Electrical resistivity tomography of the Douala-Massoumbou Paleocene- Eocene aquifer (Cameroon Atlantic Margin)

The Paleocene aquifer of the Douala-Massoumbou sub basin is a rhythmic sequence of sand and shale deposits. Resistivity pseudosections and profiles of half-distance between current electrodes of 350m were acquired at 20 different sites of the Douala-Massoumbou subbasin. These data coupled with mud and gamma-ray logging of deep groundwater boreholes led to the identification of two major sand sequences through the Paleaocene-Eocene stratigraphic section. The upper sequence associated with Ypresian 30-50 m thick unconformity made up with semi-conductive records, is punctuated by lenses of substantially water bearing sand (WBS). While the lower sequence locally associated with H2S and iron pollution plumes, displays high conductive records. In such sequences, the distribution of sand and clay deposits settled by channel incision appears fundamental in predicting reservoir geometry and the hydrological potential of the Paleocene WBS.


INTRODUCTION
According to Regnoult (1986), the Paleocene to Recent sedimentary cover of the Douala-Massoumbou Subbasin appear as a thick series of shale deposits (Fig. 1) with intercalated layers of sand that denotes a turbidite sequence.Many studies of this sequence have been carried out in terms of their petroleum and hydrogeological potential (Dumort, 1968;Chiarelli, 1978).In relation to its hydrogeological resources, records of pressure and well-logging analysis (Chiarelli, 1978) have shown that the Paleocene of the Eastern Douala-Massoumbou sag basin is divided into two layers: upper clayey sand (CS) and lower sand over clay (SOC).
From Martin (1979), the thickness of the Paleocene SOC sequence in the RZ2 borehole is about 100 m, while its uppermost part is made up of a dark shale layer.To the West of the RZ2 borehole, the Paleocene SOC sequence gives way to a marine unit which is basically clayey.It appears that, the rework of basement faults (i.e.Logbesou and Bisombè flexure) has an influence on the sand distribution during the Paleocene SOC deposition (Regnoult, 1986).By the same token, the intercalations of colluvial deposits in the Cenozoic sequence as well as in the other series were recognized (Martin, 1979).However, given the results of water chemical analysis (BRGM, 1981), it has been suggested that the Paleocene SOC sequence might represent the most interesting hydrogeological prospect of the Douala sag basin.Thereafter, the groundwater supply project of the Douala neighborhood, including 9 deep monitoring and 12 pumping wells (greater than 30 cm in diameter) have been carried out along the Dibamba-Masoumbou border (Fig. 1).In as much as this project produced flow rate lower than predicted, further investigations by BRGM (1983) andFrey (1985) have also revealed lateral and rapid variations of the Paleocene SOC lithofacies.Foregoing, the purpose of this paper is to help the reader to better understand the depositional setting and sediments distribution within the Paleocene SOC layer or aquifer.We interpret and discuss the Paleocene SOC electrical resistivity in the light of post well-logging and provide keys elements for the Douala-Massoumbou groundwater assessment.
The study area covers the eastern part of the Douala sag basin (Fig. 1).Morphologically, the topography is undulating and comprises submeridian ridges attaining 100 m.From a geodynamic point of view, the Douala sag basin is a faulted structure with a N045 direction.Its origin and evolution are related to basement dilatancy in response to transcurrent faulting between the Beti-Fang and Brazilian-West-African plates (Reyre, 1984;Nely & Vaillant, 1993;Mbida, 2012).
Correlation between boreholes data have helped to reconstruct the regional hydrogeological crosssection (Fig. 2) of the Douala sag basin.Following groundwater studies (Martin, 1979;BRGM, 1981;BRGM, 1983;Frey, 1985 andMbida, 2004), it has been suggested that the Paleocene aquifer represent the most interesting prospect of the Douala sag basin.To improve knowledge concerning the hydraulic potential of this prospect, resistivity pseudosections and profiles have been acquired within and along the Paleocene-Holocene deposits.Martin, 1979).The regional geology (color area) is modified from Dumort (1968).Tectonic features are shown according to Mbida (2012).Red square: study area containing the location of regonal hydrogeology cross-section of figure 2

MATERIALS AND METHODS
Preliminary project of field data acquisition involved site recognition and survey design setting.Following this program, geophysical lines including vertical sounding (VES) and multidimensional pseudosections (MPS) were acquired along 7 localities.To increase survey data accuracy, VES to welllog calibration was first performed.Accordingly, field measurements were carried out using Schlumberger electrode configuration (Halvorson & Rhoades, 1976;Barker, 1989;Pozdnyakov et al. 1996;Banton et al., 1997), with a maximum current spacing of 750 m.For MPS imaging survey (Marescot et al., 2003) data were taken with three cable sets of potential difference along a trace of line.Following field investigations, processed pseudosections and surface contours were generated using the least-squares inversion program (Loke, 1995;Loke and Baker, 1996) and geological modeling.

RESULTS
Detail analysis of the resistivity pseudosections led to the identification of tree layers labeled: L1, L2 and L3 (Fig. 3) through the Paleocene formation.L3 appears as a thick resistant sequence with significant low conductive anomalies.Underlain by a top-discordant relation, L2 displays linear shaped patterns with local variation of thickness and a resistivity range from 250 to 610 &!.m.L1 seems to be a thick and folded conductive sequence with resistivity records lower than 150 &!.m.Together, results from post wells logging (Fig. 4) show that L2 (Paleocene SOC) and L3 (Paleocene CS) electrical patterns corresponded to clastic sequences (Pettinggill & Weimer, 2002;Carvajal et al., 2009;Covault et al., 2011) with prominent water-bearing sand.Because of its electrical and large-scale folded pattern, we interpreted L1 as the over pressured shales of the Nkapa Formation (Reyre 1981; Regnoult, 1986).As shown in figure 4, the Upper Paleocene aquifer (CS) is composed of interbedding red shaly sand and shale layers, while the lower Paleocene aquifer (SOC) exhibits grey to white sand bank over clayey units.Given these colorations, it appears that the sedimentary regime during the Paleocene CS accumulation was dominated by stepped sea level regression, while the Paleocene SOC deposition was controlled by a basin wide transgression episode (Reyre 1981;Regnoult, 1986).Following this assumption, relationships can be viewed with the presence of H 2 S and iron pollution plumes occurrence within the UPA (BRGM, 1981;BRGM, 1983;Frey, 1985).

DISCUSSION AND CONCLUSION
In keeping with water chemical analysis (Frey, 1985;Labogenie, 2013), the present study shows that the Paleocene SOC represent the major water-bearing unit of the Douala-Massoumbou sag basin.However, results from geological modeling (Fig. 5) indicate that its depositional system is associated with channel incision and by time-space migration of stream bed (Salvador et al., 2005;EL Ghachi, 2007).This result provides a relatively complete and readily dated record of depositional setting, spatial distribution and structure of the Paleocene aquifer, compared to previous studies (Martin, 1979;BRGM, 1981;BRGM, 1983;Frey, 1985 andMbida, 2004).
Apart from hydrogeology, the present study indicates that CO 2 and methane gas records within the LPA might be link to deep hydrocarbon prospects.Despite the low penetration of processed pseudosecctions, the results in this paper (study) show highlighted significant disparities within the Paleocene aquifer.Improved understanding of this variability could significantly affect the economic development or exploitation of LPA hydrogeological resources, and urge an assessment of site conditions before boreholes setting.