Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/125 Comunicações Geológicas (2012) 99, 2, 53-59 ISSN: 0873-948X; e-ISSN: 1647-581X

Burial and thermal history modeling and petroleum potential evaluation of the northwestern Niger Delta, Nigeria Modelação da evolução térmica e do soterramento e avaliação do potencial petrolífero do noroeste do Delta do Níger, Nigéria O.J. Ojo1*, I. Akpabio2, J. Frielingsdorf3 Recebido em 21/10/2011 / Aceite em 24/01/2012

Disponível online em Janeiro de 2012 / Publicado em Dezembro de 2012

© 2012 LNEG – Laboratório Nacional de Geologia e Energia IP

Abstract: One of the major hydrocarbon exploration risks or constraints in the Niger Delta is the complex nature of the petroleum systems, as there is lack of geochemical data from deeper and older potential strata. In this study, 1D models of burial and thermal histories were constructed from stratigraphic and well-log data in order to assess the petroleum potential of part of the northwestern Niger Delta basin using Cauldron and PetroMod software. The thermal maturation of the source rock intervals was reconstructed based on crustal thinning during rift, break up, and drift during the Lower and Upper Cretaceous. Bottom-hole temperature data were used to estimate present-day subsurface temperature. Results show that the Eocene and Paleocene source rocks attained sufficient thermal maturities to contribute oil and gas into the Oligocene and Miocene clastic reservoirs. In Operation Mining License (OML) 1 and 40, the Paleocene, which is overmature (modeled Ro% ranges from 0.9 to 3 Ro %) at present, entered the oil window and expelled most of its oil during late Eocene. In the present day, it could be expelling minor volumes of dry gas. The Eocene source rock intervals appear to be the most active at present, having entered the oil window during Oligocene and attained present-day maturities in the range of 0.62 to 0.90 Ro% in most of the wells. However, in OML 38, with relatively higher sedimentation rates, the Paleocene source rocks are presently at the peak of hydrocarbon generation and expulsion whereas the Eocene source rocks in most of the wells are barely mature. The Oligocene intervals in the wells studied are not mature according to modeled vitrinite reflectance ranging from 0.4 to 0.52R% at present day. At present time, average cumulative oil generated and expelled from Paleocene source rocks in OML 1 and 40 are 98,000 kg/m2 and 77,500 kg/m2, respectively. The Eocene source rocks stand at 73,000 kg/m2 and 35,000 kg/m2, respectively. In OML 38, average cumulative oil generated from Paleocene and Eocene source rocks are 95,000 and 51,000 kg/m2, respectively. Only 76% and 1.9% of the generated hydrocarbon have been expelled, respectively. It is only from the Paleocene interval of Abiala 1 (OML 40) that a substantial amount of gas (65,000 kg/m2) has been generated.

Keywords: Paleocene, source rock, heat flow, maturation, Abiala, hydrocarbon. Resumo: Um dos principais riscos de exploração de hidrocarbonetos ou restrições no Delta do Níger é a natureza complexa dos sistemas petrolíferos, pois faltam dados geoquímicos de potenciais estratos mais profundos e mais antigos. Neste estudo, modelos de 1D da evolução térmica e do soterramento foram construídos a partir de dados estratigráficos e logs de poços para avaliar o potencial de petróleo de parte noroeste da bacia do Delta do Niger utilizando os softwares Cauldron e PetroMod. A maturação térmica dos intervalos de rocha-mãe foi reconstruída com base em adelgaçamento crustal durante o processo de rifte, abertura e deriva durante o Cretáceo inferior e superior. Dados de temperatura do fundo dos poços foram utilizados para estimar a temperatura actual sub-superficial.

