THE GEOCHEMISTRY, PALYNOLOGY AND PALYNOFACIES CHARACTERISTICS OF MAIGANGA COAL DEPOSIT UPPER BENUE THROUGH NIGERIA

ABSTRACT

Geochemical, palynological and palynofacies studies had been carried out on samples from 24 boreholes that penetrated Gombe Formation around Maiganga coal mine. The aim of the study was to establish their microfloral assemblages, age, paleoclimate, paleoenvironment and biozonation as well as evaluating the petroleum generative potentials of the Formation. The standard maceration and Rock Eval 6 methods were employed for the palynological/palynofacies and organic geochemical analyses respectively. The studied boreholes are considerably rich in palynomorphs which permitted  some  adequate  deductions,  such  as  the  age,  paleoclimate  and paleoenvironment of the formation. Majority of the pollen and spores observed from the study area included Proteacidites sigalii, Retidiporites magdalensis, Monoporites annulatus, Cingulastisporites ornatus, Rugulatisporites caperatus, Scabratriporites annellus, Proteacidites longispinosis, Distaverrusporites simplex and Foveotrilete margaritae. The recovered marker species have been used to date the studied sections Maastrichtian age. Palynomacerals types I, II and III were found in the studied sections with little type – IV and Armophous Organic Matter (AOM). The analyses of these palynofacies  were  used  to  interpret  the  paleoclimate  and  paleoenvironment  of  the studied   sections   as   well   as   zoning  the  sections   into   two   palyzones,   namely, Proteacidites sigalii – Echitriporites trianguliformis zone and Cyathidites spp- Laevigatosporites haardtii zone. The analyzed palynoforms showed that the paleoenvironment of the study sections were swamps and flood plains while the coal seam intercalation was an indication of swampy environment under anoxic condition. The paleoclimatic conditions were inferred to belong to the Late Cretaceous Palmae province based on the analyzed palynomorphs. The geochemical analyses showed that the analyzed samples were organic rich and contained kerogen types (II and III). They were  however  considered  thermally immature  based  on  the  analyzed  Tmax  values which were considered low. The results of the Rock-Eval analyses showed that the samples  in  boreholes  BA-7  and  BA-16  contain  Type  II kerogen  while  those  from borehole BA-17 contain Type III kerogen. This study also revealed that the analyzed samples especially those from boreholes BA-7 and BA-16 may constitute a good source rocks capable of generating petroleum and gas. The Rock-Eval Tmax data available for thermal maturity assessment of the samples suggest that the analyzed samples from the three boreholes are thermally immature. This assessment was consistent with the immaturity status of their coeval formations in other part of the Benue Trough. These contemporaneous formations may be related in depth and/or have experienced similar geothermal gradient with those of the Gombe Formation.

CHAPTER ONE

1.0      INTRODUCTION

1.1       Background of the Study

Palynological and Geochemical studies (Rock Eval and TOC determinations) have become valuable tools and universally practiced methods of evaluating the stratigraphy and source rocks potentiality of sedimentary basins. While palynology deals with the study of plant remains in the sedimentary successions and their applications in biostratigraphy, Rock Eval and TOC studies deal with organic geochemical evaluation of sediments for the purpose of evaluating their source rocks potentials.

A lot of contribution to the understanding of depositional environments and stratigraphy of  the  Gombe  Formation  have  been  made  by  several  previous  workers  (Ojo  and Akande, 2004). The previous studies indicated a wide range of Late Senonian to Maastrichtian age for the Gombe Formation. In another study, Lawal and Moulade (1987) assigned an Upper Maastrichtian age to the Gombe Formation.

Several geochemical studies using Rock Eval and TOC determination methods have been carried out on samples from the Gombe Formation with the principal aim to evaluating the source rocks potentiality of the formation (Idowu and Ekweozor, 1989; Obaje and Abaa, 1996; Obaje, Funtua, Ligouis and Abaa, 1998; Obaje, 2000; Obaje and Hamza, 2000; Ojo and Akande, 2002, Obaje, Attah, Opeloye and Moumouni, 2006; Tukur, Samaila, Shettima and Jauro, 2006).

