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Detritic Input Quantification in Lacustrine Petroleum Systems: Example of the Pre-Salt Source Rocks from the Lower Congo Basin (Congo)

Abstract

The aim of the study is to propose an integrated workflow to quantify detritic input into autochthone lacustrine deposit in terms of regional extend and total organic carbon (TOC) content. This workflow includes (1) organic geochemical data such as Rock Eval data together with quantitative palynofacies assessment and (2) paleogeographical maps derived from well and seismic data. This workflow was applied to the immature Barremian source rocks from the Lower Congo Basin because of the availability of a large dataset describing a complex sedimentary record with the interaction between organic rich muds, subaqueous clastic systems and carbonates deposition. From palynofacies analysis, it was demonstrated that lacustrine organic matter corresponds to HI higher than 600 – 650 mg/g C whereas detritic input (terrestrial – inertinite – fusinite) corresponds to HI lower than 300 mg/g C. The range of HI between 300 and 600 mg/g C corresponds to mixtures of organic matter in which the detritic contribution clearly increases as the HI decreases. The correlation between HI and detritic content was then applied to 50 wells and enabled to map detritic extend at local and regional scale. The geochemical data were plotted on high resolution paleographic maps for 4 stratigraphic intervals, BA2, BA2-BA3, BA3-PN and BA3-PI. Results confirm the periodic presence of lacustrine turbiditic systems allowing strong detritic inputs into the paleolake under the tropical paleoclimate, especially in BA-BA3 and BA3-PI intervals. These continental derived organic inputs degrade the quality and richness of the lacustrine source rock. These detritic influxes were related to coastal rivers whereas, in the deepest and more protected parts of the basin, the autochthonous is well preserved and not diluted. This allowed the deposition of exceptional source-rocks with very high TOC and HI values especially during BA2 and BA2-BA3 intervals. All these geochemical diagnostics are fully consistent with available paleogeographic maps. Consequently, this study demonstrated the importance of integrating geochemical data (Rock Eval data and quantitative palynofacies diagnostic) into the paleo-reconstruction of the source rock environmental deposit. It enables to really constrain paleogeographic maps in terms of detritic input as well as preservation at both regional and local scale of a given petroleum system.