Victoria F. Sachse

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.

Organic geochemistry, petrology and palynofacies of Middle Devonian lacustrine flagstones in the Orcadian Basin, Scotland: depositional environment, thermal history and petroleum generation potential

During Middle Devonian time a thick succession of organic-rich, mainly lacustrine flagstones developed within the Orcadian Basin. These petroleum source rocks crop out in northern Scotland. Nineteen samples were studied using organic petrology, palynology and organic geochemistry in order to characterize kerogen type, depositional environment, thermal maturity and petroleum generation potential. C org , carbonate and sulphur content as well as hydrogen index (HI) values are quite variable (e.g. HI from 79 to 744 mg HC/g C org ). Based on biomarker data, organic material mainly originates from aquatic organic matter deposited under lacustrine conditions with oxygen-depleted, but not permanently anoxic, bottom waters. Petrography reveals small quantities of vitrinite particles, indicating minor input of terrestrial material. This is supported by biomarker data and the palynofacies, which is characterized by a high amount of oil-prone amorphous organic matter (AOM) and generally few miospores. Maturity of the succession studied in Caithness and Orkney is between immature and oil mature. One-dimensional basin modelling shows that a significant remaining hydrocarbon generation potential exists within the Middle Devonian succession. In contrast to the low hydrocarbon generation in the onshore area, offshore oil generation was significant, especially after deposition of thick Upper Jurassic – Upper Cretaceous sediments. At the end of Cretaceous time, hydrocarbon generation ceased due to uplift. The contribution to known oil fields from the Devonian flagstones is a realistic scenario, including a contribution to the Beatrice oil field in the south of the modelled area.