Introduction

The eastern Dahomey basin of the Nigeria sector contains extensive wedge of Cretaceous to recent sediments that thickens towards the offshore up to 3000 m. The basin has been of much geological interest as a result of the occurrences of bitumen, limestone, glass sands and phosphates (Nton 2001).

Exploration for hydrocarbon commenced in this basin in 1908, near Okitipupa, east of Lagos, where bituminous sands outcrop.

Bitumen-impregnated sandstones (tar sand) outcrop in a 120 km by 6 km belt in Southwestern Nigeria from the Okitipupa ridge/ western feather edge of the Tertiary Niger Delta to as far west as Ijebu-Ode in Ogun State, Fig. 1.

Fig. 1
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Modified after Enu (1985)

Generalized geological map of the Eastern Dahomey Basin showing area extent of the tar sand deposits.

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The geology of these deposits, oil saturation and reserve estimates as well as textural characteristics of the associated sands have been described (Adegoke et al. 1980, 1981; Enu 1987). The physicochemical properties of the bitumen in relation to production and processing have been studied (Oshinowo et al. 1982; Oluwole et al. 1985). The origin of the bitumen has been discussed (Coker 1990; Ekweozor 1986). Other relevant studies on the deposit and geology of the basin include works done by (Ako et al. 1983; Ekweozor 1986, 1990; Nwachukwu and Ekweozor 1989; Enu 1987, 1990; Enu and Adegoke 1984; Elueze and Nton 2004; Akinmosin et al. 2005, 2013; Akinmosin and Osinowo 2008).

Enu (1985) distinguished two horizons: X (shallow) and Y (deeper than 50 m). The former can be exploited by open cast mining, and the latter by steam-assisted gravity drainage (SAGD).

Study area

Based on Fig. 2, the study area is located at 6.55 N, 4.45E; 6.75 N, 4.77 E. Samples used for the project were sourced from three core holes labelled Well X, Well Y and Well Z which, respectively, located in Ayede-Ajegunle with coordinate 6.58 N, 4.62 E, Araromi-Obu/Ago Alaye with coordinate 6.59 N, 4.54 E and Ajegunle with coordinate 6.60, 4.60 E.

Fig. 2
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Location map of the study area

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Stratigraphy of the Dahomey basin

Table 1 shows stratigraphic succession compiled following the works of Jones and Hockey (1984), Omatsola and Adegoke (1981), and Agagu (1985) who relied at least partly on earlier studies by Russ (1924) and Reyment (1965). In most part of the basin, the stratigraphy is dominated by sand–shale alternation, with subordinate limestones and clays (Agagu 1985).

Table 1 Age and stratigraphic relationship of the formations of the Dahomey basin
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Research methodology

Twenty (20) selected core samples were used in this study. The lithological logs of the borehole drilled in each location were generated by logging each borehole.

Representative samples were collected from each lithofacies for laboratory analyses which included: grain size analysis, thin section petrography, X-ray diffractometry (XRD), and scanning electron microscopy (SEM). Table 2 shows the locations of core holes and the depth of analysed samples.

Table 2 Core holes and samples depths
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Grain size analysis

For textural analysis, 100 g of the prepared samples were dry-seived using vibrating sieving machine for 15 min with sieves of the following mesh sizes: 2 µm, 1.4 µm, 1 µm, 0.5 µm, 0.25 µm, 0.18 µm, 0.125 µm, 0.09 µm, 0.075 µm, 0.063 µm and pan, corresponding to − 1Φ, − 0.5Φ, 0Φ, 1Φ, 2Φ, 2.5Φ, 3Φ, 3.5Φ, 3.75Φ, 4Φ and 5Φ on the logarithmic scale values, respectively. The individual cumulative weight with their percentages was determined. These data were used to plot histograms and cumulative frequency curves for individual samples on arithmetic and semi-log graphs. The parameters were computed using moment statements mean, mode, standard deviation, kurtosis, and skewness (using formulae by Folk and Ward 1957).