Os resultados mostram que as rochas-mãe do Paleocénico e Eocénico atingiram suficiente maturidade térmica para contribuir com petróleo e gás nos reservatórios clásticos do Oligocénico e Miocénico. Na Licença de Operação Mineira (OML) 1 e 40, o Paleocénico, que é excessivamente maduro (Ro modelado varia de 0,9 a 3 Ro%) actualmente, entrou na janela de formação de óleo e expeliu a maior parte do seu óleo durante o Eocénico tardio. Actualmente, poderá ainda estar a expelir volumes menores de gás seco. Os intervalos de rocha-mãe eocénica parecem ser mais activos actualmente, tendo entrado na janela de formação de óleo durante o Oligocénico e atingiu maturidades actuais na ordem de 0,62 a 0,90 Ro% na maior parte dos poços. No entanto, na OML 38, com maiores taxas de sedimentação, as rochas-mãe do Paleocénico estão actualmente no auge da geração de hidrocarbonetos, enquanto as rochas-mãe eocénicas estão pouco maduras na maioria dos poços. Os intervalos oligocénicos nos poços estudados não estão maduros de acordo com a modelação da reflectância da vitrinite, variando entre 0,4 a 0.52R% actualmente. Neste momento, a média acumulada de óleo gerado e expelido das rochas-mãe paleocénicas na OML 1 e 40 são 98.000 kg/m2 e 77.500 kg/m2, respectivamente. As rochas-mãe do Eocénico estão em 73.000 e 35.000 kg/m2, respectivamente. Na OML 38, a média acumulada de óleo gerado e expelido das rochas-mãe paleocénicas e eocénicas é 95.000 e 51.000 kg/m2, respectivamente. Apenas 76% e 1.9% dos hidrocarbonetos gerados foram expelidos, respectivamente. É somente a partir do intervalo Paleocénico de Abiala 1 (OML 40), que foi gerada uma quantidade substancial de gás (65.000 kg/m2).

Palavras-chave: Paleocénico, rocha-mãe, fluxo de calor, maturação, Abiala, hidrocarbonetos. 1Department of Geology and Mineral Sciences, University of Ilorin, Nigeria. 2Department of Physics, University of Uyo, Nigeria. 3Shell Petroleum Development Company, PortHarcourt, Nigeria. *Corresponding author / Autor correspondente: solafoluk@yahoo.com

1. Introduction

The Niger Delta basin, located on the western edge of the African continent and southern part of Nigeria, covers an area of 75,000 km2 and consists of 9,000 to 12,000 m of clastic sediments (Fig. 1). The evolution of the basin is rift related and associated with the Benue System and separation of African and Brazilian plates in the early Cretaceous times (Evamy et al., 1978). The basin is divided into five depobelts: northern Delta, Ughelli, central swamp, coastal swamp, and offshore (Knox & Omatsola, 1989). Being a prolific hydrocarbon habitat, the basin geology and hydrocarbon occurrence and distribution have been well studied and reported in various research publications and technical reports (Weber & Daukoru, 1975; Ejedawe, 1981; Ekweozor &

Artigo original Original article

54 O.J. Ojo et al. / Comunicações Geológicas (2012) 99, 2, 53-59

Daukoru 1994). In spite of the deep level of knowledge of the dynamics of petroleum plays in the Niger Delta, understanding of the petroleum charge system, with respect to the charging history and the roles of Cretaceous and Paleocene petroleum systems, retains risk and uncertainty in the exploration strategy. Haack et al. (2000) defined three petroleum system: Lower Cretaceous system (lacustrine), Upper Cretaceous to Paleocene system, and Tertiary deltaic system across the Niger Delta. Another significant constrain in the Niger Delta is the lack of geochemical data from the deeper or older source rocks, due largely to the fact that the shallow total depths rarely reach the older source rocks in the wells. In view of the need for proper evaluation of the petroleum system and to understand and successfully predict hydrocarbon distribution in the Niger delta, this study, covering OMLs 1, 40, and 38, is designed to accomplish the following objectives:

i) establishing the burial and thermal history of the parts of NW fringe of the Niger delta (study area encompasses OMLs 1, 38, and 40),

ii) modeling the timing and extent of maturation of the source rock intervals,

iii) determining the rate and volume of oil and gas generated/expelled through time, and

iv) determining the potential contributions from the Tertiary/Cretaceous source rocks.

Fig.1. Map of OMLs 1, 38, 40 and well locations. Fig.1. Mapa das OML 1, 38, 40 e localização dos poços.

2. Data Set and Methods

2.1. Input Data

The lithologies, which are mainly sandstone and shale at various depths, were properly defined using sand percentage data obtained from well logs. Biostratigraphic ages (based on pollen/spore and foraminiferal markers) at different depths were obtained from the in house data bank.

In order to assess the relative contributions of the source rock intervals in the wells, input data for source rocks such as type of organic matter, total organic carbon (TOC), and

hydrocarbon index (HI) were obtained from the literature (Ekweozor & Okoye, 1980; Udo & Ekweozor, 1988; Bustin, 1988). Kinetic models and algorithms of Ungerer et al. (1990) and Sweeny & Burnham (1990) were used.

