CLIMATIC EVOLUTION OF THE DOBA BASIN, CHAD: CONTROLS ON DEPOSITIONAL SETTING AND STRATAL ARCHITECTURE OF THE THREE FIELDS AREA, CHAD

Penny E. Patterson¹, Clive R. Jones², Maija E. Schellpeper¹, Ray Skelly², and David Lowe^2
1 ExxonMobil Upstream Research Company, Houston TX, USA
² ExxonMobil Development Company, Houston TX, USA

Doba Basin, Chad, is situated within the Central African rift system, and contains up to 9 km of Cretaceous alluvial and lacustrine deposits. The complex climatic and tectonic evolution of the region spanning Early to Late Cretaceous is uniquely recorded in the stratigraphic architecture. In this paper, we present a model of alluvial and lacustrine fill, which invokes high-frequency climatic fluctuations imposed on a longer-term climatic cycle, and variable accommodation produced by extensional processes. The evolution of the basin fill is reviewed with regard to prevailing climatic and tectonic conditions and their influence on style of fluvial and lacustrine deposition. We will discuss the application of this model to the Three Fields Area (Miandoum, Bolobo, Kome) of Chad, which is a joint development venture between ExxonMobil, Petronas, and Chevron-Texaco.

An understanding of the alluvial setting within the Doba Basin is critical in deciphering the climatic influence on the depositional environments. The Doba Basin Upper Cretaceous (Cenomanian – Coniacian) stratal successions are interpreted to represent fluvial depositional systems that systematically varied along their longitudinal depositional profile, ultimately terminating in ancestral Lake Chad. Four distinct fluvial-dominated facies belts characterize the alluvial succession. Structurally controlled sediment entry points fed proximal (up-dip) fluvial-facies belts comprised of amalgamated mid-channel to bank attached bars that developed within low-sinuosity braid channels. These channels are stacked vertically and laterally forming thick and aerially extensive channel complexes. Medial (mid-dip) fluvial-facies belts consist of semi-amalgamated bars that possess sedimentary bedforms indicative of decreasing stream competency within the low-sinuosity single-thread channels. Channel complexes in medial depositional regions are thinner and less laterally persistent. Distal (down-dip) fluvial-facies belts are characterized by a more heterolithic style of sedimentary bedding suggesting the occurrence of fluctuating low-energy discharge during deposition in single-thread braid channels. These distal channel complexes are thin, and yet more laterally extensive than those of the medial regions. The fluvial system terminates in shallow ephemeral to perennial lakes or ponds, building small, coalesced terminal splay complexes.

Within all three fluvial facies belts, channel complex elements are overlain by intervals dominated by floodplain deposits containing rare, isolated channels, and occasional ephemeral-perennial lacustrine mudstones. Within a sequence stratigraphic context, the sand-prone channel complexes are interpreted as late lowstand systems tracts of a depositional sequence and the mudstone-prone floodplain/lacustrine intervals are interpreted as transgressive and highstand systems tracts. Examination of the pedogenically modified floodplain deposits reveals that high-frequency climatic fluctuations apparently governed the development of these depositional sequences. Relatively moist temperate climatic conditions, which are recorded by vertisol development in the floodplain deposits and occurrence of lacustrine mudstones, prevailed during degradation of the alluvial landscape and subsequently through fluvial aggradation within the channel complexes. A change to drier temperate conditions followed, interpreted from the formation of alfisols, coinciding with accumulation of thick mudstone intervals dominated by floodplain deposits. Similarly, development of composite sequences, which are the larger scale alluvial architectural elements, reflect a longer-term climatic variation from moist tropical to dry temperate conditions.

Understanding the complex interplay of the Doba basins long term structural evolution vs high-frequency climatic cyclicity recorded within the Cretaceous sediments, has proven essential in the accurate prediction of reservoir / seal distribution and continuity. This understanding has proven critical in developing an effective depletion strategy for the Three Fields Area of Chad.