Abstract:

The sedimentary and diagenetic characteristics of reservoir were special in Bongor basin, Chad. Based on regional tectonic evolution and various basic data such as cores, thin sections, mud logging, test data and so on, the authors do systematic research on the sedimentary-diagenetic characteristics of reservoir sandstone. The results show that series of NW-SE depressions were formed during the early Cretaceous rifting stage by the NE-SW extension, which controlled the sediment source, the sandstone distribution patterns and formed lots of fans. These fans should be classified as subaqueous fans and mainly distributed along the boundary faults. The structure inversion of the Late Cretaceous induced large amount of erosion and uplifted the reservoir greatly. It also influenced the normal diagenesis process of the reservoir. Due to the tectonic evolution, the hydrocarbon infilling and compositional maturities, their mechanical compaction and cementation were relatively weak but the dissolution was relatively common. The diagenesis of reservoir sandstone was still under the substage of the middle diagenetic stage and has the good quality: medium porosity, mediumhigh permeability. The quantitative evaluation of the relationship between porosity and burial depth revealed that the depth limit of the effective reservoirs is much deeper than that of the present oil pay zones and the deep reservoir still has a good prospect of exploration.

Key words: Chad, Bongor basin, reservoirs, sandstone distribution, reservoir diagenesis, main controlling factors