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This work will investigates the hydrocarbon potential and thermal maturity of the Eze-Aku Shale in selected areas of South Eastern Nigeria Lower Benue Trough, by sedimentological analysis and Rock -Eval VI pyrolysis. Total Organic Carbon of the Eze-Aku shales from previous studies show sediments contain (TOC) the above threshold value of 0.5 wt percent of rock, suitable for a potential source rock. This study integrates the use of vitrinite reflectance and maceral composition derived from the rock samples in conjunction with TOC/Rock-Eval pyrolysis generated set of data to further evaluate and identify the organic rich source interval to predict future prospects in the basin.
The investigated area forms part of the Lower Benue Trough of South Eastern Nigeria. South Eastern Nigeria lies roughly between 4Â°25 ÌN and 7Â°00Ì N of the equator; and Longitudes 6Â°30ÌE and 9Ì 30E of Greenwich meridian (fig. 1.1). It is situated mainly within the Tropical Rain Forest belt and occupies a landmass of about 50,000sqkm. The Lower Benue Trough represents a fracture of the Benue Trough. The Benue Trough is a unique rift feature on the African continent that trends north easterly to attain an approximate length of 800km,with a width ranging from 130-150 km and is crammed with Cretaceous rocks ranging in age from Middle Albian to Maastrichtian (Ofoegbu, 1984).
In an inaugural lecture at my past college by Akande (2003) titled "Mineral and Fossil fuel Discovery: The Adventure of Exploration", the Anambra basin that forms part of the Lower Benue Trough was discovered to produce oil and gas due to the presence of the necessary source rock, reservoir and trapping conditions. The estimated oil potential of the area was set at ca. 1billion barrels of oil with an additional accumulation of about 30 trillion cubic feet of gas have been reported by (Dublin, Green and Agha, 1999 cited in Akande 2003:35). In spite of these findings and estimation, several unknown areas still remain in respect to the main hydrocarbon source rock, their level of maturity and the principal hydrocarbon reservoirs cited (Akande, 2003:35).
Elsewhere, several studies by various scholars are still ongoing to further determine the thermal maturation and the source rock qualities of the rock in the area to generate hydrocarbons.
Fig.1 Location of South East Nigeria Efiong-Fuller (nd)
In view of this, the aim to determine the organic geochemical studies in the selected areas of South Eastern Nigeria came of interest. This will determine the economic importance of the Eze-Aku Formation exposure in the area, with respect to its qualities as a potential source rocks, therefore predicting an insight into several sequences favourable to hydrocarbon exploration in the various places where they exist. This will also establish the fact that the outcropping sediments of the Eze-Aku Formation are traceable and mapable in these areas, to further describe the lithologic components in order to access the source rock potential and thermal maturity of the rocks recovered from the area.
The objectives of this study include:
To investigate the lithofacies / lithologic units of the Eze-Aku Formation that exists in the area.
To investigate the Hydrocarbon source rock potential and thermogenetic maturation of the Eze-Aku shales in the study area, if sufficient enough to be economically viable, thus attract foreign investment in the region.
REVIEW OF PREVIOUS LITERATURE
A number of researchers have conducted a wide range of organic geochemical analysis on various groups of shale samples in the Southern Benue Trough to assess their source rock characteristics.
The Eze-Aku Formation (Turonian), the Agwu shale (Turonian-Coniacian) and the NKporo shale (Compano-Maastrichtian) have received most of the attention ( Agagu and Ekweozor, 1982; Petters and Ekweozor, 1982; 1982b; Unomah and Ekweozor, 1987, 1993; Akande and Viczian, 1996; Akaegbobi and Schmitt, 1998 cited in Onuoha et al. 2008:2). The studies suggest that the sediments contain total organic content (TOC) above the 0.5% threshold value expected for a potential source rock with the highest TOC value recorded from the Nkporo shale (Onuoha et al. 2008:2).
However, another school of thought has developed new techniques of determining the organic content of sedimentary rocks through the use of wireline logs for rapid assessment of organic content in various sedimentary basins (Schmoker 1979, 1980; Schmoker and Hester et .al.. 1983; Meyer and Nderlof, 1984; Hester et al. 1990 cited in Onuoha et al .2008:2). Wireline logs believed to be quick in assessment of rocks, readily obtainable and continuously records data by eliminating problems of limited samples and sampling bias, when compared with organic geochemical analysis techniques which is time consuming, expensive and are subject to problems of sample bias (Onuoha et al.2008:2). Nevertheless wireline logs are hindered by the fact that shales with comparable degree of compaction are likely to have equal water of saturation; for this reason, variation in density would be a function of the amount of organic matter present (Schmoker,1979; Meyer and Nederlof,1984 cited in Onuoha et al. 2008:2).
