Age, depositional environment, and hydrocarbon potential of the Bonalea strata in the northern Douala basin, SW Cameroon: An integrated organic and mineralogical approach

Abstract

This study uses the lithofacies analysis, palynology and palynofacies, as well as bulk organic geochemical by Rock-Eval analysis, and mineralogical analysis to date and assess the hydrocarbon potential, maturity, and depositional environment characteristics of the strata from the Bonalea area in the northern part of the Douala sub-basin. The sediments are palynomorph-poor silts/sandstones, with key palynomorph species (e.g., Longapertites sp., Cyathidites sp., Laevigatosporites sp., Echitriporites trianguliformis, Retitriporites sp., Monocolpopollenites sp. and Psilatriletes sp.) indicative of the Paleocene-Eocene N'kapa Formation. These materials exhibit weak to moderate compaction and are distinguished by the predominance of detrital minerals such as quartz, kaolinite, smectite, and illite, as well as the initial phase of chloritization of illite and weak thermal alteration index (TAI) values, indicating the thermal immaturity of the deposits. The present-day high total organic carbon (TOC) contents (3.79–13.20 %), associated with high hydrogen index (HI: 594–796 mg HC/g TOC), S1 (1.91–24.23 mg HC/g rock) and production index (PI: 0.07–0.24) values, as well as low Tmax values and the existence of significant S1 peaks and asymmetric S2 peaks, reveal natural contamination of these deposits by migrated oils. These oils impregnate the rocks, resulting in stains with jellified aspect that may be misinterpreted as amorphous organic matter (AOM), leading to incorrect interpretations. Based on the combination of silty-sandy characters of the deposits, relative abundance of phytoclasts and presence of Botryococcus braunii, the paleodepositional environment appears to be a proximal continental shelf environment in which deposition would have occurred following the channel-bank collapse under well-oxygenated conditions, with slumped bank materials transitioning from landslip masses to sediment-gravity flows. Therefore, the Bonalea deposits, which are affected by the movement of oils from external sources and have the potential to be a full reservoir, could be a promising option for future petroleum exploration in the Douala sub-basin.