DETERMINATION OF THE SEDIMENTOLOGICAL AND GEOCHEMICAL CHARACTERISTICS OF THE SEDIMENT IN THE NIGER DELTA BASIN
The mineralogical and chemical composition of clastic sedimentary rocks are controlled by various factors, including (1) the composition of their source rocks, (2) environmental parameters influencing the weathering of source rocks (e.g., atmosphericchemistry, temperature, rainfall and topography), (3) duration of weathering (4)transportation mechanisms of clastic material from source region to depocenters, (5)depositional environment (e.g., marine versus fresh water), and (6) post-depositional processes (e.g., diagenesis, metamorphism) (Hayashi et al., 1997). Numerousinvestigations are substantiating the above aspects pertaining to genesis of both ancient andmodern siliciclastic sediments (e.g., Dickenson et al., 1983; Nesbitt and Young, 1982, 1984; Bhatia, 1983; Roser and Korsch, 1988; McCann, 1991; Condie et al., 1992; Condie, 1993; McLennan et al., 1993; Nesbitt et al., 1996; Cullers, 2000; Hessler and Lowe 2006; Nagarajan et al., 2007; Spalletti et al., 2008). Several studies have also been focused on the identification of palaeotectonic settings of provenances based on geochemical signatures of siliciclastic rocks (e.g., Dickinson and Suczek, 1979; Bhatia, 1983; Bhatia and Crook, 1986; Roser and Korsch 1986; McLennan and Taylor, 1991). Among the terrigenous sedimentary rocks, shales are considered to represent the average crustal composition of the provenance much better than any other siliclastic rocks (e.g., McCulloch and Wasserburg, 1978). Shales retain most of the mineral constituents of the source and their bulk chemistry preserves the near-original signature of the provenance and more faithfully reveal palaeoweathering conditions (e.g., Pettijohn, 1975; Graver and Scott, 1995). The present note examines the geochemistry of sediment from part of the subsurface Niger Delta Basin province, attempts to constrain there paleo redox and tectonic setting and provenance. Owing to limitations of analytical facilities, the present work is based on chemical analyses data of major and select trace elements of the investigated sediment of the study area.
1.2 AIM AND OBJECTIVE
To determine the sedimentological and geochemical characteristics of the sediment in the study area.
1. To deduces the characteristics of the source area from the composition and textural properties of sedimentary rock
2. To determine the tectonic setting of the basin
3. To determine the provenance of the sediments
1.3 LOCATION OF STUDY AREA
The study area (Fig.1) is located in the Niger Delta Basin
Fig.1: Location Map of the Study Area. Inset: Map of Africa and Nigeria showing the Niger Delta (Corredor et al. 2005).
1.4 REVIEW OF PREVIOUS WORK
The geochemical characteristics of clastic sedimentary rocks (which include sandstones) are useful in determining the tectonic, depositional setting and its associated provenance. The study of sedimentary provenance interfaces several of the mainstream geological disciplines and it includes the location and nature of sediment source areas, the pathways by which sediment is transferred from source to basin of deposition, and the factors that influence the composition of sedimentary rocks (e.g., relief, climate, tectonic setting). Information on transport history, paleo-environment of deposition and energy of transport medium can be deduced from mineralogical studies and the incorporation of this into data from inorganic geo
Provenance studies on clastic sedimentary rocks have been carried out using a number of analytical techniques such as geochemical modal analysis of arenites (Dickinson, 1970; Dickinson and Suczek, 1979; Ingersoll and Suczek, 1979; Zuffa, 1985, 1987; Johnsson, 1993). The composition of arenites is mainly controlled by source lithology, tectonics, climate, topography, weathering, transport and depositional environment, thus, the provenance is not easily interpreted from petrographic analysis alone (Johnsson, 1993; Critelli et al., 2007). A more integrated approach that includes heavy mineral analysis is used to provide detailed paleo-geographic reconstruction of source or basin systems. According to Pettijohn et al. (1987), the aim of provenance studies is to deduce the characteristics of the source areas from the compositional and textural properties of sedimentary rocks, supplemented by information from other lines of evidence. Heavy mineral analysis is an effective tool for reconstruction of sediment provenance (Mange and Otvos, 2005). Geochemical and heavy mineral analysis of sediments in the southern Nigeria sedimentary basins have been previously limited to the Benue – Abakaliki Trough and the Anambra Basin (Hoque, 1977; Hoque and Ezepue, 1977; Odigi, 1986; Amajor, 1987; Tijani et al., 2010). Heavy mineral assemblages in the Albian–Turonian sediments show major contributions from plutonic and metamorphic rocks of the Oban and Adamawa Massifs (Odigi, 1986), collectively called the Cameroon basement (Hoque, 1977). Amajor (1987) suggested a major sedimentary source and a minor contribution from the crystalline basement of Oban Massif for the Maastrichtian Ajali Sandstone of the Anambra Basin. This was recently supported by Tijani et al. (2010); they integrated X-ray fluorescence (XRF) with grain-size analysis and petrography. Hoque (1977) considered the sandstones of the Abakaliki Basin to be first sedimentary cycle deposits (from the Cameroon basement) based on the dominance of feldspathic sandstone. Sediments of the Anambra Basin are considered to be a product of a second sedimentary cycle with contributions from both the Abakaliki folded terrain (as a sedimentary source) and the granitic complex of Cameroon basement. Published petrographic and heavy mineral studies of the
Paleogene strata of the outcropping Niger Delta are limited to the Nanka Formation (Hoque, 1977; Nwajide, 1980). Nwajide (1980) noted the high proportion of ultrastable minerals, zircon, tourmaline and rutile and also the presence of kyanite, staurolite and sillimanite which suggest a medium to high grade metamorphic provenance.
The aim of this work is directed at carrying out comprehensive provenance and tectonic studies of samples collected from Niger Delta Basin, by means of inorganic geochemical studies with a view to determining paleo-tectonic settings of fine grained sediment.This study also acts as research tool for further research work.
SCOPE AND METHOD OF STUDY
This study is focused on determining the geochemical and sedmentolodical attributes of sediments of subsurface Niger delta Basin.
1. Laboratory studies involving geochemical analyses of acquired samples.
2. Integration geochemical data consisting of major, trace and rare earth elements of the shales and sand of subsurface Niger delta sediment to study and delineate various parameters including provenance, tectonic setting of the environment etc.
3. Laboratory studies involving sedimentological and petrographic analysis of the collected samples.
4. Interpretation of results.
2.0 REGIONAL GEOLOGY OF THE NIGER DELTA
Situated on the continental margin of the gulf of guinea in southern Nigeria(fig.2.), Niger delta Basin borders the Atlantic Ocean and extend from about latitude 3° and 6° N longitudes 5° and 8° E, covering a sub distance of about 75,000km and extending more than 300km from the apex to the mouth. The Niger Delta is ranked among the world’s most prolitic hydrocarbon province just as it is reputed to be the largest delta in Africa (Knox and Omatsola, 1989; Reijers det al.1997; Adegoke, 2012). The Niger Delta has been classified as a wave and tide dominated delta appearing to be constructive at the centre and destructive on either flask (Doust &Omatsola, 1990; Brigg, 2007; chinotu et al 2012)
2.1 Geological setting
The Niger Delta Province is delineated by the geology of southern Nigeria and southwestern Cameroon. The northern boundary is the Benin flank--an east-northeast trending hinge line south of the West Africa basement massif. The northeastern boundary is defined by outcrops of the Cretaceous on the Abakaliki High and further east-south-east by the Calabar flank a hinge line bordering the adjacent Precambrian. The offshore boundary of the province is defined by the Cameroon volcanic line to the east, the eastern boundary of the Dahomey basin (the eastern-most West African transform-fault passive margin) to the west, and the two kilometer sediment thickness contour or the 4000-meter bathymetric contour in areas where sediment thickness is greater than two kilometers to the south and southwest. The province covers 300,000 km2 and includes the geologic extent of the Tertiary Niger Delta (Akata-Agbada) Petroleum System. (Michele L. W, Tuttle, Ronald R.Charpentier, Michael E.Brownfield et al)
2.2 TECTONIC SETTING
Like most other sedimentary basins of southern Nigeria, the origin and development of Niger Delta is closely associated with the tectonic events of the Benue Trough. The Benue trough developed as a tectonic depression during the break-up of the Gondwana (fig 3) supercontinent during the Aptian-Albian times (burke, 1976; knox and Omatsola, 1989; Briggs, 2007; Oyanyan et al. 2012; Nwajide, 2013). The Benue Trough is widely considered a failed arm of a triple rift (RRR)with the two (2) other arms thought to have followed the south-western and south-eastern coast of Nigeria, developing into collapsed continental margins due to mantle convection resulting in triple rift junction developed at hot-spots with failed rifted arms leading to continental embayments (Burke,1967; Nwajide, 2013). Typically, deltas usually prograde down from such failed arms into oceans. The Niger Delta is thought to have formed thus just as other major deltas such as Mississippi, Rhine, St. Lawrence, casmaance and amazon deltas are all believed to have originated from similar processes (Nwajide, 2013).
