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

2018 AAPG International Conference and Exhibition

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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 autochtone 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 its efficient tropical paleoclimate in terms of detritic fluxes from continent to the deepest part of the offshore basin. Two specific stratigraphic levels were selected. The first one is the Pointe-Noire Formation with exceptional hydrogen indices (HI) up to 900 mg/g C and TOC from 3 to 30%. The second level corresponds to the transition Pointe-Noire to Pointe Indienne characterized by very large range of HI from 20 to 800 mg/g C. A set of about 50 wells with geochemical data was selected and covers the basin from the proximal domain to the deeper depositional environment of the paleolake. From quantitative palynofacies analysis, it was demonstrated that only pure lacustrine organic matter corresponds to HI higher than 800 mg/g c whereas detritic input (terrestrial – inertinite – fusinite) corresponds to HI lower than 250 mg/g C. The range of HI between 250 and 800 mg/g C depends on the detritic input and it contribution clearly increases as the global 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. Results confirm efficient detritic input under the tropical paleoclimate. The continental derived organic input degrades the OM matter quality of the lacustrine autochtone OM. A systematic dilution of the overall TOC by sediment fluxes is also observed. This detritic influx is related to coastal rivers whereas in the deepest and more protected parts of the basin, the initial organic matter is well preserved with minor or no detritic input. All these geochemical diagnostics are fully consistent with available paleogeographic maps. In conclusion, the 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 at both regional and local scale of a given petroleum system.