Thin section preparation

Thin section preparation was carried out at the Geological Laboratory of the Obafemi Awolowo University Ile-Ife, Nigeria. The sample was placed in a container and Canada balsam spread on the sample. The sample was then placed on a hot plate and heated for about 15 min subsequently removed and left overnight (to solidify). The next day, the sample was placed and cut on a lapping slide glass, was left for 24 h. The araldite allowed the sample to stick to the surface of the glass /mounting slide. The sample was grounded with the use of a grinding machine to a considerable thickness. The grinded sample was then placed on the lapping slide with the use of microscope. This was to ensure that the sample was not too thick or thinly prepared so that the minerals can be seen visibly. This is then covered back with silicon carbide, cover slide and Canada balsam on the lapping slide. This was placed on the hot plate for about 15–20 min and left to get cold for about 1 h. The sample was then washed with acetone and water with detergent and left to dry before it was labelled.

Laboratory analysis: XRD

XRD analyses was carried out at the Nigerian Geological Research Laboratory (NGRL) in Kaduna, Kaduna State. The NGRL Kaduna uses Empyrean diffractometer DY 674 (2010) with a copper anode material manufactured by Analytical Holland. It works with a combination of other components such as the water chiller which cools the X-ray tube and maintains a uniform temperature. There is also the compressed air that helps in opening and closing of the cabinet door. The goniometer forms the central part of the Empyrean diffraction system. It contains the basic axes in X-ray diffractometry: the omega and 2 theta axes. The X-ray tube, incident beam optics, the sample stage and the diffracted beam optics including the detector are mounted on specific positions on the goniometer. The goniometer is set up in vertical mode and can be configured for theta–theta and alpha-1 diffraction geometries. The goniometer radius is 240 mm. The diffractometer consists of three basic elements: an X-ray tube, a sample holder, and an X-ray detector.

Laboratory analysis: SEM

The model used for the SEM analysis was SEM-Aspex 2300 (Fig. 12). Vent system by lifting the lever on the left side of the microscope; wait for the drawer to drop. The machine has a chamber with a stage inside it and a tube that is also attached. The stage is divided into segments; the segment used for this analysis is 3 by 3. There is also a magnifying lens used to view the microstructure of the clay sample. There are different interphases using a program perception model. There is beam energy (acceleration voltage) of 15 kV; the filament is driven at a percentage of 66.9% of the total voltage which results into current flow. The emission current is 50.7 µA. It is set to different magnification. The stage is divided into X, Y, Z. The beam: beam energy, filament drive, emission current. The vaccum has interlock which has to read ready before scanning and the pressure must be at 3.2 × 10 −5 torr. The pressure must be at least 5 × 10 −5 torr. Sample pressure is set to high vaccum for images to be seen.

Result and discussion

Lithological descriptions of the core samples from the study area

Lithological description of the study area was based on core samples collected from the three locations. The columna lithological descriptions of the studied wells are shown in Figs. 3, 4 and 5 respectivily.

Fig. 3
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Litho-section of core samples at Ayede-Ajegunle (Well X)

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Fig. 4
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Litho-section of core samples at Araromi-Obu/Ago-Alaye (Well Y)

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Fig. 5
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Litho-section of core samples at Ajegunle A4 (Well Z)

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Grain size analysis

Textural parameters such as roundness, sphericity, grain size and sorting, derived from granulometric analysis, can be used to correctly deduce the provenance of sediments and further characterize the reservoir. The use of grain shape to identify sedimentary environments assumes that grain morphology reflects environmental history (Krinsley and Doornkamp 1973; Bull 1981). Table 3 shows a comprehensive result of the grain size analysis carried out on the analysed samples.

Table 3 Results of grain size analysis
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Deductions from grain size analysis

Data from both the lithological description and granulometric (sieve) analysis confirmed that the core samples for Ayede-Ajegunle location (Well X) range from fine to very fine grained sizes (Fig. 6). However, samples at depths between 69 and 72 m have their grain sizes ranging from medium grained to very fine grained. The grains are moderately sorted to very well sorted, symmetrical skewed and platykurtic to mesokurtic (90–93 m).

Fig. 6
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Grain shape and size as observed from the petrographic microscope from Well X at a depth of 54–57 m

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The particle size for core samples from Araromi-Obu/Ago-Alaye (Well Y) range from fine to very fine grain sizes. The grains are generally moderately sorted, symmetrical to strongly coarse skewed and mesokurtic to very platykurtic. Similarly, the sediments in this location show a relatively narrow range of grain size (2.400–3.101). This also suggests that the sandstones of this location are relatively closer to the distal portion of the basin as compared to the proximal portion. The close range of grain sizes, also depicts a single source of sediment supply as it is been revealed by the predominant unimodal.

