Ditch cutting samples from Kasade-1 well were processed for microfauna, nannofossil and palynomorphs to throw more light on biostratigraphic events in the northern portion of the Bornu Basin, Nigeria. The lower section of the well within interval 990 - 1565 m yielded fairly abundant foraminifera species, while the upper section was barren. Palynomorph distribution follows the same pattern as observed for foraminifera except that few fresh-water algal species, Botryococcus braunii , were recovered from the upper section. The samples are devoid of nannofossil but contain few ostracod species, echinoid remains and fish tooth. Recognition of two planktonic foraminifera zones, Globotruncanella havanensis and Globotruncana aegyptiaca , enabled the delineation of the Campano-Maastrichtian and Maastrichtian boundary. The Cretaceous Tertiary boundary was inferred based on the top occurrence of palynomorph Dinogymnium euclaense. An abrupt truncation in the distribution of foraminifera species at a depth of about 990 m suggests unconformity, a widely reported event that followed the Santonian squeeze in the entire mega-Chad Basin.
The renewed search for hydrocarbon in the Bornu Basin, Nigeria (
significance. The oldest dated sequence in the basin, termed pre-Bima, is Albian [
Significant outcrop studies both within and outside the basin (because of its rare occurrence in the basin) have been carried out to make biostratigraphic deductions in the basin. [
This study attempts to integrate the study of both microfauna and microflora for a better resolution of biostratigraphic events in the Bornu Basin. It is hoped that the study would serve as a data base for establishing a globally acceptable biozonation scheme for the basin.
Kasade-1 well is one of the relatively shallow wells drilled to a total depth (TD) of 1510 m in search of petroleum in the Bornu Basin. The Bornu Basin constitutes the Nigerian portion of the Chad Basin that is made up of some oil producing basins like the Termits, Doseo, Doba and Bongor. The Chad Basin is an inland rift basin which is part of the West and Central African Rift System (WCARS) first clearly described by [
Throughout the Cretaceous, rifting and sedimentation occurred in a variety of tectonic settings within the West and Central Africa forming fault bounded sub-basins, mostly of a “pull apart” type; thus the lithostratigraphy of the sub-basins differs from one area to another. The sedimentary successions in the sub-basins were deposited under varying sedimentary environments-continental to marine. In the area of Lake Chad and parts of Sudan, sedimentary sections are over 10,000 m thick. In other areas, values between 4000 m and 7000 m are common [
Two hundred and eighty six ditch cutting samples retrieved within interval of 60 - 1565 m of Kasade-1 were used for this study. The ditch cutting samples were portioned into four for sedimentolodic, palynologic and micropa-
laeontologic studies. The remaining portion was kept as reserve.
The operations employed in the sedimentological studies of the cuttings include a gentle wash of each of the samples with running water, as well as physical observation, microscopic examination and chemical tests of the samples using dilute hydrochloric acid. No fluoroscopic examination was carried out.
The two hundred and eighty six ditch cutting samples retrieved within interval of 60 - 1565 m of Kasade-1 well were composited to forty three and processed for foraminifera and accessory microfauna. Twenty grams of each of the dried samples were soaked in kerosene for 20 minutes. Samples were decanted of kerosene and then soaked in soap solution overnight to allow for sufficient disintegration of the samples. Soaked samples were subsequently washed through a 63 μm sieve under jet of water until samples were freed of mud. The resultant residues were dried in an oven at temperature below 100˚C. Thereafter, foraminifera species were recovered into slides (cellules) and identified under a stereoscopic reflected light binocular microscope. Scanning Electron Photomicrograph of identified specimens was taken.
Shale samples were carefully selected for this study. Two grams of crushed sample were dispersed in water in test tubes. The resultant suspension from each sample was then pipetted into 22 × 40 mm glass slide cover slips. The pipetted suspension on cover slips was dried at about 60˚C - 70˚C. Dried suspension on cover slips was then mounted on glass slides using Norland Adhesive. The slides were cured under ultra violet light and examined for nannofossils under polarized light, stereoscopic microscope.
The samples for palynological analysis of the Kasade-1 well were selected and composited at 25 m intervals for their palynomorph content. Spores and pollens were recovered by treating 20 g of each sample with hot hydrochloric acid to remove carbonates. Samples were then digested in hydrofluoric acid inside a fume cupboard before they were treated with hydrochloric acid (HCl) and wet-sieved through a 5 μm mesh polypropylene sieve. The Branson Sonifier 250 was employed during sieving to facilitate complete removal of silt and clay particles. Sieved residues were oxidized using concentrated nitric acid (HNO3). Oxidized residues were mounted on glass slides using Norland Adhesive and then cured in ultraviolet (UV) light. Slides were examined for palynomorphs under polarized light, stereoscopic microscope.
