TECTONIC EVOLUTION OF THE DOBA AND DOSEO BASINS, CHAD: CONTROLS ON TRAP FORMATION AND DEPOSITIONAL SETTING OF THE THREE FIELDS AREA, CHAD

David J. Reynolds¹ and Clive R. Jones^2
1 ExxonMobil Exploration Company, Houston TX, USA
² ExxonMobil Development Company, Houston TX, USA

The Doba and Doseo basins in Chad are part of the Central African rift system. These basins contain up to 10 km of non-marine sediments which record the complex tectonic and climatic evolution of the region from the Early Cretaceous to the present. These basins were originally identified using gravity data, but three decades of subsequent exploration activity has significantly increased our understanding of the basins evolution. We present a model of basin development that calls on multiple extensional and inversion events to produce the structures observed today. Our interpretations differ significantly from earlier models that invoke widespread wrench faulting. In this paper, we discuss the evolution of the basin in terms of trap development within the Doba basin and focus on the influence on non-marine and lacustrine depositional settings of rift structures and topography created by Cretaceous extension. In particular, we will apply these models to the Three Field Area; a joint development venture between ExxonMobil, Petronas and Chevron-Texaco, which formed the economic basis for the construction of the Chad-Cameroon pipeline.

In order to understand the distribution of play elements within these basins, it is necessary to first unravel their evolution and identify those features which influence depositional environments during the time in question. The basins were sites of extension on a Pan African shear zone beginning in the Neocomian. In the Doba Basin these faults trend northeast-southwest and develop en-echelon southeast dipping half-grabens. In the Doseo Basin, most major faults are located along the northern margin and dip to the southeast creating northwest-dipping basins. Between these basins, a large northeast-southwest accommodation feature named the Borogop fault zone was created. Changes in extension direction in the Aptian induced oblique extension and created new northwest-southeast trending fault systems. Accommodation and relay zones are present in the Doba Basin where these faults intersect the Neocomian faults. These features played an important role in creating sediment input points to the basin. In Doseo, a series of north-dipping normal faults were created along the basins southern margin, while faults along the northern margin amalgamated to form a long, complex fault zone. A sag phase commenced about 110Ma and continued with minor extension until 84Ma when changes in Atlantic spreading directions induced compressional stresses throughout the African plate. This created inversion of many central African basins. In Doba, inversion was focused in the pre-existing, Aptian-aged accommodation zones and large breached relays. The maximum inversion in these regions is on the order of 100-200m. Minor inversion continued until about 40Ma when the African plate was placed under large compressional stresses once again, this time due to convergence of Africa and Europe during the Pyrenean orogeny. In the Doba Basin, inversion of the Neocomian-age normal faults is common and several northeast to southwest-trending anticlines are present. In some places these earlier faults are reactived while in others, new reverse faults are formed. Maximum inversion is approximately 500-700m on these structures. The Doseo Basin has similar deformation associated with these inversion events as well. Beginning about this time, the western portion of the Doba Basin was being uplifted in response to heating associated with the Cameroon volcanics belt. The Doba and Doseo basins were next deformed by regional northeast-southwest extension beginning in the Late Oligocene to Early Miocene. These north-northwest to south-southeast trending fault systems often reactivate pre-existing Aptian-aged faults, sometimes for only a segment of their length, before continuing on newly-formed fault strands. In other cases, the faults are detached in an Upper Cretaceous shale interval. Regional uplift associated with the African superplume has created a widespread erosional surface with an age of approximately 10Ma that has beveled the entire section in the basins so that no surface expression of the most recent structures is observable.