GEOLOGY, STRATIGRAPHY AND INORGANIC PETROLEUM GEOCHEMISTRY OF THE BORNU BASIN AT ITS BOUNDARY WITH UPPER BENUE TROUGH, NE NIGERIA

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Date
2021-12
Authors
Yakubu Bata, MOHAMMED
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The study area is situated in the southwest part of the Nigerian sector of the Bornu Basin, which falls between latitudes 10o 52I 30II to 11o 15I N and longitudes 11o 37II 30II to 12o E. A detailed geological mapping coupled with sample collection was carried out within the study area so to analyze the samples. Geological map was produced distinguishing different lithological units on the basis of their field relationships using various geological parameters. Petroleum geochemical analytical methods were employed using AAS and MREAS to determine source rocks maturation level of some selected shale samples for trace elements and transition metals that are usually incorporated into the petroleum charge system. A new proposed stratigraphy from the Chad (Bornu) Basin is here in presented to give a detail and comprehensive outlook of the study area, which in turn reflects a true image of the entire basin. For the first time, the basin is being classified into group and members in order to revise and correct the stratigraphy on the basis of anomalies observed by this work and previous researchers. The oldest unit in the basin Bima Formation is now regarded as a group in the Bornu Basin because of some new field evidences that were found worthy to describe it as Bima Group and subsequently subdivide the lithological units into formations, vis; Lower, Middle, and Upper. Yolde Formation, which is only recognized in the Gongola Basin, has been mapped and now considered to be overlying the Bima Group in the study area. No any report or detail field evidence has ever been presented on the basin to consider the Yolde Formation as part of its stratigraphy; this is the first time it is reported as a stratigraphic unit within the basin. The previously known Gongila and Fika Shale Formations in the basin are now regarded as members within the Pindiga Formation in the Gongola Basin, because field evidence have shown that the Gongila village lies on the boundary between Middle Bima, Yolde Formation and Kanawa Member of the Pindiga Formation. In the present study Fika Shale is considered as a member because of the revised stratigraphy of the Gongola Basin. The Gongila Formation was previously considered to consist of two lithological units, i) the lower shale/limestone unit which is now considered as the Kanawa Member and ii) the upper shale/sandstone unit which is now regarded as the Dumbulwa Member. In the present study both the Gongila Formation and Fika Shale are now regarded as part of the Pindiga Formation. Upper conglomeritic sandstone forming a marked formational boundary was observed within the Kerri- Kerri Formation in the study area. The Chad Formation outcrops in the study area and overlie the Cretaceous sediments extending into the basin blanket over most of the Cretaceous sediments. Detailed facies analysis revealed facies groups: 1) Gravel Facies, consisting of sub- facies units, i) clast- supported conglomerates (Gcs), ii) clast- supported conglomerates grading into sandstones (Ggs), 2) Sandstone Facies (S) which consists of i) massive sandstones, (Ss) ii) crossbedded sandstones (Sx), iii) normally- graded sandstones (Sng) and 3) Mudstone Facies (M) which consists of i) silt- clay streaked layers (Msm) and ii) massive mudstone (Mm), and lastly 4) Carbonate Facies (C) mail composed of carbonate- rich layers. Analysis of architectural sedimentary elements also showed five elements which are; channels (CH, HO), sediment gravity- flow deposits (SG), sandy bedforms (SB), downstream- accretion macroforms (DA) and flood plain fines (FF). These elements are defined by their geometries and bounding surfaces. The overall petroleum geochemical results showed that Fe and Mn (siderophile) elements have the highest concentration value of inorganic geochemistry, which generally does not corresponds with most global results obtained in relation to elemental analysis with regards to crude oils and vii source rocks. In general, source rocks have higher geochemical values than crude oil samples if they mature, but in this study the concentration of the elements are mediocre which may be as a result of shallow depth of burial and lack of sedimentary cover over the targeted source rocks. The biophile elements Ni and V which are principal actors used in assessing petroleum potentials have lower values because of exposure and subsequent obliteration of their main constituent. Therefore, it can be concluded that, both the siderophile (Fe, Co, Mo, Cr, and Mn), chalcophile (Cu, Zn, Pb, and Cd) and biophile (V and Ni) elements have relatively low geochemical values for petroleum generation. Representative sandstone samples from different formations such as the Bima Group, Yolde Formation and Dumbulwa Member of the Pindiga Formation in the study area were classified on the basis of their mineral composition. Samples 4, 5, 7, 24, 27, 28, 34, 35, 36, 38, 40 and 41a are classified as arkosic arenites due to the high percentage of feldspar and quartz than rock fragments in majority of the samples, whereas samples 12, 21, 25 are classed as lithic arenites because rock fragments have higher mineral aggregate than feldspar. The compositional maturity of the sandstones was identified, the matured composed of arkosic arenites and the immature ones are related to lithic arenites. It can therefore be concluded that the Bornu is same as the Upper Benue Trough (Gongola Basin) based on the geological map produced, field relationship and geological setting. The Bornu Basin is no different from the adjacent basin except the sedimentation of the Quaternary Chad Formation
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A THESIS SUBMITTED TO THE SCHOOL OF POSTGRADUATE STUDIES AHMADU BELLO UNIVERSITY, ZARIA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD OF THE DOCTOR OF PHILOSOPHY, GEOLOGY (PETROLEUM GEOLOGY) DEPARTMENT OF GEOLOGY, FACULTY OF PHYSICAL SCIENCES, AHMADU BELLO UNIVERSITY, ZARIA, NIGERIA
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