LITHOFACIES AND ORGANIC MATTER ENRICHMENT IN MADINGO FORMATION OF BLOCK M, LOWER CONGO BASIN

The source rock of the Madingo Formation in the Block M of the Lower Congo Basin is selected as the research object of this paper. Mineralogy, paleontology and lithofacies of the source rock were analyzed using X-ray diffraction, thin sections, backscatter and energy spectrum. Special attention is paid to major elements and organic matters, and the enrichment mechanism of organic matters discussed. The results show that the source rock is mainly composed of clay and silica minerals, followed by calcite, feldspar, dolomite and pyrite. Micro fossils and nanofossiles, such as foraminifera and radiolarian, are rather popular. Based on mineralogy, paleontology, phosphate content and microstructure, six types of lithofacies are identified, i.e.: foraminifera-bearing fine deposits, radiolarian-bearing fine deposits, phosphatic fine deposits, argillaceous fine deposits, silty fine deposits and calcareous fine deposits. The deposition of the source rock in Madingo Formation was under the joint control of sea level eustacy, upwelling current, paleo-climate and paleogeography. Several cycles of transgression and regression events are identified in the Formation. Sea level eustacy constrained the deposition of lithofacies through changes in water depth, redox conditions and sediments types. Foraminifera-bearing fine deposits, phosphatic fine deposits and radiolarian-bearing fine deposits with high organic content are formed in two cycles of upwelling. Depressions in the inner shelf were filled with evaporate deposits during the period while the Madingo Formation was deposited. The relatively closed environment benefited the preservation of organic matters. The paleo-climate experienced three stages of change from dry to relatively humid and to humid climate during the period of Madingo Formation. The abundance and types of organic matter were dependent upon to some extent the input of terrigenous organic matter.