Despite the work by earlier workers, there exist some stratigraphic and geochemical evaluations gaps that require further studies to be filled up. These gaps include the non inclusion of the coal facies in the stratigraphy of Gombe Formation and the analysis of these coal facies and the subsurface Formation samples to ascertain their geochemical characteristics. These gaps may be attributable to lack of subsurface samples and data which were not available to the earlier authors as occasioned by lack of available boreholes that penetrated Gombe

Formation as at the time of their studies. The search for coal and its subsequent mining in the Maiganga area has however paved way for access to subsurface samples and data from the Gombe Formation. Hence, this study is undertaken to fill the missing stratigraphic and geochemical gaps.

It is therefore justified for the continous study on the Gombe Formation to improve on the stratigraphic and source rocks prospectivity of the Formation. Gombe Formation belongs to the Cretaceous period and forms part of the Nigerian inland basins.

Apart from the Cenozoic Niger Delta which have been studied in detail because of its presumed potentiality for source rocks, other Nigerian inland basins have not been studied intensively which could also be prospective for mineral resources. These inland basins should also be explored since their coeval basins of the world have proved to be viable geologic province for mineral resources.

Among  the list of composite methods for stratigraphic and source rock evaluation of an area,  Rock  Evaluation  method  using  Rock  Eval  and  Palynological  studies  remain leading and of priorities in Sedimentary basin studies (Obaje et al. 2006). While Rock- Eval provides information on the quality and quantity of the organic components of the source rock and its potentiality for oil and gas generation, palynology provides basis for dating the formation, their paleoclimatic conditions, paleoenvironments and possible correlation with known Formations.

1.2    The Study Area

The area chosen for this study is located within the Maiganga coal mine, Gombe state. Maiganga coal mine falls within the Gombe Formation in the Upper Benue Trough of Nigeria. The core samples used for the analyses were collected from the core store room at Ashaka Cement Factory, Ashaka. Twenty-four (24) exploratory boreholes with about 400 samples were collected for the studies. Out of these, three (3) boreholes (BA–7, BA–16 and BA–17) with seventy-three (73) samples were collected and used for the study. Borehole BA–7 penetrated to a depth of 48 m, BA–16 to a depth of 60 m and BA–17 to a depth of 45 m. The Maiganga coal mine is bounded by latitudes 009⁰N to 12⁰N and longitudes 10⁰E to 12⁰E (Figure 1.1). The Maiganga coal mine is accessible through the Gombe–Yola Road. The mine is located at 8 km, off the 22ND km Gombe – Yola Road  in  Akko  Local  Government  Area of Gombe State.  The villages  in  the vicinity of the mine include Kumo, Duba Fulani, Kalshingi and Pindiga.

1.2.1.   Climate, relief, drainage and hydrology

Gombe state which hosts the study area is part of Central Nigeria, but the flat landscape in the northern and southern parts of the state have isolated hills (Gombe State, 2013). While the elevation of the plain is at about 600 m above sea-level, the hills reach between 700 m and 800 m. This relief is typically displayed by the Killang Hills at Kaltungo. The Gongola River is the main drainage system in the area, running approximately north south towards the Benue River Basin, but with principal tributaries draining from west to east into river Gongola. Gombe state lies mostly within the poor groundwater provinces in the Gombe sandstones, the Pindiga shales, Kerri-Kerri Formation  of  Tertiary age  and  the isolated  hill  region  of the  northern  central  and southern parts (Zaborski, 1997). Groundwater occurs between 180 m  depth on the sandstones. In the Gongola River Basin, because of the Cretaceous sandstones and shales, groundwater is generally erratic and meager.

Indeed, dams from suitable surface drainage systems are more favoured than groundwater and boreholes as sources of water for domestic, agricultural and industrial uses (Gombe ADP, 2013). River Gongola is the only major river that flows through the state in the north and east through Dukku, Nafada and all the eastern LGAs, to join the River Benue at Numan. At Nafada, the River Gongola bends southwards and flows through the eastern boarder of the state before it joins River Benue at Numan. It is the sixth longest River in Nigeria, stretching about 530 km much of which lies in Gombe state (Gombe ADP, 2013). It has numerous tributaries and smaller streams including Rivers Dukul, Ruhu, Guji, Balanga and Dadin Kowa.

1.2.2    Soils, vegetation and land use

The   topography  of   Gombe   state   has   exerted   an   enormous   influence   on   soil development; hence over half of the state (central) that is underlain by the Kerri-Kerri Formation has shallow to moderate impoverished soils, with sandy loams on iron pan. In the Chad Formation in the northern part of the state, the soils are deep but sandy and clayey, but are mostly blanketed by sand dunes. Soils in the eastern parts of the state are shallow to deep loamy, sandy clay, loam and vertisols with cracking clays that have weathered from shales (Gombe ADP, 2013).