The particle size for core samples from Ajegunle A4 (Well Z) are generally fine grained. The grains are moderately sorted, fine skewed and platykurtic. In the same vein, the grain size ranges of Ajegunle are also of close range to each other (Fig. 7).

Fig. 7
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Sorting and cement development as seen under petrographic microscope for Well Z at a depth of 54–57 m

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Petrographic analysis

Thin section analysis was carried out on the sandstones of the study area to further describe the textural parameters of the samples, their composition, cementing materials, as well as grain contacts. The following mineralogical compositions were identified from the petrographic study:

Quartz Thin section petrographic analyses revealed quartz as the predorminant mineral (Table 4). Its dominance cuts across the three bore hole (core) samples (Figs. 8, 9) of the present study. The dominance of quartz minerals, relatively gave the sandstones of the study area a kind of mechanical stabilty.

Feldspar Unlike quartz, Feldspar minerals were poorly represented (Table 4). In comparison to other minerals, detrital feldspar was generally low (Figs. 10 and 11). Like quartz mineral, its low percentage, cuts across the three core samples location. Detrital feldspars, contributed to some of the diagenetic alteration in the study area.

Micas Mica grains make up some of the accessory minerals observed in the study area. However, it was generally low in percentage in the entire study area (Figs. 12 and 13).

Rock fragments Rock fragments in the whole study area was generally low (Table 4).

Fig. 8
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Quartz grain in Araromi-Obu/Ago-Alaye at depth 57–60 m under crossed polarised light (i.e. nicol). Qtz quartz, Por pore space

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Fig. 9
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Quartz grain in Araromi-Obu/Ago-Alaye at depth 81–84 m under crossed polarised light (i.e. nicol). OPMN opaque mineral, PDF partial dissolution of feldspar, POR pore space

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Fig. 10
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a, b Feldspar grain particularly orthoclase in Araromi-Obu/Ago-Alaye at depth 60–63 m and 75–78 m, respectively, under crossed polarised light (i.e. nicol). INTPOR interconnected pore space, OPMN opaque mineral, PDF partial dissolution of feldspar, Por pore space, QTZ quartz

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Fig. 11
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a, b Showing Plagioclase Feldspar grain in Ajegunle A4 at depth 63–66 m under plane polarised light (PPL) and at Ajegunle A4 at depth 75–78 m under cross polarised light (nicol)

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Fig. 12
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a, b Biotite in Araromi-Obu/Ago-Alaye at depth 81–84 m under plane polarised light (PPL) and at Ajegunle A4 at depth 54–57 m under plane polarised light (PPL)

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Fig. 13
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a, b Showing accessory mineral particularly apatite grain in Ayede-Ajegunle at depth 75–78 m under plane polarised light (PPL) and at Araromi-obu/Ago-Alaye at depth 84 m under plane polarised light (PPL)

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Table 4 Frequencies of the framework elements and binding material
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X-ray diffraction (XRD) and clay minerals analyses

The different sandstone samples of the Afowo Formation analysed consist dominantly of quartz and kaolinite. Subordinate amounts of syvite (KCL), montmorillonite, gypsum, calcite and jarosite (KFe3 + 3(SO4)2(OH)6) were found. A representative diffractogram of one of the analysed sample is shown in Figs. 14 and 15.

Fig. 14
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XRD diffractograms of core sample from Ajegunle A4 at a depth range of 75–78 m

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Fig. 15
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XRD diffractograms of core sample from Araromi-Obu/Ago-Alaye at a depth range of 81–84 m

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Scanning electron microscopy (SEM)

Scanning electron microscopy was used to identify the clay species, examining the orientation of clay to framework grains and to identifying features, such as pores, that cannot be seen under the petrographic microscope (Fig. 16).

Fig. 16
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SEM photomicrograph for Ayede-Ajegunle at depth range of 54–57 m. F fracture, FL fracture line, MP micro pores, SK sheet kaolinite, KF K-feldspar, VK vermiform kaolinites. Magnification a = ×50, b = ×100, c = ×750, d = ×2500

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The occurrence of clay minerals adversely impacts the permeability and porosity of a reservoir rock. Clays can occur as matrix, grain coatings, clay intraclasts and replacement grains. Thus, clays can completely occlude the porosity if they occur as matrix, or hinder the permeability if they occur as grain coatings. Factors such as the source rock, type of weathering and depositional environment can determine the primary clay mineralogy and habit in the sediments. Other factors related to the sandstone composition, its diagenetic evolution, burial/thermal histories can also determine the clay type in the sandstones.

