; these were early sealed by broadly spread Upper Eocene clays, whereas, source-rock maturation and hydrocarbon migration might have occurred during the Tortonian–Pliocene Atlassic compression.