High-resolution Biostratigraphic Studies of Step-1 Well, Offshore, Western Niger Delta

The lithologic description was carried out by examining the samples under a hand lens in order to identify the constituents, describe and name the lithology. This was supported by the GR log to complement the lithologic description based on its deflection away from the Shale Baseline (SBL).

The ditch cuttings were prepared for foraminifera, calcareous nannofossils and palynomorph contents using standard preparatory methods. Fifty samples were selected at 12 m intervals for foraminifera preparation; the standard weight (20 g) of each sample was soaked overnight to ensure proper disaggregation and liberation of microfossils present within the cuttings. The disaggregated samples were then washed under a shower of water over a 63 μm sieve, rinsed in liquid detergent to remove drilling mud and dried over a hot plate. The forms were picked with a picking needle under an Olympus binocular microscope. Preparation and identification of individual foraminifera were guided following the works of [11, 12, 13], among others.

Fifty samples were selected at 12 m interval for calcareous nannofossils; about 2 g of each of the samples were scraped onto a glass microscope slide. The slides were labelled sequentially and two blobs of Norland Optical Adhesive were affixed onto the cover-slip. The slides were dried and later studied under a transmitting light microscope in polarized light. This was done following Martini [14].

Twenty-five samples were selected and composited at 24 m intervals for palynological analysis. A constant weight (30 g) of each sample was initially given a 5% dilute hydrochloric acid treatment to remove carbonates prior to complete digestion in concentrated hydrofluoric acid (HF) under a fume cupboard. The samples were decanted thrice at an interval of 1 h each through the Branson Sonifier and with the aid of a 5 μm sieve to filter away the inorganic matter. A LOCTITE mounting medium was used for the residues, which are the palynomorphs. Identification and preparation of the specimen was done following Germeraad et al. [15].

The lithostratigraphic section of the studied well was produced from ditch sample descriptions and the deflection away from the shale base line on the gamma ray log. The total thickness of the analysed sample is 609.6 m (i.e. between 1,828.8 m and 2,438.4 m intervals). From the lithologic description, the samples are mainly shale with a little sand. The shale is fissile, greyish to black, while the sand is fine to medium grained. The observation revealed that the studied well is largely made up of a sequence of fine-grained shale alternating with fine- to medium-grained sandstone in the lower part while the upper part is mainly shale (Figure 3).

Figure 3

The well recorded fairly rich and diverse assemblages of planktonic and benthonic foraminifera at the upper part (1,902–2,109 m), with 55 species recorded. Of these, 22 (40%) species are calcareous, while 27 (49%) are arenaceous. Two foraminiferal “zones” were recognized in the studied section. The Cenozoic chronostratigraphic scheme of Berggreni et al. [13] and the Global Sequence Cycle Chart of Hardenbol et al. [16] were adopted for this study. The zones are characterized briefly below.

Figure 4

The results of calcareous nannofossil analysis show high abundance and diversity of species (Figure 5). Biozonation and age determination of the well was based largely on calcareous nannofossils assemblages, abundance and diversity. The chronostratigraphic scheme adopted follows the usage of the worldwide zonation schemes of Okada and Bukry [17] and Haq et al. [18]. Considerable effort was made to identify and define zonal tops with the FDOs of diagnostic marker species, abundance and species diversity peak as these form the most reliable events [14]. The highest nannofossil peaks were dated using important marker species such as Sphenolithus abies, Discoaster pentaradiatus, Ceratolithus cristatus, Ceratolithus armatus, Discoaster quinqueramus and Discoaster berggrenni (Table 2). The stratigraphic distribution of the recorded species along with the significant datum, MFSs, nannofossil zones and age interpretations are presented in distribution charts (Figure 5). The highlights of the results are summarized below.

Figure 5

Twenty-five palynomorphs were selected at 24 m intervals. The palynomorphs are well preserved and fairly diverse (Figure 6). These include Zonocostites ramonae, Monoporites annulatus, Sapotaceae, Retitricolporites irregularis, Pachydemites diederixi, Acrostichum aureum, Pteris spp, Laevigatosporites spp, Verucatosporites spp, Stereisporites spp, Cyperaceaepollis spp, Sapotaceae and Psilatricolporites crassus. Dinoflagellatecysts recorded are Selenopemphix spp, Polysphaeridium zoharyi and Leiosphaeridia spp.

Figure 6

The STEP-1 sediments are assigned to the section within the Pantropical Echitricolporites spinosus zone of Germeraad et al. [15] and the P800 zone of Evamy et al. [1]. The zone was further subdivided into P840 and P830 sub-zones. Details of the zone and sub-zone identified are represented in Tables 3 and 4, and explained briefly below.

The ages for this well were established using three bioevents (Table 5): these include recorded peak faunal abundance (MFS), sequence boundary (SB) and the occurrence of some index fossils such as Cyclammina cf. minima and Discoaster beggrenii. The chronostratigraphic position of STEP-1 well is given in Table 6.

Palynofossils are preserved mainly in continental basins. The characteristics of great quantity, wide distribution and different preserved lithofacies are the unique advantage of these fossils [19]. For this reason, palynomorphs are now very important for reconstructing palaeoclimatic conditions at the time of sediments deposition. The climate of an area is reflected by its vegetation type [20]. The three important palynomorphs used for this study are Monoporites annulatus, Zonocostites ramonae and Acrostichum aureum. Monoporites annulatus abundance is often used as an indicator of a large degree of landscape openness [21, 22], and increased aridity [23, 24]. Zonocostites ramonae has been used as an indicator for mangrove vegetation and hence, wet climatic condition [25]. Acrostichum aureum is used as an indicator of wet climatic condition [26].

From Figure 7, the dominance in abundance of Monoporites annulatus over the relative abundance of Acrostichum aureum favours a dry climatic condition for the interval 1,853–2,048 m. While the sediments are believed to have been deposited in the Savanna setting, interval 2,073–2,146 shows alternating wet and dry climate.

Figure 7

From Figure 8, the dominance in abundance of Acrostichum aureum over Monoporites annulatus from interval 2,170–2,438 m indicates a wet climatic condition; hence, they are of mangrove species, although interval 2,243–2,265 m relatively shows alternating wet and dry climatic conditions. Figure 9 shows photomicrographs of foraminifera, calcareous nannofossil and Palynomorps recovered from the STEP-1 well.

Figure 8

Figure 9