According to Reyment (1965), vegetation in Gombe state is predominantly wooded shrub land in the central part, with the plant community comprising Anogeissus/Combretum/ Afrormosia/ Detarium. The northern part of the state exhibits a mosaic of shrubbed grassland and grassed shrubland with the preponderance of Acacia. In the hilly southern areas, the vegetation is shrubbed woodland with mostly Afrormosia and Detarium.

Gombe state has the cultivated and urban areas constituting over 35 % of the land use/land cover. Agricultural land uses include planting of cereals, fruit crops and graze lands for pastoral farming.

1.3    Statement of the Problem

According to Obaje et al. (2006), current production of oil and gas in Nigeria comes entirely  from  the  Niger  Delta  onshore  and   offshore.   Some  palynological   and geochemical studies have been undertaken in other sedimentary basins of Northern Nigeria including Gombe sub-basin with the aim to have a better understanding of the stratigraphy  and  geochemistry  of  the  basins  which  could  boost  the  oil  and  gas knowledge and production base of the Country.

Also as earlier stated, Ojo et al. (2004) have posited that a lot of contributions to the understanding of the stratigraphy of Gombe Formation have been made. However, these inland  basins  (Upper  Benue  Trough  which  host  Gombe  Formation  inclusive)  have proved to have a missing gap in terms of palynological and geochemical data. This has left the detailed understanding of the stratigraphy and geochemistry of the inland basins unaccomplished.

Gombe Formation have been chosen for this research work for two reason;(i) Discovery and mining of coal in the area and (ii) access to core samples from the boreholes drilled in the area for coal exploration. The results obtained from this study will complement the  existing  knowledge  about  the  stratigraphy  and  geochemistry  of  the  Gombe Formation especially with the use of subsurface samples from the area as against the previous surface outcrops used for similar studies in the area.

1.4    Aim and Objectives of the Research

1.4.1    Aim

The primary aim of the research is to carry out the Palynological, Geochemical and Palynofacies analyses of Gombe Formation. This is with a view to elucidate more on the stratigraphy and source rocks prospectivity of the formation.

1.4.2    Objectives

The objectives of the study therefore include;

(i)    To determine the paleoclimate and paleoenvironments of the Gombe Formation based on the palynological and palynofacies analyses.

(ii) To evaluate the source rock prospectivity of the Maiganga coal and the shale strata.

(iii) To compare and correlate the Gombe Formation with other coeval formations such as Patti Formation, Bida Formation and Mamu Formation based on the analyzed palynomorphs.

(iv) To fill the missing stratigraphic gap in the Gombe Formation by the inclusion of the coal facies in the stratigraphy of the formation.

1.5    Justification for the Research

Previous workers have carried out stratigraphical studies of the Nigerian Benue Trough

(Reyment, 1965; Lawal, 1982; Adegoke et al., 1986; Popoff et al., 1986; Lawal et al.,

1987; Nwajide, 1989; Dike, 1995; Zaborski et al., 1997; Zaborski et al., 1998; Ojo et al., 2004; Ojo, 2009; Odedede et al., 2011; Olaburaimo et al., 2011; Adebanji, 2012). These previous authors have dated the Gombe Formation to range from late Senonian to Maastrichtian. This wide range may be attributed to the use of outcrop samples for analyses as basis for the dating rather than the use of subsurface samples.

It is also on record that many authors have carried out geochemical analyses on samples from the Gombe Formation and their results well documented (Idowu et al., 1989; Obaje et al.,2000; Akande et al., 2002; Ojo et al.,2002; Akande et al., 2005; Obaje et al., 2006 Moumouni et al., 2007;). Their works were carried out on outcrop samples. However, the advent of Maiganga Coal Mine have given rise to the drilling of some coal exploratory boreholes which gave rise to core samples which are curiated at the Ashaka Cement Factory. These core samples have afforded this study the relative advantage  of  analyzing  subsurface  samples  as  compared  to  the  outcrop  samples analysed by the previous workers, hence a perceived more accurate results.

The Gombe Formation in the Upper Benue Trough was chosen for this study for two reasons as earlier stated.The present study, is aimed as an input to the understanding of general stratigraphy and organic geochemical data of the Upper Benue Trough.

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