Fracture kinematics, cementation and burial history of a structurally polyphase sedimentary basin, Tunisia
Basin analysis of over 1000km of 2-D seismic profiles and thirteen exploration wells from the North and South Kairouan Permits in the foreland region of the Tunisian Atlas has led to the identification of a number of late Mesozoic to recent basin systems. The generation of these basin systems resulted in the development of fractures and emplacement of diagenetic fluids and hydrocarbons in the early Campanian to early Maastrichtian (83.5-70Ma) chalks of the Abiod Formation. Recognition of the basin-forming events has been based on the identification of regional unconformities, subsurface thickness variations of mid-Cretaceous to recent strata, and changes in the location and size of depocentres. The basin systems correspond to: (1) early Albian to early Campanian (110-83.5Ma) extension related to convergence across Tethys during the opening of the North Atlantic; (2) early Campanian to late Langhian (83.5-l5Ma) basin inversion and salt diapirism related to the convergence between Africa and Europe; (3) late Langhian to late Tortonian (15-7Ma) basin inversion, salt diapirism and foreland basin sedimentation associated with the southeastwards propagation of the Atlas Mountains thrust-system through the Mejerda Zone, Intermediate Atlas and into the North-South Axis; (4) late Tortonian to Late Pliocene (7-1.8Ma) extension related to the opening of the Strait of Sicily and foreland basin sedimentation from the North-South Axis; (5) Late Pliocene to recent (1.8- 0Ma) basin inversion related to the final stages of deformation along the North-South Axis. Hydrocarbons in the North and South Kairouan Permits are produced from open and partially open fractures in the upper Abiod Formation. Petrographic analysis has allowed the identification of ferroan calcite, ferroan dolomite and barite cements in fractures of the upper Abiod Formation which were emplaced in that order. The integration of burial history modelling and fluid inclusion analysis suggests that the pressure-corrected homogenisation temperatures (trapping temperatures) for the ferroan calcites are 59-660°C and for the ferroan dolomites are 78-93°C. Even though the polyphase fracturing events range from the late Mesozoic to recent, the integration of burial history modelling, petrographic, Formation MicroScanner/MicroImager and fluid inclusion analysis have constrained the timing of reservoir charging by diagenetic fluids to the: (1) late Langhian (15Ma) for the ferroan calcite cements; (2) Late Pliocene to recent (l.8-0Ma) for the ferroan dolomite and barite cements. Stable carbon and oxygen isotope analysis on samples of the fracture fills, in conjunction with fluid inclusion trapping temperatures, suggests that the [delta][sup]180 and [delta][sup]13C ratios of the fluids responsible for the recipitation of calcite cements ([delta][sup]18O[sub]H2O = +3.70% to +3.80% V-SMOW, [delta][sup]13C[sub]HCO3 = +0.02% to +0.08% V-PBD) were connate waters derived from marine carbonates. Dolomite cements ([delta][sup]18O[sub]H2O = -3.00% to +0.30% V-SMOW, [delta][sup]13C[sub]HCO3 = -2.53% to -1.16% V-PDB) were precipitated from meteoric waters with an input of soil-derived organic carbon. The meteoric waters were probably derived from an area of recharge that was located to the northwest of the study area towards the thrust-front of the Atlas Mountains, during the development of the North-South Axis. Burial history modelling indicates that hydrocarbons would have reached mid-maturity in source rocks beneath the Abiod Formation (Mouelha Member, 110-99Ma; Bahloul Horizon, 95-93.5Ma), from the Late Pliocene onwards (<1.8Ma). Petroleum was probably transported into the fractured upper Abiod Formation coeval with the fluids responsible for the precipitation of the ferroan dolomite cements and/or for the precipitation of the barite cements during the final stages of basin inversion (1.8-0Ma).