Data from petrographic, and SEM analyses document a diagenetic sequence. The following diagenetic features were recognized; quartz-overgrowth, calcite cement and pore-filling kaolinite. XRD analysis, reveals not only the presence of calcite cement, but also minor calcite occurrence. These features strongly indicate that porosity in these sandstones is partly secondary in origin. Preservation of high porosities after the dissolution of grain-supporting carbonate cements suggests that the pressure seal became effective about the same time that carbonate cement began to dissolve.

Quartz overgrowth surfaces examined under SEM reveal a network of ridges that form a box work texture suggestive of local quartz dissolution (Fig. 17). Carbonate cement dissolution textures are observed in few samples (Fig. 18) while in thin section, patches of discontinuous calcite cement with synchronous extinction are observed, suggesting a previous connectivity that was disrupted by dissolution. Kaolinite occurs locally in normally pressured, high-porosity sandstones and typically fills illite lined pores in samples that exhibit intergranular grain penetration. Authigenic illite often occurs as a fibrous pore-filling mineral which strongly reduces the permeability in reservoir sandstones.

Fig. 17
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Quartz overgrowth in Ayede Ajegunle at a depth range of 63–66 m. QO quartz overgrowth. Magnification = ×100

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Fig. 18
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Development of calcite in the analysed sample. C calcite

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Porosity and permeability

The high porosity noted in the study area is attributable to the dissolution of minerals principally kaolinite and to the early precipitation of iron oxide cement which prevented further significant compaction (Christopher et al. 2017). The porosity and permeability data were derived empirically since sample size did not meet the criteria for measurement using the permeameter and porosimeter. The values range from 15.5% (medium) to 33.6% (very high), as seen in the Table 5, the porosity value decreases with depth except for Well X at a depth of 90–93 m. This is probably due to complete dissolution of feldspar grain at high temperature and pressure due to overburden (Baiyegunhi et al. 2017). Permeability ranges from 270 mD (low) for the sandy shales to 4800 mD (medium) for the sandstones. The results of the porosity and permeability determination are shown in Table 5.

Table 5 Side wall core analysis result with the porosity and permeability values
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Generally, the porosity in the study area showed no distinct trend. This is probably due to shallow depth which is still within the eogenetic stage (i.e. early stage diagenesis or shallow diagenesis) and or dissolution of minerals which majorly accounted for the secondary porosity noticed. These features were observed in Well X at depth range 69–72 m, 90–93 m, Well Y at depth range of 60–63 m, 66–69 m, 75–78 m and Well Z depth range 63–66 m and 75–78 m.

Conclusion

Due to the high viscous forces retaining tar within the reservoir pores, heavy crudes are often characterised by extremely low mobility under reservoir conditions. Consequently, successful in-situ techniques, primarily thermal methods, are premised on downhole reduction of the viscosity. Steam-based processes, most of which generate steam by burning fossil fuels, currently account for some 98% of commercial production from thermal techniques.

Success of steam-assisted gravity drainage technique among other things, depends largely on the textural attributes of the reservoir as well as on clay types. These in turn determine the quality of porosity and permeability of such reservoir. Inclusion of clays that are vulnerable to swelling when in contact with water affects the success of this technique. The study has, however, shown that some of these types of clay though present, are not well formed.

Consequently, this technique will no doubt be suitable for recovery of the Nigerian oil sand deposits. Although, the need for considering the cost implication should still not be down played.

Moreover, data from petrographic and SEM analyses create graphic view of the diagenetic sequence of the studied samples. The following diagenetic features were recognized; quartz-overgrowth, calcite cement and pore-filling kaolinite. XRD analysis, reveals not only the presence of calcite cement, but also minor calcite occurrence. These features strongly indicate that porosity in these sandstones is partly secondary in origin. Preservation of high porosities after the dissolution of grain-supporting carbonate cements suggests that the pressure seal became effective about the same time that carbonate cement began to dissolve.