In addition, Ehinola et al (2005:690) studied Nkporo shale which oversteps the Awgu shale and the Eze-Aku shale of the Lower Benue Trough, to access its petroleum potential by sedimentological analysis and Rock-Eval pyrolysis. The type localities along the Enugu-Porthacourt express way of South Eastern Nigeria. Their findings show (TOC) range from 0.4 to 3.01wt % suggesting potential petroleum source rocks.
Moreover, Akande and Erdtmann (1998) described the sediments of the Eze-Aku Formation as having deposited as a result of a renewed transgression into Trough during the second depositional cycle. Dessauvagie et al. (1972) also maped these sediments of the Eze-Aku Formation and described its lithology to comprise of shales, sandstones and limestone.
Similarly, Ehinola et al.(2005:690) further described these sediments to consist of black calcareous shales, shelly limestone, siltstones and sandstone deposited as a result of a renewed transgression into the Trough during the second depositional cycle.
South Eastern Nigeria dates back to Precambrian age. The opening of the South Atlantic Ocean initiated tectonism in the region and led to the advancement of the Benue Trough (Wright, 1996; Nyong, 1995:14 cited in Efiong-Fulleret et al. (nd)). The growth of the Benue Trough provided the main structural control and gallows for subsequent geologic evolution of South Eastern Nigeria, which represents most areas of the South Benue Trough. The Southern Benue Trough comprises of the Abakaliki Anticlinorium, Afikpo Syncline, Ikom-Mamfe Embayment and Ogoja Sub-Basin (Fig.2).
Fig. 2. Structural setting of South Eastern Nigeria (Efiong-Fuller nd)
A number of researchers studied the origin of the Lower Benue Trough, among are Murat (1972), Petters (1978), Simpson (1954), Tattam (1944). These workers considered the sedimentary history as resulting from short-lived extensive transgression, which was succeeded by regressive cycle in view of dominance of marine, paralic and continental facies. Murat (1972) observed that subsequent to the Santonian uplift, there was marine transgression in the Lower Benue Trough in the Campanian.
Furthermore, Ehinola et al (2005:690) described the Lower Benue Trough to comprise of tectonically inverted Abakaliki Anticlinorium and the flanking Anambra Basin and Afikpo Syncline to the west and east respectively. More so, De Klasz and fayose (1976) attempted using microfossils as biostatigraphic tools for a subdivision of Mid-Cretaceous sediments in the Lower Benue Trough.
In addition, Agagu and Adighije (1983) affirmed the Cretaceous sediments fill in the Lower Benue Trough is about 5000m thick approximately 2000m of these sediments where deposited in the Anambra Basin.
Ehinola et al (2005:690) documents the factors responsible for this sedimentation within the Lower Benue Trough during the Cretaceous were the progressive sea level rise from Albian to Maastrichtian leading to transgression, regression and local tectonics. This resulted to four major cycles of deposition. Albian -Cenomanian, Turonian- Coniacian, Companian - Maastrichtian, and palaeogene sequence have been recognised in the Lower Benue Trough. The various lithostratigraphic units contained in the Lower Benue Trough from the oldest to the youngest are as follows as shown in Fig.3.
Fig.3. Geologic Sketch map of South Eastern, Nigeria. (Effiong-Fuller, nd)
ASU RIVER GROUP
Asu River Group represents the earliest clastic fill in the Southern Benue Trough (Petters and Ekweozor 1982). The sediments consist of 2000m of poorly bedded shales .The brand localities of the sediments can be found in areas like Nkalagu quarry, Isiagu and Lokpanta areas of Nigera.
The Odukpani Formation consists of dark grey to black calcareous shales, which has been assigned a Cenomanian age based on ammonite fauna (Ehinola et al 2008)
Overlying the Asu-River Group, the Eze-Aku shales consisting of black calcareous shales, shelly limestone, siltstones and sandstone which were deposited as a result of a transformed transgression into the Trough during the second depositional cycle (Ehinola et al 2005). The Formation thickens Southerly and attains a thickness of 1000m towards Owerri-Abba areas of Abia state, representing shallow water deposit.
The Awgu Formation overlies the Eze-Aku shale. This Formation consists of bluish-grey shales with interbeds of fine-grained calcareous sandstone and black bioclastic limestone. Ojoh (1990) describe the sediments of the Agwu Formation to be deposited as a result of marine influence of the Turonian which was succeeded by a Coniacian highstand.