2.2. Calibration Data

Bottom-hole temperature (BHT) data were used for calibration for the validity test of the models. The continuous temperature data were interpreted from the temperature logs and also obtained from the well files. The continuous temperature data were considered reliable and to be an excellent indication of the actual subsurface temperature because they were recorded after stabilization, months after mud circulation had stopped.

2.3. Calibration Data

Two different types software were used to run the input data and generate the models for the various wells studied. Cauldron (Shell in-house software) is capable of reconstructing the burial history of the decompacted formations and paleothermal conditions of the stratigraphic intervals through time. PetroMod software (Schlumberger software) was also used to run 1D models using the same data set. This checks the validity of the results and serves to complement the capabilities of Cauldron. Results of modeling are presented visually and numerically, facilitating interpretation of the processes. The results compared favourably.

2.4. Geological Concepts / Assumptions / Controls

Crust Parameter – The heat flow and thermal history of a basin is directly related to its dominant regional tectonics. The basic geological model for the origin of the Niger Delta is that it is a rift basin in which rifting and subsequent thinning was assumed to have started 120 Ma with present crust thickness of about 27 km.

Top boundary condition – For the onshore Niger Delta, within which the area of study falls, present-day surface temperature of 27ºC is assumed, therefore, in all the wells studied, a constant temperature of 27ºC was used.

Bottom Boundary condition – In the Cauldron program, a range of top mantle heat flow of 35+/- 5 mW/m2 was used for the wells. Radiogenic heat production at top of the crust varies from 1.5 to 2.5 µW/m3. For the PetroMod software, the BHT plot was used to calibrate the input by modifying the recent heat flows until the measured and the calculated BHT curves fit the general trend.

Thermal Conductivity – Thermal conductivity of sand is higher than that of shale and, conversely, heat production and geothermal gradient is higher for shale. Thermal conductivity (default) for the sandstone and shale are 3.4+/-1.9 and 1.1+/-0.69 W/m K, respectively.

Source Rocks - Four source rock intervals; Oligocene, Eocene, Paleocene, and Upper Cretaceous were delineated for hydrocarbon potential evaluation. Their estimated geochemical parameters are presented in table 1.

Table 1. Source rock input data.

Tabela 1. Dados introduzidos para as rochas-mãe.

Petroleum potential of the Niger Delta, Nigeria 55

3. Results and Discussion

3.1. Burial History and Heat Flow

OML 1 (Pologbene Field)

In the burial history presented in Figure 2, rapid sedimentation occurred between 48 – 50 Ma, leading to a sudden reduction in the heat flow. The model, using the best match of the temperature data and modeled temperatures, indicates that the present heat flow in Pologbene 2 is 60 mW/m2 (Fig. 3).

Fig.2. Burial history of Pologbene 2, OML 1. Blue line indicates boundaries of decompacted formations, Red indicates isothermal lines and Green indicates thermal maturation. Note the rapid burial between 48-50Ma. Fig.2. Evolução do soterramento de Pologbene 2, OML 1. A linha azul indica fronteiras de formações descompactadas, a vermelha indica linhas isotérmicas e a verde indica maturação térmica. De notar o rápido soterramento entre 48-50Ma.

Fig.3. Modeled heat flow and temperature-depth of Pologbene 2 (PetroMod programme). Fig.3. Fluxo de calor modelado e temperatura-profundidade de Pologbene 2 (Programa PetroMod).

OML 40

In Tsekelewu 1, Opuama 6, Abiala 1, and Tongarafa 1, two periods of rapid sedimentation and burial between 53 – 50 Ma and 29 – 33 Ma and their implications on source rock thermal maturity are predicted (Fig. 4). The modeled present day heat flow is higher (average 70 mW/m2) in Opuama 6 and Tsekelewu 1 whereas a lower heat flow regime is predicted for Abiala 1 and Tongarafa 1 (Fig 5).

The relatively high present day heat flow scenarios in OML 1 and 40 may be due to closeness to basement highs and high amount of shaly units. Tuttle et al (1999) observed higher geothermal gradient in the proximal parts of the Northern Niger Delta.

