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Palynofacies Analysis, Organic Thermal Maturation and Source Rock Evaluation of Sedimentary Succession from Oligocene to Early Miocene Age in X2 Well, Greater Ughelli DepoBelt, Niger Delta Basin, Nigeria

Lucas F.A, Omodolor Hope E*

Department of Geology, University of Benin, P.M.B. 1154 Benin City, Edo State, Nigeria

Abstract: One hundred and ninety (190) ditch cutting samples of depth range within 20ft- 11820ft (6.09m-3603.7m) from X2 Well Greater Ughelli DepoBelt, Niger Delta Basin were subjected to palynological and palynofacies analysis combined with sedimentological studies, with a view to define its organic thermal maturation and source rock potential. The analysis of the ditch cutting samples were carried out using reflected light microscope for lithologic description while the palynofacies analysis was carried out using transmitted light microscope. The sedimentological analysis reveals forty-nine (49) lithozones and seven (7) lithofacies units, which were deduced based on their mineralogical composition, textural properties, fossil content, homogeneity and heterogeneity of the lithofacies units. The major lithofacies units penetrated in the well are sandstone, shaly sand, sandy shale, clay, sandy clay, clayey sand and shale. Its associated minerals include quartz, feldspar, calcium carbonate and glauconite. Identification of the petroleum play elements and hydrocarbon potential of the X2 Well were equally established. The result from the palynofacies analysis reveals miospores (pollen and spores), woody plant materials (black wood and phytoclast), amorphous organic matter which were used to characterize the well of its kerogen type and maturity. Fifty (50) lithofacies units were subjected to palynofacies analysis, this indicates a potential for hydrocarbon generation. Presence of the dark brown to black wood plant material (black wood and phytoclast) suggest gas-prone for this well. However, the high abundance of light brown to brown colour index of spore/pollen shows that the organic matter of X2 Well falls within the zone of mature main phase liquid hydrocarbon generation (Spore Color Index of 4-6). Palynofacies analysis of X2 Well shows that the well can yield about 70% of oil and 30% of gas (kerogen type II).

Keywords: palynofacies, thermal maturation, kerogen, source rock, sedimentology

Received June 28, 2018; Revised August 01, 2018; Accepted August 17, 2018

Citation: Lucas F.A, Omodolor Hope E, Palynofacies Analysis, Organic Thermal Maturation and Source Rock Evaluation of Sedimentary Succession from Oligocene to Early Miocene Age in X2 Well, Greater Ughelli DepoBelt, Niger Delta Basin, Nigeria. Journal of Geosciences and Geomatics, 6 (2). 85-93. January 2018. doi:10.12691/jgg-6-2-6

1. Introduction

The term palynofacies is “the total complement of acid resistant particulate organic matter recovered from sediments by palynological processing techniques”. [1]. However, [9], provided the most recent and widely used definition of the palynofacies term as “the total particulate organic matter assemblage contained in a body of sediment thought to reflect a specific set of environmental conditions or to be associated with a characteristic range of hydrocarbon generating potential”. The latter definition can be used in paleoenvironment interpretation as well as in source rock evaluation and will be used here because it links palynofacies types to sedimentary sequences. Several palynological investigations have dealt with the paleoenvironmental interpretations of the Niger Delta Basin. However, there is still a lacking in use of detailed palynofacies analyses in interpreting paleoenvironmental settings. The present work aims to study the palynological facies of X2 well with more detailed analyses of the percentage distribution of the palynological organic matter (POM) assemblages to infer the Thermal Maturation, Source Rock potential, and Sedimentology

1.1. Background of Study

The X2 well, is located in the Greater Ughelli Depo Belt, Niger Delta Basin. The Greater Ughelli is one of the Depo Belt in the Niger delta Basin [5]. The Niger Delta is in the Gulf of Guinea on the west coast of Central Africa. The Cenozoic Niger Delta is located at the intersection of the Benue Trough and the South Atlantic Ocean where a triple junction developed during the separation of South America and Africa in the Late Jurassic [10].

