Mike_stratigraphygeologyKribiCampo1.pptx

Ce travail présente la stratigraphie des dépots anté cénomaniens dans le bassin de Kribi-Campo, au large de la marge atlantique du Cameroun

1. Stratigraphical nature and Palaeogeographical significance of the Top Albian surface in the South Cameroon Atlantic Margin.

2. Author speaker information Mike-Franck Mienlam Essi Earth Sciences Department, Faculty of Science, University of Yaoundé I/Cameroon

3. Introduction : Study Area Location The South Cameroon Atlantic Basin is located in the Gulf of Guinea This geological domain corresponds to the Douala/Kribi-Campo Basin ( Ntamak-Nida et al., 2010 ) The Douala/Kribi-Campo Basin is subdivided into two sub basins, namely Douala sub basin at the north and Kribi-Campo sub basin to the South( Loule et al., 2018 ). Fig.1 Location of Cameroon in Africa B- Segmentation of Cameroon Atlantic Margin (CAM) C-Location of the study area ( Mienlam Essi et al., 2021 ).

4. Introduction : Problem Statement The Top Albian surface marks the transition between Continental Lower Cretaceous and Marine Late Cretaceous( Cenomano -Turonian) ( Batupe, 1997; wornadt et al., 1999; Haq, 2014; Iboum Kissaaka et al. , 2016 ) 1 The Lower Cretaceous consists of continental and marine deposits, with a highstand sea-level reached at the Middle Cretaceous (Aptian-Albian) ( Ostanciaux, 2012; CGG Robertson, 2015; Mienlam Essi et al., 2021 ) . 2

5. Introduction : Main objective R e-examine the stratigraphical nature of the Top Albian surface in order to understand the Lower Cretaceous palaeogeoagraphy of the study area or the West African Margin

6. Stratigraphical Context Lower Mundeck (Berriasian to mid-Aptian) Upper Mundeck (Albian to Cenomanian) Logbadjeck(Turonian to Lower Campanian) Logbaba (Mid-Campanian to Maastrichtian) N’kapa (Upper Danian to Lutetian) Matanda and Wouri (Pliocene to Recen t). Fig.2 Lithochronostratigraphic chart of the Studied Basin (modified from CGG Robertson, 2015 ). Souellaba (Oligocene to Mid-Miocene) Kribi (mid-Miocene)

7. Palaeogeography Context Fig.3 Jurassic and Cretaceous tectonostratigraphic evolutions of the DKC Basin (modified from Abolo, 1996 ) . Roman numerals represent unconformities and Arabic numerals are geological formations. Pre-Cenozoic unconformities are emphasized (I: Top Albian, II: Campanian and III: Cretaceous-Tertiary ) ( Mienlam Essi et al., 2021 ) The Pre-rift and synrift stages ( Late Jurassic to Late Barremian : an arid climatic regime, ( SPT, 1995 ). 1 The Aptian period : the onset of marine incursion occurred. The final breach was effective a t 104 Ma, Heine et al., 2013 ). This phase corresponds to the Sag and Early post-rift phases. 2 At the latest Cretaceous, an uplift and erosion occurred in this domain with removal of pre-Campanian sediments, and a progressive westward progradation of the shelf, which lead to an eustatic sea level fall, precisely during the Maastrichtian ( Nguene et al., 1992 ). 3

8. Datasets Distribution of wells and seismic lines in study area, C.A.R.: Central African Republic. Fig.4

9. M ethods And Theory : Wells analysis Gathering of lithochrostratigraphic data above and below he Top Albian surface issued from each well; 1 Identification of wells top formations; 2 Correlation of lithochronostratigraphic intervals between wells tops; 3 Construction of wells section; 4 Establishment of 3D model of this surface over the study area, using GDM software provided by the BRGM-France. 5

10. M ethods And Theory : Seismic analysis The methodology can be summarized as follows: Well-to-seismic tie Identification and description of seismic sequences (including sequence boundaries and main seismic Facies) Correlation of seismic analysis results with the geological context of the study area based on previous studies.