Fig. 3: De Ruiter, 1977, Doust and Omatsola, 1990). Schematic of the seismic section from the Niger Delta continental slope/rise showing results of internal gravity tectonics on the sediments at the distal portion of the depobelts. The late cretaceous-early tertiary has a low velocity gradient, probably marine shale, whereas the late tertiary has a normal velocity gradient, suggesting a much sandy facies (Modified from Lehner and De Ruiter, 1977, Doust and Omatsola, 1990).
The Cenozoic Niger delta began developing during the Middle Econe (knox and Omatsola, 1989; Asseez, 1989). The pro delta is thought to have developed in the northern part during the Campanian transgression ending with the Paleocene transgression, with the formation of the modern delta commencing during the Eocene (Asseez, 1982). After the mid-Miocene, the Niger-Benue and cross delta systems merged with a marked reduction in the influence of the upper Cretaceous events. From the mid-Miocene onwards, the rate of development of the delta was mainly influenced by the rate of erosion of uplifted blocks in the hinterland and the eustatic changes in sea level (weber and Daukoru, 1975). Climatic variations, proximity and nature of sediment source areas affected the growth of the delta (Reijers et al, 1997; Reijers, 2011).
Meanwhile, as the delta developed, growth faults and roll-over anticlines were developed alongside. The development of the growth faults has been attributed to rapid sedimentation along the delta edge resulting in faulting contemporaneous with sedimentation, thus producing abrupt thickness of sediments across the fault line on the down thrown block. This kind of growth fault has also been reported in the gulf coast of America (Asseez, 1989). At first, submergence was slower (due to shallower basement) with thin mobile shale and less sediments supply then in later stages of the delta development. The greater reworking of sediments resulted in the rather sandy deposits typical of the ‘northern delta’ (knox and Omatsola, 1989). The tectonic structures in the Niger Delta Basin are very typical of an extensional rift system, but the added shale diapirism due to compression makes this basin different. The main method of deformation is due to gravitational collapse of the basin, although the older faulting and deformation in the basin are related to the continental breakup and rifting of the African plate and South American plates.The overall basin is divided into a few different zones due to its tectonic structure (fig: 4). There is an extensional zone, which lies on the continental shelf that is caused by the thickened crust. There is a transition zone, and then there is a contraction zone, which lies in the deep sea part of the basin (Tuttle et al., 2015)
Fig. : 4 Evolution of Niger Delta subbasins as mobile shale migrate towards the continental slope (Ajibola and Brian, 2006)
2.3 STRATIGRAPHY OF THE NIGER DELTA
TheTertiary deltaic fill of Niger Delta Basin (Fig.5.) is represented by astrong diachronous sequence (Eocene – Recent) which is usually divided into three lithofacies units (Short and Stauble 1967). They are Benin, Agbada and Akata Formations. (Burke, 1976)
Fig.3:Stratigraphy of the Niger Delta (Doust and Omatsola 1990).
2.3.1 BENIN FORMATION
The Benin Formation (fig: 6) extends from the west across the whole Niger area and southward beyond the coastline. It is over 90% sandstone with shale intercalations. It is coarse grained, gravelly, locally fine grained, poorly sorted, sub angular to well - round and bears lignite streaks and wood fragments. It is a continental deposit of probable upper deltaic depositional environment. Various structural units (points bar, channel fills, levees, back swamp deposits, oxbow fills) are identifiable within the formation, indicating the variability of the shallow water depositional medium. In the subsurface, it is Oligocene in age to the North, progressively younger southward. In general, it ranges from Miocene to Recent. The thickness is viable but generally exceeds 6000ft, very little hydrocarbon accumulation has been associated with the formation (Avbovbo, 1978).
