Mineralogic Characterization and Petroleum Potential of Clays
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International Journal of Geosciences, 2012, 3, 696-709 http://dx.doi.org/10.4236/ijg.2012.34070 Published Online September 2012 (http://www.SciRP.org/journal/ijg) Mineralogic Characterization and Petroleum Potential of Clays (Shales) of the N’Kapa Formation (Paleocene-Eocene) in the Douala Sedimentary Sub-basin (South-West Cameroon) Paul Lambert Kenfack1*, Pierre Ricard Njike Ngaha1, Georges Emmanuel Ekodeck2, 2 Gabriel Ngueutchoua 1Faculty of Sciences, Department of Earth Sciences, University of Dschang, Dschang, Cameroon 2Faculty of Science, University of Yaounde I, Yaounde, Cameroon Email: *[email protected] Received May 20, 2012; revised June 23, 2012; accepted June 30, 2012 ABSTRACT The aim of this research work was to report a facies analysis of the N’Kapa formation, identified the clay minerals pre- sent in those facies and evaluate their oil potential. For that to be done, Lithostratigraphic descriptions were performed on three natural outcrops chosen in three different localities of the northern border of Douala sedimentary basin. Ten shaly samples were then collected on those outcrops and submitted to X ray diffraction and Rock-Eval pyrolysis. Lithologically, the N’Kapa formation is made up of dark to grey shales and fine to coarse sandtones. The mineralogic content of the shales is made up of Kaolinite, dickite, low quartz and vaterite. Those shaly facies present high amount of immature organic matter (average TOC content around 2%). The petroleum potential is fair to poor (average S2 for all the samples around 3.33 mg HC/g of rock) though some samples (M1 and M2) presenting a good petroleum potential up to 6.62 kg HC/t of rock and 6.44 kg HC/t of rock respectively. They have undergone a low degree of diagenesis (early to burying diagenesis). This is evidenced by the predominance of kaolinite and dickite, low quartz and vaterite which are minerals stable at low temperature. Keywords: Mineralogy; Organic Matter; Petroleum Potential; N’Kapa Formation; Douala Sub-Basin 1. Introduction the scientific interest, but also and predominantly the economic interest of the facies. Reference here is made The Cameroon Atlantic basin located in the Gulf of to oil and natural gas. Guinea is one of the West African coastal basins. It com- prises two major basins: The Douala-Kribi/Campo basin All those works for the most part were very globa- and the Rio Del Rey one. lizing, ignoring the specificities of each sector of the ba- However, these two sedimentary units are separated in sin. However, the structural dynamics that led to the es- onshore by the volcanic massif of Mount Cameroon form tablishment of the sedimentary basin of Douala Kribi/ only one sedimentary basin in their offshore extension Campo like other basins of the West African coast has from which the name coastal atlantic basin of Cameroon resulted in the sequence variability of deposits by sector is deduced. The Douala/Kribi-Campo (Figure 1) basin is considered. Moreover, many of the works are unpub- in its turn subdivided into two sub-basins: the Kribi- lished because of the known confidentiality characterize- Campo sub-basin in the south and the Douala sub-basin, ing the oil research domain. the northern component. Indeed, the establishment of the Douala sub-basin, Owing to the economic potential of this area, several linked to the gradual opening of the South Atlantic, studies have been carried out by scientist and petroleum resulted in a diachronism of deposits from South to North. industry, leading to the typification and the stratigraphy Another consequence of this was a temporal and spatial of the deposits. (Amonst: [1-19]. These works were variation of sedimentary environments along the West based on many important field campaigns motivated by African coast and consequently facies characteristics. This situation does not permit to easily extrapolate re- *Corresponding author. sults obtained on one sector to another. Reason why Copyright © 2012 SciRes. IJG P. L. KENFACK ET AL. 697 Figure 1. Location map of Cameroon (West Africa) and outlines of the Douala and Kribi/Campo sedimentary sub-basins. (Modified from SNH (1999)). Cameroonians sedimentary geologists and foreigners has carried out a research topic on the palynostratigraphy decided to conduct research program in each sector of the of the Cretaceous facies in the eastern border of the Cameroon Atlantic basin. It is in this context that Ntamak Douala sedimentary sub-basin. No specific work had yet et al since few years has being developing an important been carried on the Tertiary sedimentary facies of the scientific production in the Kribi-Campo sub-basin, [19] northern part of the Douala sedimentary sub-basin. Copyright © 2012 SciRes. IJG 698 P. L. KENFACK ET AL. Moreover, as mentioned above, there is no published data the onshore part towards the South up to Londji (North existing concerning