11. Results : Well Log Analysis The Top Albian (red line) is an Unconformity which delineates the Middle Cretaceous ( Aptian and Albian ) and Late Cretaceous (Cenomanian to Maastrichtian). The lithologies of Aptian-Albian times mainly consist of sandstones, shales and evaporitic units . The occurrence of dark shales indicates marine deposits ( Arthur and Sageman, 1994 ), suggesting a sea level rise perhaps related to subsidence caused by margin uplift or to seafloor spreading . A- General cross line section of the 23 wells : the red color represents inner land, orange is continental shelf, yellow to green indicates bathymetry up to 200 km . B- 2D chronostratigraphic wells section showing the distribution of Aptian to Recent deposits. 1- Pliocene to Recent 2- Miocene to Recent 3- Miocene 4- Eocene 5- Paleocene- to Eocene 6- Paleocene 7- Maastrichtian 8- Campanian to Maastrichtian 9- Santonian to Campanian 10- Coniacian to Campanian 11- Coniacian 12- Turonian to Campanian 13- Cenomanian to Lower Campanian 14- Albian 15- Aptian to Albian 16- Aptian 17-Upper Tertiary to Recent 18-Paleocene to Eocene 19-Upper Cretaceous 20-Middle Cretaceous. Fig.5

12. Results : Well to Seismic-tie Seismic Line L 9 -well W 23 calibration to distinguish top Albian surface on seismic section, Unc: Unconformity. Fig.6 The main surfaces identified : Top Albian, Danian and Mid- Miocene unconformities . 1 The Top Albian U/C seals Aptian -Albian deposits , which show seismic characters defined by variable internal and external seismic configurations. 2

13. Results : Seismic Analysis The seismic analysis reveals the existence of seismic boundaries: The first surface is associated with Erosional truncation terminations ; The seismic package beneath this surface shows also Downlap and Onlap terminations , suggesting the presence of sedimentary cover in deep domain . Erosional Truncation Onlap Downlap Seismic a tt ribu t es identified on seismic line Fig.7

14. Results : Seismic stratigraphy characterization Cenomanian to Recent High Geological signifiances of boundaries and seismic packages Fig.8 The geological correlation reveals that the Top Albian surface shows two characters: erosive and concordant. 1 This target surface delimits successive transgression and regression phases. 2 The Top Albian u/c follows a Downlap surface, suggesting a significant sea-level period before the Albian times on the South Cameroon Atlantic Margin 3

15. Results : Geochronological Significance Pre -rift and syn-rift deposits ( Late jurassic to Barremian) Sag sediments ( Early to Mid- Aptian ) Post-rift sediments (Mid- Aptian to Top Albian) Cenomanian to Recent Mid-Aptian to Albian Early to Mid-Aptian Late Jurassic to Barremian Basement Salt Top Albian Gargasian Early Aptian Geochronological signifiances of boundaries and seismic packages Fig.9

16. Results : 3D Modeling The spatial distribution of the Top Albian show that it is well developed and it could be affected by incised valleys, submarine channels and submarine fan lobes, on proximal part of the basin . Spatial distribution of the top Albian in South Sanaga area, using seismic and Wells data. A) NW-SE view; B) upset view. Fig.10

17. Conclusion The Top Albian u/c resulted of a sea-level drop, related to margin uplift. 3 The presence of terrestrial and marine sediments within the Aptian-Albian stage in the shallow waters, implies a marine incursion before the late Albian epoch. 2 The Top Albian surface shows two characters. Erosive in proximal domain and concordant in deep marine. 1

18. Acknowledgements / Thank You / Questions Authors are thankful to the National Hydrocarbon Corporation of Cameroon (NHC) for providing data, as well as Bureau de Recherches Géologiques et Minières (BRGM) for GDM Suite Software. We are also grateful to Professor Christopher Aiden-Lee Jackson from Manchester universi t y, Professor Franke Dieter from the BGR in Germany, Professor Igor EFFIMOF of Houston-Texas in United States of America and Doctor Yuan Huan from SINOPEC Petroleum Exploration and Production Research Institute in Beijing-China, for their strong and constructive remarks and reviewers who accept to revise this paper. This work is dedicated to Irène Victoire Tatiana Bobiyate Essi who left this world early in 2020.