ABSTRACT: Chalk deposition characterizes the Late Cretaceous period in many areas of the world. In Tunisia it occurred predominantly during Campanian - Maastrichtian time, resulting mainly in Abiod Chalk Formation deposits that show high potential as a petroleum reservoir. Cored intervals of the uppermost Aleg and Abiod formations (around 100 m) have been logged and sampled in detail from three wells (Miskar W1, W2, W4) in the Miskar Gas Field, Gulf of Gabes, offshore eastern Tunisia. The uppermost Abiod and the base of the overlying El Haria Formation have been logged and sampled in one additional well (Miskar W3). In this area, the Abiod Formation mainly consists of homogenous chalk successions that, while adequately constrained in terms of biostratigraphy, are inadequately correlated between wells, so that the need remains for additional integrated studies that combine geochemical and isotopic methods. Such studies would further confirm the validity of chemostratigraphy as a reliable correlation tool. The Abiod Formation is subdivided lithologically into two units in Miskar W3, three units in Miskar W4, and four units in the Miskar W1 and W2 wells. The Aleg and El Haria formations are designated as separate lithostratigraphic intervals. A low-resolution biostratigraphic study was carried out in which comparisons to previous studies (Moody et al. 1995; Bailey et al. 2000a, 2000b) indicated an Early Campanian-earliest Maastrichtian age for the Abiod Formation, a probable Late Turonian-Coniacian age for the uppermost Aleg Formation, and an Early Maastrichtian age for the base of the El Haria Formation. Strontium isotope (87Sr/86Sr) stratigraphy (SIS) supports the biostratigraphic dating. Geochemical determinations of nineteen elements in the Miskar wells at high stratigraphic resolution show significant vertical variation. Five geochemical curves (Ca, Al, Mn, Ti, Zr) appear to be suitable for correlation. Based on these elements, the uppermost part of the Aleg (in Miskar W1, W2, and W4), and the lower part of the El Haria Formation (Miskar W3) can be geochemically characterized. The Abiod Formation is subdivided into eight geochemical units in Miskar W1, six in Miskar W2, four in W4 and three in W3. The proposed geochemical correlation agrees with the biostratigraphic interpretation, and in addition, some of the lithological boundaries lie close to the geochemical unit boundaries. However, the established geochemical correlation between wells suggests that detailed geochemical subdivisions of the formation can be established and laterally correlated without being entirely related to specific lithological variation. Study of the carbon and oxygen isotope profiles in the Miskar wells confirms the existence of an unconformity between the Aleg and Abiod formations that is indicated by biostratigraphic and SIS studies. Isotopic correlation of the Abiod Formation between Miskar W1 and W4 is based on the definition of eleven isotope units in W1, which appear to be suitable for correlation elsewhere. A good agreement exists between the isotopic units and the identified geochemical unit boundaries. In sum, the integrated chemostratigraphic study in this paper identifies correlatable geochemical discontinuities and recognizes stratigraphically significant surfaces and marker beds, as well as small-scale chemostratigraphic events in a presumed homogenous chalk succession.This study offers comparative data for the hemostratigraphic correlation and characterization of the Upper Cretaceous successions elsewhere in the southern Tethys.
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