SEDIMENTARY TRANSPORT OF ORGANIC MATTER IN LACUSTRINE SYSTEMS: A CASE STUDY FROM A TROPICAL RIFT LAKE

Although the majority of the petroleum produced from shales to-date has been derived from marine settings, lacustrine petroleum systems may contain significant unconventional hydrocarbon accumulations in certain regions of the world (most notably China). This study uses an extensive suite of sediment cores and surface-sediment grab-samples collected throughout Lake Malawi, East Africa combined with additional published datasets to study the sedimentological processes that control the organic geochemistry of sediments in large tropical rift lakes. Sediment samples were analyzed for total organic carbon (TOC), total nitrogen (TN), and by Rock-Eval pyrolysis assay. TOC values average 2.4% with a maximum value of 8.8%. The kerogen is highly variable ranging from type II to III; consistent with other large East African lakes. Whereas the average TOC value is lower for all sites above the chemocline (~200 m) than for deeper water sites, the difference is not statistically significant due to the high degree of variability within both groups. This indicates that anoxic preservation of organic matter (OM) does not sufficiently explain the distribution of organic facies in Lake Malawi. The most profundal environments in Lake Malawi do not have the highest TOC content nor do they contain the most algal OM. Moreover, sedimentary TOC and TN concentrations are strongly correlated, which can be explained by a homogenized source of OM being diluted by inorganic sediment. We propose that the homogenization of sedimentary OM in Lake Malawi is the result of the selective resuspension and downslope transport of OM throughout the lake. This inference is supported by sediment trap and core data that indicate that a significant amount of OM is being delivered to offshore sites via processes other than direct sedimentation from the water column above. Water-column particulate-carbon data confirm that near lake-bottom resuspension of OM occurs on a regular (perhaps seasonal) time scale. Furthermore, the extent of separation of organic and inorganic sediment is dependent on the slope of the lake floor, with shallower bathymetric gradients leading to greater partitioning. These results have important implications for the potential formation of petroleum source rocks and unconventional hydrocarbon resources in lacustrine systems.