Figure 1. Map showing the distribution of Depo Belts within the Niger Delta and the location of the study area within the Greater Ughelli Depo belt [3]

2. Geology of the Study Area

The Niger Delta Basin occupies the Gulf of Guinea continental margin in equatorial West Africa between Latitude 3⁰ N and 6⁰ N and Longitude 5⁰ E and 8⁰ E. The clastic wedge of the Niger Delta formed along a failed arm of a triple junction system (aulacogen) that originally developed during the break-up of the South American and African plates in the late Jurassic [5].

3. Materials and Method

The data and interpretations presented in this study were based on detailed examination of 190 ditch cuttings samples from X2 Well which represent the sedimentary succession of depth range within 20ft-11820ft (6.09m-3603.7m). Standard methods for processing and concentration of organic matter were employed. These involved the use of Hydrochloric (HCl) (35-38 %) and hydrofluoric (HF) acids (40 %) to digest the carbonates and silicate content of the sediments respectively and the release the organic matter from the rock martrix. The residues were sieved through a 10-μ mesh and washed using ultrasonic cleaning for preparing slides. Polyvinyl Alcohol (PVA) was used as a mounting medium. For discriminating among the different palynofacies characteristics, particulate organic matter and palynomorph particles were counted and used to calculate relative abundances.

4. Results and Discussion

The result of the sedimentological analysis is as shown in [Figure 2 to Figure10] below.

4.1. Lithofacies Description and Interpretation

The sedimentological analysis revealed forty nine (49) lithozones and seven (7) lithofacies units, deduced based on their mineralogical composition, textural properties, fossil content, homogeneity and heterogeneity of the lithofacies units (Figure 2 to Figure 10). The major lithofacies units penetrated in the well are sandstone, shaly sand, sandy shale, sandy clay, clayey sand and shale. Its associated minerals include: quartz, feldspar and glauconite.

4.2. Maturity

The maturity of sediment encountered in X2 well were determined based on their textural (sorting and rounding) and compositional maturity (minerals present).

The sediment found in X2 well are mainly medium to coarse grain, subrounded to subangular, moderately to well sorted. Thus, the sediment in X2 well can be describe to be mature base on their textural and compositional properties (presence of a stable mineral i.e. quartz).

Figure 2. Lithostratigraphy analysis of samples from X2 Well, Greater Ughelli Depo-Belt(6.09m-4817m)

Figure 3. Lithostratigraphy analysis of samples from X2 Well, Greater Ughelli Depo-Belt (500m-1067.1m)

Figure 4. Lithostratigraphy analysis of samples from X2 Well, Greater Ughelli Depo-Belt(1065.4m-1628m).

Figure 5. Lithostratigraphy analysis of samples from X2 Well, Greater Ughelli Depo-Belt(1969.5m-2268.3m)

Figure 6. Lithostratigraphy analysis of samples from X2 Well, Greater Ughelli Depo-Belt(2286.6m-2567.1m)

Figure 7. Lithostratigraphy analysis of samples from X2 Well, Greater Ughelli Depo-Belt (2585.4m-2932.9m)

Figure 8. Lithostratigraphy analysis of samples from X2 Well, Greater Ughelli Depo-Belt(6.09m-3603.7m)

Figure 9. Lithostratigraphy analysis of samples from X2 Well, Greater Ughelli Depo-Belt (29512m-3384.2m)

Figure 10. Lithostratigraphy analysis of samples from X2 Well, Greater Ughelli Depo-Belt (3384.2m-3603.7m)

4.3. Palynofacies Assemblages

The palynofacies components in this study are broadly classified as AOM, phytoclasts and palynomorphs, for quantitative analysis. The detailed classified methods and criterions can be referred to previous research [4,6,7]. Analysis of 50 (fifty) slide containing palynofacies were sampled from depth rang of (1640m-11760m) based on their quantitative composition of the particulate organic matter content.

Plate 1. 1(spinizonocolpites sp), 2 S. (baculatus), 3, 4 and 9: Palynomorphs(pollen, spore and dinocyte. ,from the X2 WELL. All images ×480. Plate 3: Magnastriatites hawardi. 4: Striatmonocolpites catatumbus (spore) and Arecipites exilimuratus (280) (pollen). 5: dominated by brown and black wood. 6: AOM, TAO (transparent amorphous organic matter) and marine palynomorphs. 7: Magnastriatites hawardi. 8, and 15: phytoclasts. 16: AOM. (All Magnification X400)

Table 1. Showing Spore Color index [4]

The AOM components are common and represent an important organic matter type in this study. Based on the shape, color and fluorescence, the AOM can be classified into granular or gelified forms. The granular formis mainly yellow to brown under natural light and exhibits irregular aggregated shapes formed by fibrous and ultramicroscopic organic particles [9,11]. (Plate 1).

