Detrital input quantification in lacustrine petroleum systems: An example of the pre-salt source rocks from the Lower Congo Basin (Congo)

This study describes an integrated workflow designed to quantify detrital input into lacustrine deposits in terms of regional extent and total organic carbon content. This workflow includes (a) organic geochemical data such as Rock Eval and palynofacies and (b) palaeogeographical maps. This workflow was applied to the immature Barremian source rocks from the Lower Congo Basin because of a large available data set describing a complex sedimentary record. From palynofacies analysis, it was demonstrated that lacustrine organic matter corresponds to a hydrogen index higher than 600 mg/g C whereas detrital input corresponds to a hydrogen index lower than 300 mg/g C. The range seen in the hydrogen index of between 300 and 600 mg/g C corresponds to mixtures of organic matter. The correlation between the hydrogen index and detrital content was then applied to 50 wells which allowed the extent of detrital input to be mapped at local and regional scales. The geochemical data were plotted on high-resolution palaeogeographic maps for four stratigraphic intervals, BA2, BA2-BA3, BA3-PN and BA3-PI. Results confirm the periodic presence of lacustrine turbiditic systems allowing strong detrital inputs into the palaeolake under the tropical palaeoclimate, especially in the BA2-BA3 and BA3-PI intervals. These inputs of continental-derived organic material degrade the quality and richness of the lacustrine source rock leading to a decrease of the initial hydrogen index values. These detrital influxes were related to coastal rivers whereas, in the deepest parts of the basin, the autochthonous organic matter is well-preserved and not diluted, which allowed the deposition of exceptional source rocks especially during the BA2 and BA2-BA3 intervals. All of these geochemical diagnostics are consistent with available palaeogeographic maps. This study demonstrated the importance of integrating geochemical data into the palaeo-reconstruction of the source rock depositional environment. It enables palaeogeographic maps to be constrained in terms of both detrital input and preservation at both regional and local scales for a given source rock.