NKPORO SHALE/ENUGU SHALE
The Nkporo shale lays uncomformably on the Agwu shale. It represents the brackish marsh and fossilferrous prodelta facies of the Late Campanian- Early Maastrichtian depositional Nkporo cycle proposed by Regers and Nwajide (1998).
Ekweozor and Unomah, (1993) described the Nkporo shale to be carbonaceous and barren of foraminifera occasionally, but it generally rich in fossills. Reyment, 1965 and Agagu et al. 1985 predicted a shallow environment due to presence of foraminifera and ammonites (cited in Tijani et al. 2008).
The formation consists of blue or dark grey to back shales with occasional thin beds of sandy shale and sandstone (Onuoha et al.2008:2).
The Nkporo group is overlain by the Mamu Formation. Tijani et al. (2008) in a recent paper described the late Campanian sedimentary unit also known as the "Lower Coal Measures" to consist mainly of dark blue to grey shales/mudstone units with alternating sandy unit and coal seam horizons forming a stripped rock unit. Reyment (1965) observed the Lower Coal Measure in the central part of the basin consisting of five coal seams with a range in thickness from 4cm-3.5cm as part of the Formation in the Enugu area of Nigeria. The Formation is Indicative of a shallow water of the paralic facies of a deltaic complex (Cratchley and Jones, 1965 cited in Tijani et al.2008).
Ajali Formation known as the "Middle Coal Measures" described by Ehinola et al (2005) is a Maastrichtian Sandy unit overlying the Mamu Formation Conformably. It consists of white, thick friable, poorly sorted cross bedded sands with thin beds of mudstone near the base (Tijani et al 2008).
The "Upper Coal Measures" which lies comformably on the Ajali Formation. The Unit consists of interdigitation of very fine grained sandstones, dark shales and coal indicating paralic environment of Maastrichtian to Palaeogene age (Ehinola et al 2005). Reyment (1965) dated Nsukka Formation (Maastrichtian) using Afrobolumonatra fossils.
Ehinola et al. (2005) found that the Imo Formation and Ameki Formation where deposited as a result of the Palaeogene transgression and Eocene regression. Imo Formation consists of clay shale with clay ironstone and sandstone bands which lies comformably on the Nsukka Formation.
The Imo shale is overlain by the Ameki Formation, that represent the uppermost succession of the basin, consisting of highly fossil ferrous greyish -green, sandy clay, with calcareous concretions and white clayey sandstone (Ehinola et al. 2005) as shown in Fig.4.
Fig.4.Cretaceous -Tertiary Straigraphy, in South Eastern Nigeria. (Akande et el. 2007)
This study will focus on four states, located in South East of Nigeria in the Lower Benue Trough: Abia state, Cross River, Akwa Ibom and Imo state. These states have been selected under setting conditions. Efiong-Fuller (nd) reported the states to be mostly affected by mass degradation, which have ruined most of the infrastructure and posed arm to the socio-economical stand of these states.
To define the lithologic units that exists in the area, a fieldwork exercise will be carried out at various locations of the states .The various equipment to be used for the field exercise will include the following.
Compass: A theodolite compass which will be used to measure the horizontal and vertical angles of various structures on field. Also to provide the direction on the field
Measuring Tape: It will be used to measure the thickness of various lithologic units when mapping.
Hammer and Chisel: it will be used to get samples from outcrops for sampling and laboratory analysis.
Camera: To snap various sedimentary structures, and observe bed relationship of various outcrops exposed.
Log Book: This will be used to record various observation of the outcrops, ranging from sampling, description and drawing of various lithologic units analysed.
GPS: It will be used to estimate the distance from the nearest village or landmark to the possible outcrop sections in the area.
The field mapping aspect will involve bed to bed measurement of different outcrops, sampling, description and collection of various rock samples. The thickness of the various exposed section will be measured and the description of their lithologies in terms of texture, sedimentary structures, thickness, and geometry and bed relationship will be drawn out. Therefore, a framework of the lithostratigraphic unit of the Eze-Aku Formation believed to be traceable and mapable in the area will be established after the following procedure.
The laboratory aspect of the project will be concerned with organic petrography and geochemical analysis to evaluate the shale samples retrieved from the study areas, to access their thermal maturity and petroleum potential capabilities.
(A) Organic petrography is a technique that investigates organic matter present in sedimentary rocks; in particular coal and petroleum source rocks. This has been widely used for several decades to establish and predict the thermal maturity and hydrocarbon generation of petroleum source rocks across several sedimentary basins of the world.
Organic petrography will be carried out with the use of a sophisticated instrument used in the laboratory called Reichert Jung Polyvar photo microscope equipped with stabilized halogen and HBO lamps and a photomultiplier (Akande et al. 2007). Vitrinite reflectance measurement and maceral description/composition of the rocks will be determined in the process.