The sequences penetrated within this well consists of shales, occasionally with subordinate carbonates; shaly sand; sandy shale; and coarse grained, poorly sorted sand that are occasionally ferruginized. Absence of core samples hampered proper lithofacies definition for the well. The upper section of the well (down to a depth of 60 m) where samples were not available are considered to be the prevalent Recent alluvium deposits that covers most of the surface area of the basin.
From the forty-three (43) composited ditch cutting samples taken within the interval of 60 - 1565 m of Kasade- 1 well and processed for foraminifera and accessory microfauna, only fifteen (15) foraminifera species were recovered. Five (5) of these were planktonic species while ten (10) were agglutinating species. Few ostracod species, echinoid remains and fish tooth were also recovered. The stratigraphic distribution, age and paleobathymetric ranges of the species are presented in a Stratabug spreadsheet (
Two planktonic foraminifera zones have been recognized based on the Cretaceous planktonic foraminiferal
zonation scheme of [
Stratigraphic Interval: 1245 - 1565 m
Planktonic Foraminiferal Zone: Globotruncanella havanensis and possibly older zone
Age: Campano-Maastrichtian (Upper Cretaceous)
Diagnostic Criteria:
The top of this interval is delineated by the First Appearance Datum (FAD) of Heterohelix navaroensis at 1245 m. The interval is characterized by the occurrence of Upper Cretaceous planktonic foraminifera species such as Heterohelix globulosa, Pseudotextularia elegans and species of Heterohelix. Agglutinating foraminifera species such as Ammotium nwalium, Haplophragmoides bauchensis, Ammobaculites benuensis and species of Ammobaculites were recorded within this interval. These species have been recorded in Cretaceous sediments of the Benue Trough and Chad Basin by [
Stratigraphic Interval: 1145 - 1245 m
Planktonic Foraminiferal Zone: Globotruncana aegyptiaca zone
Age: Maastrichtian (Upper Cretaceous)
Diagnostic Criteria:
Top of the interval is marked by the Last Appearance Datum (LAD) of Heterohelix globulosa at 1145 m. The interval is characterized by the occurrence of Upper Cretaceous planktonic species such as Heterohelix globulosa, H. navaroensis, Pseudotextularia elegans, Guembelitria cretacea and species of Heterohelix. The agglutinating foraminifera species Ammobaculites bauchensis, Ammobaculites benuensis, A. jessensis, A. coprolithiformis, Haplophragmoides bauchensis and species of Ammobaculites, Ammotium, Milliammina and Haplophragmoides were also recorded within this interval. The base of the interval is marked by the FAD of Heterohelix navaroensis at 1245 m.
Stratigraphic Interval: 990 - 1145 m
Planktonic Foraminiferal Zone: Non Diagnostic
Age: Maastrichtian (Upper Cretaceous)
Remark: The top of this interval which is placed at 990 m is suspected to be an erosional or unconformity surface owing to abrupt truncation in the distribution of foraminifera species. The interval is devoid of planktonic species. It is characterized by relatively abundant agglutinating species such as Ammotium nwalium, Haplophragmoides bauchensis, Ammobaculites bauchensis, A. benuensis, A. coprolithiformis, A. jessensis and species of Ammobaculites, Ammotium and Haplophragmoides.
These species have been recorded by [
Stratigraphic Interval: 60 - 990 m
Planktonic Foraminiferal Zone: Non Diagnostic
Age: Indeterminate
Remark: The interval is barren of foraminifera species. Hence, palynological evidence is relied upon for the dating of this interval.
A total of forty two (42) samples were processed for calcareous nannofossils study. Specifically shale samples were carefully selected at various interval of 10 - 50 m. The result of the analysis shows that the entire well section was barren of nannofossils. This absence of nannofossils in the well section is thought to be closely related to a non-marine to shallow water environment of deposition. This can be confirmed from the paleobathymetric data from foraminifera, as well as the distribution of dinocysts (from palynological analysis) in the well section.