OML 38

In this study, we have selected six wells (Ajalomi 1, Mosogar 2, Ovhor 1, Asuokpu, Orogho 2, and Umutu 2) where reliable input and calibration data are available. For ease of reference, OML 38 is located about 100-km east of OMLs 1 and 40 (Fig 1). Sedimentation rate is relatively moderate to rapid in most of the wells. Breaks in sedimentation (hiatus) were observed at various times during the burial of sediments in Mosogar 2 (40-35 Ma) (Fig. 6), Orogho 2 (36 Ma), and Asuokpu 1 (10-20 Ma). Rapid burial was predicted in Umutu 2 (between 49-51 and 35-37 Ma), Ajalomi 1 (33-36 Ma), and Orogho 2 (49-52 Ma). The heat flow is generally lower in this area relatively to OMLs 1 and 40 (Fig. 5). Present-day heat flow in the wells range from 25 to 43 mW/m2, the highest recorded in Okporhuru 3. According to Frielingsdorf et al. (2008), rapid sedimentation may cause depression of isotherms and consequently lower heat flows and thermal maturity.

Fig.4. Burial history of Opuama 6, OML 40. Fig.4. Evolução de soterramento da Opuama 6, OML 40.

Fig.5. Heat flow pattern in OML 38 and 40. Note heat flow is higher in OML 40 than OML38. Heat flow is highest in Opuama 6 and Tsekelewu 1. Fig.5. Padrão de fluxo de calor no OML 38 e 40. De notar que o fluxo de calor é superior no OML 40 do que no OML38. O fluxo de calor é maior em Opuama 6 e Tsekelewu 1.

56 O.J. Ojo et al. / Comunicações Geológicas (2012) 99, 2, 53-59

Fig.6. Burial history of Mosogar 2, OML 38. Note the near uniform burial rate and hiatus between 40-35Ma. Fig.6. Evolução do soterramento em Mosogar 2, OML 38. De notar a taxa de soterramento quase uniforme e o hiato entre 40-35Ma.

3.2. Thermal Maturity and Petroleum Potential

According to Magoon and Dow (1994), a petroleum system refers to the complex and dynamic interaction among the essential ingredients and processes that are fundamental and responsible for accumulation of hydrocarbon in a certain stratigraphic position in commercial quantity. These ingredients include source rock, reservoir, traps, and seals. In this work, the Upper Cretaceous, Paleocene, Eocene, and Oligocene are evaluated with respect to their maturation history and hydrocarbon generation and expulsion efficiency through time.

OML 1 (Pologbene Field)

The charge model, using default kinetic models of Ungerer et al. (1990) and Sweeney and Burnham (1990), shows that the Paleocene source which entered the oil window at 0.6 R% during late Eocene (43 Ma) at a depth of 3000 m, would have expelled its hydrocarbons during the early Oligocene. The results indicate that this source-rock interval could have contributed significant hydrocarbons to Oligocene reservoirs, but at present thermal maturity of 3.8 Ro % it is generating minor volumes of gas (Figs. 7&8). The Eocene source rock entered the oil window during the late Miocene and at present time is active and generating black oil (1.25 Ro %). The Oligocene source rock is immature at present. The Cretaceous is already spent, and therefore could not have contributed significantly to the Tertiary reservoirs.

At the present time, the cumulative oil generated and expelled from the Paleocene is 40,000 kg/m2 each whereas the Eocene source rock has cumulative oil generated and expelled of about 135,000 and 78,000 kg/m2 respectively. The gas generated and expelled from the Paleocene and Eocene source intervals stand at 27,000 and 8,000 kg/m2, respectively (Table 2).

Table 2. Cumulative hydrocarbon generated / expelled in OML 1 and 40.

Tabela 2. Acumulado de hidrocarbonetos gerados /expelidos na OML 1 e 40.

Fig.7. Temperature and modeled vitrinite reflectance with depth, Pologbene 2, OML 1. Lower Eocene source rock attained maturity of 1.25 Ro % while Paleocene maturity stands at 3.8 Ro %. Fig.7. Temperatura e reflectância da vitrinite modelada com a profundidade, Pologbene 2, OML 1. As rochas-mãe do Eocénico inferior atingiram a maturidade de 1.25 Ro % enquanto a maturidade do Paleocénico encontra-se em 3.8 Ro%.

Fig.8. Burial history and hydrocarbon zones of Pologbene 1 (OML 1). Fig.8. Evolução do soterramento e zonas de hidrocarbonetos do Pologbene 1 (OML 1).