The rank of maturation of the rock samples will be determined by the reflectance measurement of the vitrinite particles. The mean random reflectance of the vitrinite particles in oil (Râ‚’ m %) will be measured using monochromatic (546nm) non - polarised light in conjunction with a 40Ã- oil immersion objective (Bustin et al 1983 cited in Akande et al 2005).
Bustin et al. (1983) found that (cited in Akande et al.2005) "in a randomly oriented grains the random reï¬‚ectance of vitrinite measured in non-polarised light is a true reï¬‚ection of the average the minimum and maximum reï¬‚ectances expressed as Râ‚’m = 1/2(Râ‚’max + Râ‚’min)."
Where Râ‚’ max = maximum reflectance
Râ‚’ min = minimum reflectance
Maceral description and composition will be determined by means of a Swift automatic point counter and a mechanical stage using white and blue light extraction (Akande, 2007). The number of counts each maceral analysis is based on relates to the phytoclasts distribution in the rock samples. Vitrinite reflectance and maceral description will be used in combination with TOC / Rock-Eval pyrolysis as effective screening tools for the identification of organic rich source intervals.
(B) Geochemical analysis
Rock-Eval pyrolysis has been used for many years as a principal technique for determining the amount of organic matter, identifying the type of kerogen and assessing the thermal maturity of the organic matter present in rocks (Espitalie et al., 1984, 1985; Bordenave et al., 1993 cited in Akande et al. 2005).
Rock-Eval pyrolysis VI instrument will be used for the proposed project. The new equipment provides more functions and parameters over existing models, thereby reducing problems of previous Rock-Eval systems (Lafargue et al. 1998). Pyrolysis of 100 mg of samples at 300 Â°C for 3min will be followed by programmed pyrolysis at 25 Â°C/min to 850Â° C in an inert atmosphere of helium. The Rock Eval instrument generates the parameters S1, S2, S3 peaks, Tmax and Total Organic Content (TOC) of the oil shale. Parameters such as Hydrogen Index (HI), Oxygen Index (OI) and Production Index (PI) will be calculated from the pyrolysis data recorded by the instrument.
The Rock Eval technique will be accessed based on the following criteria's.
(a) Hydrocarbon Source Rock Potential:
Akande et al. (2005) described the "Petroleum genetic potential referred to as (SP) represents the amount of petroleum (oil and gas) which can be generated by any rock during its thermogenic maturation." To obtain the petroleum generating potential of the shales to be analysed, three factors will be involved in evaluating the petroleum potential of the rock samples. These are the amount of the organic matter, type of kerogen and thermal maturity.
(i) To obtain the genetic potential, deï¬ned as the sum of S1+ S2 values derived from Rock-Eval pyrolysis:
S = the yield of hydrocarbons released by heating
S1 = the quantity of free hydro-carbons, which have already been generated within the source rocks vaporizable at about 300Â°C , measured in mg HC.g/rock (equivalent to kg HC. tonne/rock).
S2= hydrocarbons resulting by the cracking of both the kerogen and heavy extractable components, such as resins and asphaltenes during simulated catagenesis (between 300Â°C and 850Â°C) measured in mg HC.g/rock (equivalent to kg HC. tonne/rock). After Akande et al. (2005)
The following groups have been established based on (Tissot and Welte1984; Dymann et al., 1996 cited in Akande et al. 2005).
Rocks with SP less than 2 kg/t (2000 ppm) suggest insigniï¬cant oil but some gas potential.
Rocks with SP ranging between 2 and 6 kg/t (2000 ppm-6000 ppm) are classiï¬ed as moderately rich source rocks with fair oil potential.
Rocks with SP Above 6kg/t (6000 PPM) are classified as good to excellent source rock potential.
(ii) Amount of Organic matter: The amount of organic matter in the rock samples will be measured as the total organic carbon content (TOC) expressed as a percentage of the dry rock. Studies on worldwide samples of shales of different ages and origin have led to the conclusion that the minimum TOC value required for an immature source rock is 0.5 wt%, (Hunt, 1979; Hedberg and Moody, 1979; Tissot and Welte, 1984 cited in Akande et al. 2005).
(iii) Type and source Organic Matter:
To obtain the type and quality of organic matter in a source rock will be based on Rock-Eval generated data, Hydrgen Index (HI) and Tmax plot of values, which are both functions of kerogene and thermal history represented in the equation below.
