The history of the Congolese coastal sedimentary basin is linked to the Cretaceous opening of the South Atlantic between Africa and South America. The work of Desthieux et al. [1] presents a geodynamic evolution divided into three phases: antesaliferous, saliferous and post-saliferous. This basin, whose economic interest is linked to oil exploitation, has been the subject of numerous studies in the various fields of geology (tectonics, structural geology, stratigraphy, sedimentology, and micropaleontology) which provided interesting details on the lithological succession and the age of the facies. The biostratigraphic and micropaleontological studies, Kouyoumontzakis; M’Boro; N’Gatse; Massala; Giresse et al.; Giresse & Baloka; Mbani [2] - [9] carried out are mostly of Cretaceous age. Available data on Tertiary biostratigraphy, including Miocene biostratigraphy, are rare and fragmentary. Thus, few biostratigraphic studies have been carried out and published on the Miocene of the Congolese coastal basin. A large part of the existing studies on Miocene result from the correlation of the work carried out in the sub-region (Angola, Gabon and Cameroon). Although the Miocene was established during the oceanization, it is currently an interesting prospect in the oil exploration of the Congolese offshore. From the Oligocene to the Miocene, sedimentological data have prompted the search for ancient fluvial channels. These paleo-channels contain sands that have proven to be high-quality traps for migrating hydrocarbons. With a view to further exploration, a biostratigraphic study was carried out on four boreholes in the North Deep Sea block of the Congolese Atlantic Basin. This study area is located offshore in the Congolese Atlantic baisn with water depths varying from 500 m to 3000 m. The area is characterized by a thinning of the antesaliferous series and by the presence of chenalising structures (Figure 1). The objectives of this study are:

· To carry out a chronostratigraphic calibration from the biozones;

· To characterize the deposit environments based on lithological analysis and on the determination of biozones. This study presents the invotory of biozones, the chronostratigraphic calibration of remarkable biostratigraphic events at the scale of the interval studied, and the palaeoenvironmental interpretation of sedimentary deposits.

Study material includes 92 samples of side cores, 120 drill cuttings and 5 electrical logs (Gamma Ray) from four wells called BM01, NM01, RM01 and SGM01, whose total depth varies between 1670 m and 3400 m.

The methodology consists in collecting the samples of loose sediment every 10 or 20 m for the foraminifera, and every 10 m for the nannofossils. Quantitatively, 30 grams of each sample are subjected to defloculation and sieving operations. Samples are carefully washed through a 63 µ sieve. The dried residue passes through a sieve set of 500 µ, 500 - 250 µ, 250 - 125 µ and less than 125 µ. Taxonomic analysis

and identification of planktonic foraminifers are based on the documents and illustrations depicted in the work of Blow [10] [11] with modifications by Bolli & Saunders [12], Loeblich and Tappan [13], followed by recent amendments by Hardenbol et al. [14], Berggren et al. [15] and Olsson et al. [16], and in comparison to the work of Mboro et al. [17]. The micropaleontological preparations of the lateral cores were selected to study their total micropaleontological contents, and the samples of the drill cuttings were prepared for quantitative micropaleontological and nannofossil analysis. At the end of the biostratigraphic analyses, the results were integrated into the diagraph and lithofacie data in order to allow a strategic interpretation of the diagraphs, on which the stratigraphic and paleo-environmental cutting off the North Deep Sea block is based.

1) LITHOSTRATIGRAPHY

The lithological description of the core samples and cuttings associated with the analysis of the Gamma Ray logs shows that the interval drilled concerns the Paloukou formation (clay-silty, sandy and limestone). The sections studied generally consist of three units in the direction of drilling:

· Unit 1: Upper Miocene (1816 m - 2556 m) It generally has an alternation of greenish grey to dark grey clay, silty, transparent to opaque sand, very fine to coarse, and light grey siltstone. This unit is very fossiliferous with a good diversity of fossils. Its thickness varies from 250 m to 740 m approximately.

· Unit 2: Medium Miocene (2556 m - 2926 m) This section is dominated by an irregular alternation of greenish grey clay, weakly indurated, brown grey siltstone, loosely consolidated more or less clayey, and translucent sand with fine to locally coarse, sub-rounded quartz grain. The medium Miocene is moderately fossiliferous and contains abundant glauconite. This interval is between 130 m and 370 m thick.

· Unit 3: Lower Miocene (2926 m - 3400 m) This interval is composed of grey to beige clay, silty to sandy silto-sandy, little to well indurated; followed by grey to whitish siltstone; of whitish-white sand, quartzitic, medium to coarse grained, sub-angular to rounded. This unit is low fossil and has a reduced diversity of fossil species. It is about 450 m thick.

2) BIOSTRATIGRAPHY

120 species of foraminifers, including 98 planktonic (81.67%) and 22 benthic (18.33%), have been identified and used for dating, biozonation and palaeoenvironmental interpretation. From the different species of foraminifers collected in the North Deep Sea block, the different stratigraphic cuts of the Miocene are defined on the basis of the Blow [10] [11] classification (Table 1).

Upper Miocene Limit—Lower Pliocene

In the Deep North Sea block, the upper Miocene—lower Pliocene boundary is marked by the first appearance of Globorotalia plesiotumida and the last appearance of Globorotalia margaritae located at 1816 m of depth. The foraminifers Globorotalia juania,Globorotalia conoide andGloborotaloides variabilis are also recorded slightly below this depth.

a) Upper Miocene (1816 m - 2556 m)

· Messinian (1816 m - 2150 m)

It is found in three wells. In the direction of drilling, the summit of the Messinian is encountered at 1816 meters of depth by the first appearance of Globorotalia tumida plesiotumida. The base of this interval is marked by the appearance of Globorotalia merotumida at 2150 m depth. The fossil harvest is abundant and makes it possible to distinguish in the direction of drilling, the presence of planktonic foraminifers Globorotalia tumida,Globorotalia panda,Globorotalia venezuelana,Neogloboquadrina acostaensis,Globigerina angustiumbilicata,Globigerina obesa,Orbulina suturalis,Praeorbulina glomerosa,Globigerinoides conglobatus,Orbulina universa,Neoglobigerina pachyderma, and Neogloboquarina acostaensis, all characteristics of this interval.

Benthic foraminifera Martinotiella communis, Sphaeroidinellopsis disjuncta,Heoglundina elegans,Pullenia bulloides,Cibicidoides pseudoungerianus,Bulimina alazanensis are also collected in this interval. In addition, we also note the subsequent appearance at the descent of the fossils Discoaster quiqueramus at 2060 m and Discoaster berggrenii at 2100 m of depth.

· Tortonian (2150 m - 2556 m)

Micropaleontological harvest is good. In the direction of drilling, the top is marked by Globorotalia merotumida at 2298 m depth and the base is at 2556 m depth with the first appearance of Globigerina nepenthes. This area is also characterized by the first appearance of Globorotalia mayeri,Globorotalia acostaensis