OML 40 (Abiala 1, Tsekelewu 1, Opuama 6 and Tongarafa 1)

Thermal maturity of source rock intervals of four wells in OML 40 (Abiala 1, Tsekelewu 1, Opuama 6, and Tongarafa 1) was evaluated. The 1D charge model results show that the Paleocene

Petroleum potential of the Niger Delta, Nigeria 57

source rock in the wells (except Tongarafa 1 which entered oil window later at 40 Ma) attained sufficient thermal maturity between 45 to 50 ma and generated/expelled most of its oil during this period. At the present time, with vitrinite reflectance ranging from 2.3 – 3.8 Ro%, the Paleocene would be generating minimal volumes of dry gas (Figs. 9-11). The Eocene source rocks in the four wells, according to the most likely heat scenario, attained present day thermal maturities ranging from 0.65 to 0.90 Ro%, with the exception of Tsekelewu 1 (1.5 Ro%). Therefore, at the present time they are actively generating volatile oil and wet gas (Figs. 9-11). The Oligocene source rocks having present-day maturity levels between 0.45 and 0.52 Ro% are immature to marginally mature.

Fig.9. Present day maturity levels of source rocks in Abiala 1 and Tsekelewu 1, OML 40. Note: Eocene source interval is at the middle stage of hydrocarbon generation and Paleocene is overmature. Fig.9. Níveis de maturidade actuais das rochas-mãe em Abiala 1 e Tsekelewu 1, OML 40. Nota: as rochas-mãe eocénicas estão no estágio intermédio de geração de hidrocarbonetos e as do Paleocénico estão demasiado maduras.

Fig.10. Present day maturity levels of source rocks in Opuama 6 and Tongarafa 1, OML 40. Note: Eocene source interval is at the middle stage of hydrocarbon generation and Paleocene is overmature. Fig.10. Níveis de maturidade actuais das rochas-mãe em Opuama 6 e Tongarafa 1, OML 40. Nota: as rochas-mãe eocénicas estão no estágio intermédio de geração de hidrocarbonetos e as do Paleocénico estão demasiado maduras.

Fig.11. Burial history and hydrocarbon zones in Abiala 1 and Tsekelewu 1, OML 40. Fig.11. Evolução do soterramento e zonas de hidrocarbonetos em Abiala 1 e Tsekelewu 1, OML 40.

The Paleocene source intervals in Abiala 1 and Tsekelewu 1 have generated cumulative oil volumes of 155,000 kg/m2 and 41,000 kg/m2 respectively and have also expelled the same amount of oil (Table 3). The amounts of cumulative gas generated and expelled from this interval in Abiala 1 and Tsekelewu 1 are 65,000 and 10,000 kg/m2 respectively. In Abiala 1, the Eocene source rock has cumulative oil generated and expelled volumes of 95,000 kg/m2 and 10,000 kg/m2 respectively, whereas in Tsekelewu 1 it is about 51000 and 50,000 kg/m2. The higher amount of expelled hydrocarbon in Tsekelewu 1 relative to Abiala 1 may be due to higher maturity at present day, and it is probably responsible for cracking of oil and generation of dry gas. The cumulative oil generated and expelled from Paleocene source rock of Opuama 6 is the same standing at 42,000 kg/m2 and the cumulative gas expelled is 12,500 kg/m2. In Tongarafa 1, Paleocene source interval has cumulative oil generated and expelled of 35,500 and 35,000 kg/m2 respectively. The Eocene source intervals in Tongarafa 1 and Opuama 6 have higher volumes of cumulative oil generated (about 98,000 and 82,000 kg/m2 respectively) but has only expelled little oil (18,500 and 46,000 kg/m2) at present day compared to Paleocene source rocks (Table 2). Substantial amounts of cumulative gas (65,000 kg/m2) have been expelled from the Paleocene of Abiala 1, whereas the cumulative gas expelled from the Paleocene in the other three wells ranges from 3400 to 12,500 kg/m2. The Eocene source rocks in the four wells, have cumulative generated gas volumes ranging from 3,500 to 11,200 kg/m2, with an average of 6,000 kg/m2 (Table 2).

OML 38

Assessment of the maturation history of the wells in OML 38 allowed the prediction of the timing of the hydrocarbon generation and expulsion. The models indicate that the Paleocene source rocks in the Mosogar 2, Umutu 2, Ovhor 1 Asuokpu 1, and Ajalomi 1 wells entered the oil window during early Miocene, and presently are at a middle mature stage generating volatile oil and wet gas (Figs. 12 & 13). The Eocene source rocks are immature or barely at the onset of black oil generation at present and, therefore, could not have generated or expelled any significant oil or gas in the past (Fig. 14). However, in Okporhuru 3 and Ajalomi 1, it appears that the Paleocene source rock entered the oil window during late Oligocene and at present is generating volatile oil and wet gas. The Eocene source rock, with present day vitrinite reflectance of 0.7 Ro%, is currently generating black oil (Fig.15).

