Chemostratigraphy and correlation of a chalk petroleum reservoir : the Upper Cretaceous Abiod Formation Tunisia
Chalk deposition characterises most of the Late Cretaceous period in many areas of the world. In Tunisia it occurred predominantly during the Campanian - Maastrichtian stages. The facies, characteristic of the Abiod Chalk Formation, shows high potential as a petroleum reservoir rock. Cored intervals of the uppermost Aleg and Abiod formations (around 100 m) have been logged and sampled in detail from three wells (Miskar 6, A-3, A-11) 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 A-7). Around 500 m of sediments were logged and sampled in detail through an onshore section at Le Kef (NW Tunisia) covering the uppermost Le Kef Formation, the entire Abiod Formation, and the base of the El Haria Formation. A small additional section (Kodiet ez Zarbia) near Le Kef was also examined. Low-resolution biostratigraphic studies were carried out on the Miskar A-3 well and the Le Kef section and compared to previous studies. The Abiod Formation in the Miskar Field is Campanian - lowest Maastrichtian, the uppermost Aleg is upper Turonian - Coniacian, indicating an unconformity between the two formations. The El Haria Formation is lower Maastrichtian. The Abiod Formation at Le Kef and Kodiet ez Zarbia is Campanian - lower Maastrichtian. A strontium isotope ([sup]87 Sr/[sup]86 Sr) stratigraphy (SIS) study confirms the biostratigraphy. l. A geochemical investigation of 19 elements in the Miskar wells and 12 elements at Le Kef has allowed the vertical geochemical profiles to be constrained, five of which (Ca, Al, Mn, Ti, Zr) are suited for correlation. Eight chemostratigraphic units are defined in the offshore wells. Lateral correlation between Le Kef and Miskar 6 has been achieved, and nine geochemical correlation sequences (GCS) are established. This correlation demonstrates that although non-deposition occurred during part of the early Campanian in the Miskat area, the Abiod Formation is here a condensed chalk succession corresponding to most of the Abiod at Le Kef. Carbon ([delta][sup]13 C) and oxygen ([delta][sup]18 O) stable-isotope curves were obtained through the Le Kef section and the Miskar 6 and A-11 wells, with additional data for selected samples from A-3 and A-7. The stable isotope study of the Miskar wells confirms the unconformity between the Aleg and Abiod formations indicated by biostratigraphic and SIS studies. An isotopic correlation of the Abiod Formation between Miskar and Le Kef has been achieved with the definition of eight isotope correlation sequences (IsS). The identification of geochemical and isotopic discontinuities validates the use of chemostratigraphy as a tool for the recognition of stratigraphically significant surfaces and marker beds. The [delta][sup]13 C curve for the Abiod Formation at Le Kef is correlated to other Tethyan, Boreal and NW Pacific Ocean sections. Fourteen correlation sequences (CC) including the eight isotope correlation sequences (IsS) are identified, two of which are considered to be major isotopic events: one positive (Mid-Campanian Event) and one negative (Upper Campanian Event) carbon isotope excursion. These are dated at 78.7 Ma and 74.8 Ma respectively, and lasted 600-750 kyr each. The correlation confirms that carbon isotope .trends reflect global rather than local processes. Correlations can be used to test the validity of Campanian global biostratigraphic frameworks, and improve our understanding of the nature and timing of Late Cretaceous sea-level change. The [delta][sup]13 C curves are correlated to local and global sea-level curves, indicating that positive [delta][sup]13 C excursions are mainly associated with transgressive system tracts, whereas negative excursions are related to regressive systems. The manganese chemostratigraphy is correlated between Tunisia, Italy and the Danish Basin. Manganese combined with Ca, Al, Zr, and Ti data, and integrated with [delta][sup]13 C curves, provides another robust tool for reconstructing stratigraphic sequences and identifying sea-level change in pelagic - hemipelagic carbonate successions.