Petrological and geochemical investigations of potential source rocks of the central Congo Basin, Democratic Republic of Congo

Paleozoic and Mesozoic outcrop and core samples (REMINA Dekese and REMINA Samba wells) covering various stratigraphic intervals from the central Congo Basin were analyzed for total organic carbon (Corg), total inorganic carbon (Cinorg), and total sulfur content. Rock-Eval analysis and vitrinite reflectance (Ro) measurements were performed on the basis of the Corg content. Fifteen samples were chosen for molecular organic geochemistry. Nonaromatic hydrocarbons (HCs) were analyzed by gas chromatography (GC)–flame ionization detection and GC–mass spectrometry. Samples of the Alolo shales from the Aruwimi Group (Lindi Supergroup, late Neoproterozoic to early Paleozoic) are in general very poor in Corg (most samples 0.5%) and contain a high amount of degraded organic matter (OM). All samples of this group revealed a type III to IV kerogen and cannot be considered as a potential source rock. Permian–Carboniferous sediments from the Lukuga Group (Dekese well and outcrop samples) contain moderate contents of organic carbon (2%). The Tmax values (heating temperature at which the top peak of S2 occurs) indicate early mature OM, partly also a higher level of maturity because of Ro (0.6–0.7%) and production index values (S1/S1 + S2 0.2). All samples contain hydrogen-poor type III to IV kerogen with low HC generation potential, only having a very minor gas generation potential. The kinds of OM, as well as the biological markers, indicate a terrestrial-dominated depositional environment. Organic geochemical investigations on Upper Jurassic (Stanleyville Group) to Lower Cretaceous (Loia Group) samples from the Samba well and outcrops in the northeastern part of the Congo Basin reveal moderate to high contents of organic carbon (as much as 25%). The kerogen has very high hydrogen index (HI) values reflecting type I kerogen of excellent quality in the Stanleyville Group (as much as 900 mg HC/g Corg) and type I to II kerogen in the overlying Loia Group (as much as 900 mg HC/g Corg). Outcrop samples from the Stanleyville Group have variable partly high Corg contents and are also characterized by very high HI values (as much as 900 mg HC/g Corg). The samples studied are too immature for petroleum generation. Based on biomarker analysis, an aquatic anoxic depositional environment can be assumed for the Stanleyville Group, whereas a lacustrine deposition is likely for the samples of the Loia Group. Based on the geologic knowledge of the area, deposition under lacustrine conditions is most likely also for the Stanleyville Group. Both the Stanleyville and Loia groups can be regarded as excellent petroleum source rocks and could be part of a petroleum system if sufficient burial and maturation have occurred. The presence of resedimented vitrinite particles in the Lukuga Group of the Dekese well with a slightly higher maturation rank than the autochthonous vitrinites suggests that 3000–4000 m (9840–13,120 ft) of Carboniferous to Devonian sediment has been eroded from the eastern margin of the Congo Basin. Finally, Ro data were used to create one-dimensional models for the Dekese and Samba wells, giving an overview of the burial, thermal, and maturity histories of the area.