Sédimentologie, stratigraphie séquentielle et potentiel pétrolier de la série cénomanienne du bassin des Aurès

Abstract

The region between Batna in the west and Khenchela in the east covers a very distinct paleogeographic and structural domain which is the Atlas (Aures Mountains) bordering the Saharan platform. In order to determine the Cenomanian and its sub-stages, its limits, the sedimentological and tectonic conditions controlling its different deposits, as well as to decipher the paleoenvironments prevailing at that time, a multidisciplinary approach integrating lithological, paleontological, sedimentological, geochemical and structural data was carried out on samples from 3 sections (Koubaane, TaghroutAmeur; and Ktef el Hammam). Four new lithological units are identified, in a set of thickness between 700 m to 1200 m. From bottom to top: the lower unit of marl with rare intercalations of clay limestones (Unit I of lower to middle Cenomanian age), lumachelic marl-limestones (Unit II of middle Cenomanian age), alternating marl-limestone ( Unit III of middle to upper Cenomanian age) and limestone (Unit IV of lower Turonian age). These ages are obtained by applying the concepts of biostratigraphy involving the combination of pelagic foraminifera and ammonites for the first time in the Aures basin. Six ammonite biozones and five foraminifer’s biozones have been defined and calibrated; with other regions neighboring the Tethyan and Boreal realms. The biostratigraphy thus established proves to be precised and makes it possible to identify certain unknown limits such as (Albian-Cenomanian; lower to middle Cenomanian; middle to upper Cenomanian; Cenomano-Turonian). The deposition environments belong to a carbonate platform with a slight slope on which, from outside to inside, there are three major environments: (1) an open platform with calm waters of infratidal environment, (2) a medium platform of medium to upper infratidal environment that is not very open; and (3) a restricted platform, with a few evaporates, extending from infratidal to high intertidal. The sequence of different Cenomanian sediments is arranged in six mega-sequences with a regressive tendency and uneven development, limited by discontinuities of regional value. The last sequence ends with the return to the concomitant deep conditions of a eustatic rise and deposition of pelagic sediments characterizing a transgressive interval (IT) and is part of the third order Tethyan major eustatic cycle. The organic matter analysis results in the different sections showed variable (TOC) rates; 0.90% in Koubaane; 0.18%to 0.68%in TaghroutAmeur and 0.15%to 1.07 % in Khenchela). These rates revealed at the transgressive intervals characterizing the lower Cenomanian and the Cenomanian-Turonian passage. From a structural point of view, the study region went through several successive tectonic events since the Triassic; however, it mainly structured during the Tertiary by the major phases. It is formed of large NE-SW directional folds affected by major strike silp faults generally of NW-SE orientation, and a major fracturing system grouped into three directional families; the NW-SE family, the N-S family and the E-W family. A structure generated especially by the Atlas and Alpine phase