petroleum potential of this sub- of Kribi). It is limited to the South by the Cameroon vol- basin. canic line, to the East by the late Proterozoic Pan-Afri- The purpose of this article is 1) to report a comprehen- can belt. The southern limit is more difficult to define. sive facies analysis of the Tertiary deposits cropping out The onshore part of the Douala sub-basin has a trapezoic in the North of the Douala sub-basin; 2) to identify the shape and covers a total surface area of about 6955 km2 minerals present in shaly facies; 3) to evaluate the oil while the offshore part covers an area of about 5850 km2. potential of the clay facies and 4) to discuss and interpret In a chronological aspect, the history of the Douala all these in terms of level of diagenesis and oil producti- sedimentary sub-basin presents three major stages: vity. 1) All begins with the Precambrian phase of cratonisa- tion, granitisation and sedimentation followed by the 2. Geological Setting pan-african orogenesis. The Douala sedimentary basin stretches on the South 2) After this Cambrian phase, the geology of the coast of Cameroon both onshore and offshore between Douala sub-basin is only known starting from the lower latitude 3˚03'N and 4˚06'N and longitude 9˚00' and 10˚00'E, Cretaceous during which epicontinental sedimentation covering a total surface area of 12,805 km2 (Figure 1). It begins to field an afro-brazilian depression, site of the has a crescent shape starting from the south-eastern bor- future atlantic basin of Cameroon. Sediments whose age der of Mount Cameroon where the onshore part is about varies from Cretaceous to Pliocene are discordant on the 70 km wide (In the N20E direction) and extend through- Precambrian pan-african basement (Figure 2). The sedi- out the atlantic coast with a gradual decrease in width of mentation was accompanied by Plutonism (Cenomanian) Douala basin Formation SW Lithology NE Basin phase wouri Matanda Souellaba Post-rift N'kapa Logbaba Volcanic rocks Sandstones and/or conglomerate Mundeck Silstones and/or mudstone Synrift not drilled Limestone and/or dolomite Halite‐rich evaporites Prerift sag Pre-rift Pan-African Belt cratonic platform Crystalline basement Figure 2. Tectono-lithostratigraphy of the Douala sub-basin (compiled from [20,42]). Copyright © 2012 SciRes. IJG P. L. KENFACK ET AL. 699 And volcanism (Miocene) which products cover sedi- was dominated by marginal to clastic sedimentation, with ments in some area (e.g. Volcanic products of Mount shallow water carbonate deposition occurring as sporadic Cameroon). build ups between the Albian and Paleocene. From Ap- The complete tectono-lithostratigraphic sequence based tian to Turonian, the juvenile South Atlantic ocean was on oil exploration seismic and drill data offshore com- filled with anoxic bottom waters presumably caused by prises six formations from the barremian-Aptian to pre- restriction of open ocean circulation and ponding of bot- sent deposited during the pre-rift, syn-rift, rift-drift and tom waters between Walvis Ridge and Rio Grande Rise post-rift basin phases (Figure 2). in the South, and equatorial fracture zone topography in 1) Pre-rift phase: Prior to the rifting of the Afro-Ame- the North [32-37]. This led to deposition of black shales rican plate, Jurassic continental sediments were deposited in the deeper part of the basin, and an intermittent car- in an afro-brasilian depression, an intracratonic sag ba- bonate deposition on the margins, above the oxygen sin that extended over the Gabon and Douala basins[20]. minimum zone [25,31,38]. Unrestricted ocean circulation 2) Rifting phase (Jurassic-Barremian): The strati- is thought to have commenced in the Turonian, when the graphic sequence here appears to be controlled by listric North and South atlantic gateways were joined. Accord- faulting and associated roll over anticlines [21]. ing to [29], the post-rift phase comprises three stages of 3) The syn-rift phase was responsible for the fracture drift: pattern closely controlled by inherited structures of the a) The first one (Albian-Coniacian) is characterized by Precambrian basement. This phase is characterized by an rotational fault blocks in the proximal part, salt move- intensive erosion activity of the highlands and deposition ment [21,39,40], and gravity sliding. These structures are in graben previously formed. In the Kribi/Campo sub- linked to gravitational instability of the margin and the basin, these units named Lower Mundeck by [22] are presence of salt. Sedimentologically, this phase is marked exposed on the sea bottom or are shallowly buried by in the Douala basin by a major regression leading to the younger sediments [23]. They are made up of conglom- decrease of the sea level. The consequence was the ero- erates, conglomeratic sandstones, organic matter-rich sion of the Lower Cretaceous sequence thereby creating dark marls interbedded with thin limestones and black to a surface of unconformity with an irregular topography.