The cumulative oil and gas generated and expelled by the various source-rock intervals in the studied wells are presented in table 3. The models show that expulsion of oil from the Paleocene

58 O.J. Ojo et al. / Comunicações Geológicas (2012) 99, 2, 53-59

source started between 25–30 Ma and presently is at its peak, whereas Eocene source rocks are barely entering the oil window at present. Our results indicated that the Paleocene rocks have generated cumulative oil ranging from 162,000 kg/m2 to 52,000 kg/m2, with an average of 103,000 kg/m2 and expelled cumulative oil from 141,000 to 9,000 kg/m2, with an average of 24,000 kg/m2 in six wells. An exception is the Mosogar 2 well with the highest expulsion efficiency and cumulative expelled amount of 141,000 kg/m2. Cumulative gas expelled from the wells range from 3,000 to 9,700 kg/m2, with an average of 5,800 kg/m2.

Generally, the Eocene source rocks have a relatively lower proportion of cumulative oil generated, which ranges from 34000 to 81000 kg/m2, with an average of 58,000 kg/m2. The cumulative expelled is minimal, ranging from 265 to 2000 kg/m2, with the exception of Mosogar 2 and Ajalomi 1 wells which have cumulative expelled of 21,000 and 13,000 kg/m2,respectively. As expected, little or no gas (average less than 1,000kg/m2) had been expelled. The low amount of expulsion is most probably due to the low maturation status at the present time.

Fig.12. Present day maturity levels of source rocks in Mosogar 2 and Umutu 2, OML 38. Note: Eocene is barely mature while Paleocene is at the middle stage of hydrocarbon generation. Fig.12. Níveis de maturidade actuais das rochas-mãe em Mosogar 2 e Umutu 2, OML 38. Nota: as rochas-mãe eocénicas estão fracamente maduras e as do Paleocénico estão no estágio intermédio de geração de hidrocarbonetos.

Fig.14. Burial history and hydrocarbon zones in Mosogar 2 and Umutu 2, OML 38. Fig.14. Evolução do soterramento e zonas de hidrocarbonetos em Mosogar 2 e Umutu 2, OML 38.

Fig.15. Burial history and hydrocarbon zones in Ajalomi 1, OML 38. Fig.15. Evolução do soterramento e zonas de hidrocarbonetos em Ajalomi 1, OML 38.

Table 3. Cumulative hydrocarbon generated / expelled in OML 38.

Tabela 3. Acumulado de hidrocarbonetos gerados /expelidos na OML 38.

4. Conclusions

Present day heat flow is higher in OMLs 1 and 40 (proximal part of the NW Niger Delta) than in OML 38, and higher

Fig.13. Present day maturity levels of source rocks in Ajalomi 1 and Ovhor 1, OML 38. Note: Eocene is barely mature whereas Paleocene is at the middle stage of hydrocarbon generation (volatile oil and wet gas). Fig.13. Níveis de maturidade actuais das rochas-mãe em Ajalomi 1 e Ovhor 1, OML 38. Nota: O Eocénico está fracamente maduro enquanto o Paleocénico está no estágio intermédio de geração de hidrocarbonetos (óleo volátil e gás seco).

Petroleum potential of the Niger Delta, Nigeria 59

sedimentation rates occurred in OML 38 than in OMLs 1 and 40. The Paleocene and Eocene source rocks attained sufficient thermal maturities and would have contributed oil and gas into the Oligocene and Miocene reservoirs.

Paleocene source rocks in OML 38 are presently at the peak of hydrocarbon generation, whereas Eocene source rocks are not mature. Paleocene source rocks in OML 40 and 1 are over mature at present but expelled most of their oil during late Eocene. However, the Eocene source rocks are presently active in generating and expelling volatile oil.

At present, the average cumulative oil generated from Paleocene and Eocene source rocks in OML 40 are 98,000 and 73,000 kg/m2 whereas the average cumulative oil expelled from the Paleocene and Eocene source rocks are 77,500 and 35,000 kg/m2, respectively.