Phytoclasts are plant-derived fragments, including cuticles, cortex tissues, woody tissues and charcoal [2,6]. This group is the dominant composition for most of the source rock samples. Cuticles are generally translucent yellow to light yellow under transmitted natural light and show typical fluorescence (Plate 1).

Palynomorphs are all discrete HCl- and HF-resistant organic-walled microfossils. E.g., spores, pollen, dinoflagellates which are light brown to brown color.

4.4. Thermal Maturity and Kerogen Characteristics

The spore color index of 50 (fifty) slide containing palynofacies sampled from depth rage of (1640m-11760m) is within the mature main phase of liquid hydrocarbon generation [4], having about four to six (4-6) spore color index for the miospore (pollen/spores) and seven to ten (7-10) spore color index for the plant materials(black wood and phytoclast). With about 75% miospore(pollen/spores) and 30% plant material(black wood and phytoclast). This is shown from some sampled depth bellow;

From the Spore Color Index, depth 1682.9m is within the mature main phase of liquid hydrocarbon generation, having about four to five (4-5) spore color index for the miospore and eight to ten (8-10) spore color index for the plant materials. This depth has about 75% miospore and 25% plant material.

Depth 1756.1m is within the mature main phase of liquid hydrocarbon generation, having about four to five (5-6) spore color index for the miospore and eight to ten (7-9) spore color index for the plant materials. Present. This depth has about 65% miospores and 35% plant material

Depth 3170.7m is within the mature main phase of liquid hydrocarbon generation, having about four to five (4-6) spore color index for the miospores and eight to ten (7-9) spore color index for the plant materials. Present. This depth has about 75% miospore and 25% plant material

Depth 3097.6m is within the mature main phase of liquid hydrocarbon generation; having about four to five (4-6) spore color index for the miospore. This depth has about 80%

Depth 3024.4m is within the mature main phase of liquid hydrocarbon generation, having about four to five (4-6) spore color index for the miospores and eight to ten (8-10) spore color index for the plant materials present. This depth has about 80% miospore and 20% plant material.

Depth 3024.4m is within the mature main phase of liquid hydrocarbon generation, having about four to five (4-5) spore color index for the miospores and eight to ten (8-10) spore color index for the plant materials Present. This depth has about 50% miospores and 50% plant material

5. Conclusion

A total of one hundred and ninety (190) ditch cutting samples from X2 Well in the Niger Delta Basin were described using lithostratigraphic and biostratigraphic methods. The sedimentological analysis reveals forty nine (49) lithozones and seven (7) lithofacies units, which were deduced based on their mineralogical composition, textural properties, fossil content, homogeneity and heterogeneity of the lithofacies units. The sand/clay and alternation of sand and shale revealed that the studied interval is within the Benin and the Agbada Formation of the Niger Delta Basin. The sedimentological results suggest sediments deposition in a high to low energy environments, which range between continental to transitional and marine environments. Hydrocarbon play elements which comprises of reservoir rocks, source rocks and traps were described in the well which include 2(two) probable reservoir rocks (Zone 7, with thickness 18.3 meters and zone 15 with thickness 146.4 meters) and six (6)probable source rocks (Zone 2, 4, 6, 8, 10, 12).

Palynofacies analysis of the Well shows that the Well can yield about 70% of oil and 30% of gas. The spore color inference of the Well is both oil and gas but more of oil. The presence of type II kerogen indicates the presence of oil and gas in the Well. The Well also stands out as an excellent source rock potential for hydrocarbon due to the presence of organic matter, which acts as parent material for oil and gas.

Acknowledgements

The authors are grateful to Nigerian Petroleum Development Company (NPDC) and Beninand Earth Probe Ltd Lagos for their assistance in this research.

References

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