HI=1/ (aÃ- exp (bÃ- (Tmax-435)) + c)
Where a, b and c are constants used to define HI-Tmax profile of particular source rock units. After Banerjee et al. (1997)
Rock-Eval Tmax represents the temperature of the peak rate of Hydrocarbon generation. Hydrogen Index corresponds to the quantity of pyrolised hydrocarbon relative to the total organic carbon content in the sample Banerjee et al. (1997).
(iv). Thermal Maturity: The phase of maturity will be estimated using Rock-Eval (Tmax), the temperature corresponding to the maximum yield of hydrocarbon upon pyrolysis (Banerjee et al 1997). Although, is partly dependent on other factors such as the type of organic matter or mineral matrix effects (Peters and Cassa, 1994 cited in Shaaban et al. 2006).
Peters and Cassa, (1994) cited in Shaaban et al (2006) suggested an assessment of thermal maturity based on Tmax range of values.
Tmax range < 435Â°C the organic matter is considered immature at (Diagenetic depth)
Between 435 -470Â°C the organic matter is mature (Catagenetic depth)
Above or.> 470Â°C, the organic matter is said to be in a post-mature stage /indicate the wet gas zone (Metagenetic depth).
PLAN AND TASKS
Series of tasks and their respective duration have been planned to execute within the period of six months: 5th February to 12th July, 2009 as illustrated in the GANTT CHART.
Searches and Reading
Surveying the extent of exposure
Break For 5th Module
Mapping and description of sediments of the Eze-Aku Formation
Lithostratigraphy of the Formation
Sample sorting and logging
Break For 6th Module
Determination of the amount of Organic carbon
Detection of Hydrocarbon source potential
Detection of the type and source of Organic matter
Detection of Thermal maturity
Computing and Interpretation of results
Submission of Draft Copy
Final changes on draft Copy
Preparation for Presentation
FUNDING AND BUDGET
For a successful start and completion of this project, we shall request the funding from Petroleum Technology Development Fund (Nigeria), the body Awards scholarships, bursaries to post graduate student for oil and gas studies. The following is the breakdown of the estimate/budget for the project:
1trips to Nigeria via London and Return £400
Geologic Field work materials: GPS, Camera, Theodolite. £300
Feeding and Accommodation £350
2 trips to Scotland by car/train @ £50 £200
Sample Preparation £100
Sample screening £150
Rock-Eval pyrolysis £200
Kerogen analysis £150
Total requested £1,900
Limitation for this work may arise due to cost involved and time taken in performing the organic geochemical analyses in identification and characterisation of source rocks. Although, improvements and new solutions to previous models of the equipment have been made, these have enhanced the functionality and reduce the problems of previous techniques. For Instance the advent of the Rock-Eval pyrolysis (VI) equipment which marks a major development of programmed pyrolysis systems over the previous Rock-Eval instruments. This has brought about new functions and parameters that expand the reach of the technique in petroleum geosciences, thereby reducing the problems of older Rock-Eval systems (Lafargue et al. 1998)
Rock- Eval pyrolysis technique has been consistently used for about fifteen years and has become a standard tool for hydrocarbon exploration by various authors (e.g., Barker, 1974; Claypool and Reed, 1976; Espitaliéet al., 1977 and 1984; Clementz et al., 1979; Larter and Douglas, 1980; Horsfield, 1985; Peters and Simoneit, 1982; Peters, 1986 cited in Lafargue et al. 1998) to provide data on the potential, maturity and type of source the rock in different sedimentary basins of the world.
In addition, Rock-Eval method has the advantages that only small quantities of samples are needed, the procedure is simple and rapid, and the results are standardized Garcia-valles et al. (2000).
We expect the result of this work will assist in satisfying academic inquisitiveness by enriching literature and solving the puzzles of unknown hydrocarbon source rocks, their level of maturity and principal hydrocarbon reservoirs that still exists in the Lower Benue Trough. Thus, this will contribute to part of an on-going study on the petroleum source rock potential and evaluation of the Cretaceous Benue Trough and adjacent sedimentary basins initiated by the lecturers of my Alma matter, the Department of Geology, University of Ilorin.
We also anticipate that findings from this work would be useful to the Government of the states these rocks are found and studied. Therefore would allow assumptions and future predictions, most favourable for prospecting and exploration for hydrocarbons in the areas which they exist. And, thereby boost the economical stability of the states through development and creation of industry, thus impacting on the citizenry of the community at large.
The rock samples to be analysed for this work, will be mapped out from the study areas on field with appropriate approval from the Research ethic committee representative of the government of the area. Also, geochemical evaluation of the samples will be done in the laboratory, with all safety standards adhered to with minimal supervision. Therefore, we feel that there would be less or no constraints in the process of mapping the area and performing the geochemical analysis of the rock samples retrieved from study locations.