In OML 38, the average cumulative oil generated from Paleocene and Eocene source rocks are 95,000 and 51,000 kg/m2 respectively whereas 73,330 and 993 kg/m2 cumulative oil were expelled from the source intervals respectively.

Cumulative (average) gas generated in from the Paleocene and Eocene source rocks in OML 38 are 5,800 and 880 kg/m2.

Acknowledgements

This work has benefited strongly from the unceasing assistance and support of Segun Obilaja, Herwig Ganz, and Kehinde Ladipo who were involved in the conceptualization of the project and also provided useful comments and suggestions that helped improved the quality of the work. Thank is due to the Shell Petroleum Development Company management and the management of the University of Ilorin for the sabbatical arrangement for the first author. The suggestions of the reviewers which impacted positively on the quality of the manuscripts are appreciated.

References

Bustin, R.M., 1988. Sedimentology and characteristics of dispersed organic matter in Tertiary Niger Delta: origin of source rocks in a deltaic environment. AAPG Bulletin, 72, 277-298.

Ejedawe, J.E., 1981. Patterns of oil incidence and oil reserves in Niger Delta Basin. AAPG Bulletin, 65, 1574-1585.

Ekweozor, C.M., Okoye, N.V., 1980. Petroleum source bed evaluation of Tertiary Niger Delta. AAPG Bulletin, 64, 1251-1259.

Ekweozor, C.M., Daukoru, E.M., 1994. Northern Delta depobelt portion of the Akata-Agbada (!) petroleum system, Niger Delta, Nigeria. In: Magoon, L.B., Dow, W.G. (Eds), The petroleum system – from source to trap. AAPG Memoir, 60, 599-613.

Evamy, B.D., Haremboure, J., Kamerling, P., Knaap, W.A., Malloy, F.A., Rowlands, P.H., 1978. Hydrocarbon habitat of Tertiary Niger Delta. AAPG Bulletin, 62, 1-39.

Haack, R.C., Sundararaman, P., Diedjormahor, J.O., Xiao, H., Gant, N.J., May, E.D., Kelsch, 2000. Niger Delta petroleum systems, Nigeria. In: Mello, M.R., Katz, B.J. (Eds), Petroleum systems of South Atlantic margins. AAPG Memoir, 73, 213-231.

Frielingsdorf, J., Aminul Islam, Sk., Martin Block, Mizanur Rahman, Md., Golam Rabbani, Md., 2008. Tectonic subsidence modeling and Gondwana source rock hydrocarbon potential, Northwest Bangladesh Modelling of Kuchma, Singra and Hazipur wells. Marine and Petroleum Geology, 25, 553-564.

Knox, G.J., Omatsola, E.M., 1989. Development of the Cenozoic Niger Delta in terms of “escalator regression” model and impact on hydrocarbon distribution. In: van der Linden W.J.M. et al., (Eds), 1987 Proceedings KNGMG Symposium on Coastal Lowlands, Geology and Geotechnology. Dordrecht, The Netherlands, Klumer Academic Publishers, 181-202.

Magoon, L.B., Dow, W.G., 1994. The petroleum system. In: Magoon, L.B., Dow, W.G (Eds), The petroleum system – From source to trap. AAPG Memoir, 60, 3-24.

Sweeney, J.J., Burnham, A.K., 1990. Evolution of a simple model of vitrinite reflectance based on chemical kinetics. AAPG Bulletin, 74, 1559-1570.

Tuttle, M.L.W., Charpentier, R.R., Brownfield, M.E., 1999. The Niger Delta Petroleum System: Niger Delta Province, Nigeria, Cameroon, and Equitorial Guinea, Africa. U.S. Geological Survey Open File Report, 99-50-H, 65.

Udo, O.T., Ekweozor, C.M., 1988. Comparative source rock evaluation of Opuama Channel Complex and adjacent producing area of Niger Delta. Nigeria Association of Petroleum Exploration Bulletin, 3, 10-27.

Ungerer, P., Burrus, J., Doligez, B., Chenet, P.Y., Bessis, F., 1990. Basin evaluation by integrated two dimensional modeling of heat transfer, fluid flow, hydrocarbon generation and migration. AAPG Bulletin, 74, 309-335.

Weber, K.J., Daukoru, E.M., 1975. Petroleum geology of the Niger Delta. Proceedings of the Ninth World Petroleum Congress, Tokyo, Japan, 2, 209-221.