Stratigraphic architecture and depositional environment of the Paleocene-Lower Eocene aquifers system (Douala onshore basin, SW Cameroon)

Abstract

The Paleocene-Lower Eocene N'kapa Formation has long been considered as an important groundwater and hydrocarbon resource in the eastern edge of the Douala Basin. The present study’s aim is to establish a possible link between the geological nature of this formation and the quality of potential aquifers using Gamma Ray well log, cuttings and outcrops through sedimentology and sequence stratigraphy studies. The results obtained from this study show that: lithofacies are dominated by clays, silts and sands/sandstones; gamma ray electrofacies are funnel-shaped to cylindrical-shaped with some bell-shaped; depositional environments are continental to shallow marine dominated by fluvial and sometimes tidal processes; fluvial channel-fill, tidal channel-fill and delta front bars sands are potential aquifers; sequence stratigraphy analysis reveals two transgressive–regressive cycles of second order (Danian to Selandian and Thanetian to Ypresian), two Highstand System Tracts, one Transgressive System Tract and one Lowstand System Tract; the high-resolution geometry of these sandy/sandstone sedimentary bodies shows that their distribution is linked to the hydrodynamic factors and the topography of the depositional environment. A total of two types of lithological facies of the reservoir levels were defined according to the volume of clay: clean sandstone (0–15%) and low clayey sandstone (15–30%). The gamma ray well log correlation shows that the thickness of the reservoir levels is variable and that the clay content (Vsh%) increases from the NE to the SW. As a result, the best potential aquifers appear to correspond to the fluvial channel-fill sands deposited during the base-level fall of Upper Paleocene age (Lower Thanetian).

High-resolution sequence stratigraphy from well-log patterns and base-level variations of the Paleocene - Early Eocene N’kapa Formation, Douala onshore Basin (SW Cameroon)

“…This shows that the Paleocene-Eocene series develop mainly during the relative sea level fall. This also explains the coexistence of continental and marine sediments during this period (Koah et al, 2021). Kenfack (2012) shows that the Early Paleocene is a regressive period characterized by a dominantly continental ora, and the Late Paleocene to the Early Eocene period corresponds to a transgressive episode with an upwelling of water in coastal areas that are very rich in continental deposits.…”

“…The rst group of authors (Bachirou et al, 2014 ;Mbesse et al, 2021) present the Paleocene as a transgressive period and the Early Eocene as a regressive period. Contrary, the second group (Mvondo, 2010 ; Kwetche et al, 2018 ;Koah et al, 2021) shows that the Paleocene-Early Eocene of the study area was globally a prograding period, disturbed by the maximum ooding of the Ypresian age. Due to the lack or poor quality of outcrops in the Douala Onshore Basin, the sequence stratigraphic analysis from well-log data was proposed in this study.…”

“…The TS2 surface appears as one of the basin-scale isochronous surfaces on the cross-section. Koah et al (2021) show that this surface is a transitional surface between continental sediments and coastal marine sediments. It is also a basic surface for progradating continental facies (Duvail et al, 2002).…”

“…The sandstone layers are thicker than those of clay. Channel and oodplain facies develop in this unit (Koah et al, 2021). It is characteristic of a uvial system.…”

“…The syn-rift sequence affects both the Precambrian basement and the Lower Mundeck Formation of the Berriasian-Barremian age; while the post-rift sequence is characterized by the Upper Mundeck Formation (Aptian-Cenomanian) Logbadjeck Formation (Cenomanian-Campanian), Logbaba Formation (Maestrichtian), N'kapa Formation (Paleocene-Early Eocene), Souellaba Formation (Late Oligocene-Early Miocene), Matanda (Late Miocene) and Wouri Formation (Pliocene-Pleistocene). The stratigraphic architecture shows two transgressive-regressive major cycles : Early Paleocene to Middle Paleocene and Late Paleocene to Early Eocene (Koah et al, 2021). This period is mainly in uenced by tectonic activity which dominated the sea's forward and backward movements (Robertson, 1992 ;Lawrence et al, 2002).…”