AN INTRODUCTION TO THE PRECAMBRIAN PETROLEUM SYSTEM IN THE SANKURU-MBUJI-MAYI-LOMAMI-LOVOY BASIN, SOUTH-CENTRAL DEMOCRATIC REPUBLIC OF CONGO

F. Delpomdor^1,2*, S. Bonneville², K. Baert³ and A. Préat²

¹ Illinois State Geological Survey, University of Illinois, 625 E. Peabody dr., 61820 Champaign, Illinois, USA.

² Biogeochemistry and Modeling of the Earth System, Université Libre de Bruxelles, 50 av. F.D. Roosevelt, CP 160/02, 1050 Brussels, Belgium.

³ Research Group Electrochemical and Surface Engineering, Department Materials and Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium.

^* Corresponding author, email: fdelpomd@illinois.edu

This study presents a preliminary assessment of the petroleum potential of the Meso-Neoproterozoic Mbuji-Mayi Supergroup in the Sankuru-Mbuji-Mayi-Lomami-Lovoy Basin in the southern-central Democratic Republic of Congo. This basin is one of the least explored in Central Africa and is a valuable resource for the evaluation of the petroleum system in the greater Congo Basin area. Highly altered carbonates (potential reservoir rocks) and black shales (potential source rocks) are present in the Mbuji-Mayi Supergroup, which can be divided into the BI and overlying BII groups (Stenian and Tonian, respectively). For this study, samples of the BIe to BIIe subgroups from five boreholes and two outcrops were evaluated with petrographic, petrophysical and geochemical analyses.

Carbonates in the BIe to BIIe subgroups with reservoir potential include oolitic packstones and grainstones, stromatolitic packstones and boundstones, various dolostones, and brecciated and zoned limestones. Thin section studies showed that porosity in samples of these carbonates is mainly vuggy and mouldic with well-developed fractures, and secondary porosity is up to 12%. Black shales in the BIIc subgroup have TOC contents of 0.5-1%, and the organic matter is interpreted to have been derived from precursor Type I / II kerogen. The thermal maturity of asphaltite in carbonate samples is indicated by Raman spectroscopy-derived palaeo-temperatures which range from ~150 to ~260°C, which is typical of low-grade metamorphism. Raman reflectance (R_mcR_o%) values on asphaltite samples were between 1.0 and 2.7%, indicating mature organic matter corresponding to the oil and wet gas windows. Source rock maturation and primary oil migration are interpreted to have occurred during Lufilian deformation (650-530 Ma). The solid asphaltite present in fractures in the dolostones of the BIIc subgroup may represent biodegraded light oil from an as-yet unknown source which probably migrated during the Cambrian-Ordovician (~540-480 Ma). This migration event may have been related to the effects of the peak phase of Lufilian deformation in the Katanga Basin to the SE.

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