2.3.2 AGBADA FORMATION
The underlying Agbada Formation is a sequence of sandstone and shales. It consists of an upper predominantly sandy unit with minor shale intercalations and a lower shale unit which is thicker than the upper sandy unit. (Kogbe, 1989). The formation is rich in micro fauna at the base decreasing upwards and thus indicating an increase rate of deposition in the delta front. A fluviatile origin is indicated by the coarseness of the grains and the poor sorting.
Agbada Formation occurs in the subsurface of the entire delta area and may continue into the Ogwashi-Asaba and Ameki Formations of Eocene-Oligocene age. (Kogbe, 1989). It is over 10000ft thick and ranges from Eocene in the North to Pliocene/Pleistocene in the South and Recent in the delta surface. Majorhydrocarbon accumulations are found in the interval between Eocene and Pliocene age.
Fig .4:Stratigraphic equivalence of facies in the subsurface Niger Delta (Wright et al.1985).
2.3.3 AKATA FORMATION
The lower most unit (Akata Formation) is a uniform shale development consisting of dark grey sandy, silty, shale with plant remains at the top. Thin sandstone lenses occur near the top particularly near the contact with the overlying Agbada Formation. The actual thickness of the Akata Formation is not known although Doust and Omatsola (1990) opined that the formation may reach a thickness of about 7000m in the central part, Whiteman (1982) estimates a thickness ranging from 2000 to 20000ft for the formation. A Palaeocene to Recent age has been assigned to the Akata Formation (Short and Stauble, 1967). The formation (Akata shale) crops out subsea on the delta slope and continental shelf and does not outcrop onshore (Whiteman, 1982; Doust and Omatsola, 1990).
The present geomorphology of Niger delta is strongly influenced by the prevailing south-western wind and the pattern of the generated longshore currents (Weber and Daukoru, 1975). The Niger-Benue system brings a sediment load of about 0.02km3/year, usually deposited on top of the delta with a sediment thickness of 12,000m (Asseez, 1989). Sedimentation in the Niger delta has been generally described as ‘’cyclic’’, similar to the cyclic sedimentation in the rhone delta (Weber and Daukoru, 1975; Asseez, 1989; Nwajide, 2013). The continued progradation of the delta southward since the Eocene has led ton formation of depobelts ( Onyekuru et al. 2012) as shown in table 1:
Table 2.1: Depobelts in Niger Delta Basin(after Doust and Omatsola, 1990)
Depobelts Age of parallic sequence Age of alluvial sands
Northern delta Late Eocene - Early Miocene Early Miocene sands
Greater Ughelli Oligocene – Early Miocene Early Miocene
Central swamp 1 Early – Middle Miocene Middle Eocene
Central swamp 11 Middle Miocene Middle Eocene
Coastal swamp Middle – Late Miocene Late Miocene
Offshore Late Miocene Late Miocene
Each of the depobelts is about 30 – 60 km wide and are referred to Escalator Regression (Knox and Omatsola, 1989; Adegoke, 2012).
2.5 STRUCTURES IN THE NIGER DELTA
The structure of the Niger Delta continues offshore into the continental shelf (fig.5.). This “Geosynclinals” structure is believed to contain sediments of varying lithology exceeding 40000ft near Warri. The depositional environments vary from near shore to non-marine, marginal marine and deep water deposits (Kogbe, 1989).
The most striking structural features of the Niger Delta are the large syn-sedimentary growth faults, rollover anticlines and shale diapirs which deformed the delta complex (Evamy et al. 1978). The greater percentage of the on fields in the Niger Delta is associated with rollover anticlines.
Rapid sand deposition along the delta edge on top of under-compacted clay has resulted in the development of a large number of syn-sedimentary gravitational faults called “Growth Faults”. Their origin and shape can be explained on the basis of the theory of soil plasticity. Growth faults tend to envelop local depocenters at their time of formation. Their trend is thus an indication of the prevailing sedimentological pattern. The enhanced sedimentation along the growth fault causes a rotational movement which tilts the beds towards the fault. In this way, anticlinal structures known as “Rollover anticlines” are formed along the faults..