The samples for palynological analysis of the Kasade-1 well were selected and composited at 25 m intervals for their palynomorph content. The palynofloral assemblages were moderately rich and fairly diverse especially within the interval 745 and 1565 m of the well (
The Kasade-1 well penetrated the P100-P200 zones of [
P200 Zone (Apectodimium homomorphum Assemblage-Danea mutabilis zone)
Interval: 60 - 815 m
Age: Early Paleocene
Diagnosis: The top of this zone is placed at the 60 m, the shallowest depth analysed, while the base is marked by the top occurrence of Dinogymnium euclaense at 815 m. This interval is very sparse in pollen and spores. Few specimens of freshwater Botryococcus braunii were recorded within interval 185 - 415 m.
P100 Zone (Dinogymnium euclaense zone)
Interval: 815 - 1200 m
Age: Maastrichtian
Diagnosis:
The top of the zone is defined by the top occurrence of Dinogymnium euclaense at 815 m, while the base is marked by the top occurrence of Monocolpopollenites sphaeroidites at 1200 m. This zone is further characterized by the presence of Dinogymnium euclaense, Longapertites sp., Ephedripites multicostata and Cyathidites australis.
P100 Zone (Odontochitina operculata Assemblage zone)
Interval: 1200 - 1565 m
Age: Campanian
Diagnosis: The top occurrence of Monocolpopollenites sphaeroidites defines the top of this zone at 1200 m, while the base is placed at 1565 m, the deepest sample analysed. The regular records of Monocolpopollenites sphaeroidites and the presence of Hexaporotricolpites sp. further characterized this zone.
The assemblage of this interval includes planktonic species such as Pseudotextularia elegans, Heterohelix globulosa, as well as agglutinating species such as Ammobaculites bauchensis, and A. benuensis. According to [
Interval 990 - 1145 m (
The non-recovery of planktonic foraminifera species and the dominance of agglutinating foraminifera represented mainly by the genus Ammobaculites points to a shallow to marginal marine environment of deposition within this interval.
Interval 60 - 990 m (
This interval is barren of foraminifera suggesting the cessation of marine influence at the location of Kasade-1 probably during the post Maastrichtian times or may be associated with the prevalent global climatic condition leading to the Cretaceous-Tertiary (K-T) boundary mass extinctions. The occurrence of the freshwater algae, Botryococcus braunii within interval 225 - 415 m however suggests freshwater environment.
The summary of biostratigraphic events is presented in
the well, marine evidences that gradually gave way to continental/freshwater influences in the Late Maastrichtian-Early Paleocene times were observed. Some regional level unconformity surfaces have been reported in all the WCARS basins based on seismic interpretation. In Kasade-1 a fossil distribution truncation was observed at a depth of 990 m suggesting an unconformity. The occurrence of this unconformity surface just before the Palaeocene suggests its link with the post-Santonian squeeze erosion that has been widely reported in the whole Chad Basin [
The preponderance of marine planktics at the total depth of the well suggests that the continental Bima Formation is yet to be penetrated by this well. In addition, the chance of occurrence of marine shale that could serve as source rock below this depth is high, except that this study has not tested the maturity level of the shales at the total depth of the well. The implication of these on hydrocarbon exploration in the basin is important and calls for organic geochemical study of the shales.
Based on the top occurrence of Dinogymnium euclaense at 815 m in the section of the studied Kasade-1 well, the Cretaceous/Tertiary boundary was inferred. Both the foraminifera and palynomorph contents enabled the recognition of the Upper Cretaceous sections of the well. The top occurrence of Monocolpopollenites sphaeroidites was used to define the Campanian. The sedimentary sequences in the interval 990 - 1565 m yielded fairly abundant foraminifera species, suggesting shallow to open marine depositional environment during the upper Cretaceous in the basin. The absence of nannofossil in the well ditch cutting samples is probably a reflection of how shallow the level of marine events was in the basin. Because the well was stopped within marine shale bed it is suggested that the well could still be drilled further in search of hydrocarbon, although it is desirable to check first the level of the shale maturity for hydrocarbon generation.
This research work was sponsored from the PTDF 2011/2013 Research Grant. The facility of Crystal Age Laboratory was used. These are acknowledged with thanks.
Peter S. Ola,John A. Adekoya,Solomon O. Olabode, (2016) Late Cretaceous/Early Paleocene Biostratigraphy of the Northern Portion of the Bornu Basin, Nigeria. Open Journal of Geology,06,561-570. doi: 10.4236/ojg.2016.67044