498 JOUR.GEOL.SOC.INDIA, VOL.92, OCT. 2018 JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.92, October 2018, pp.498-506 Oil Shale Prospects of Imo Formation Niger Delta Basin, Southeastern Nigeria: Palynofacies, Organic Thermal Maturation and Source Rock Perspective Okeke Kachikwulu Kingsley* and Obianuju P. Umeji Department of Geology, University of Nigeria, Nsukka. E-mail: okekekingsleyk@gmail.com*; obianuju.umeji@unn.edu.ng ABSTRACT The search for oil and gas in the Nigerian inland basins and efficacy of the unconventional oil shale in hydrocarbon generation have necessitated the palynofacies characterization, thermal maturation and source rock evaluation of the Imo Formation, updip Niger Delta basin to determine its palynofacies constituents and oil shale potential. The standard acid maceration technique, palynofacies description as well as palynomorph colour index chart systematically compared to thermal alteration index and vitrinite reflectance index values were utilized for the study. The palynofacies components of the Imo Formation is dominant of yellow and brown amorphous organic matter, marine taxa, opaque particles, with few dark brown structured phytoclasts and terrestrial microflora. The source rock content is interpreted as Kerogen Type II - oil prone with abundant marine palynomorphs, amorphous organic matter and few terrestrial microfossils. The spore/pollen colour of the strata is light brown signifying mature and oil generation zone. This correlates to thermal alteration index (TAI) 2+ to 3 and vitrinite reflectance index (%R o ) values of 0.5% to 1.0%. The abundance of the amorphous organic matter of marine origin suggests good source rock which indicates good oil shale prospects. The oil shale prospects of the Imo Formation in the study area are hindered by the low abundance of the palynofacies associations. Field evidence indicate that the grey to slightly light grey shales in hand specimen, as against black carbonaceous shales rich in organic matter suggests that the environmental conditions affected the quality of the organic kerogen. The oil yields may be compensated by the great volume of marine shale deposited in the formation and the abundance of amorphous organic matter of marine origin which is known to be an excellent indicator of oil. The palynofacies model enhances the understanding of palynofacies events and oil shale quality which can be tied to the development of unconventional oil shale, oil fields and subsequent exploration drive in the area. INTRODUCTION The application of palynology in reconstructing ancient stratigraphy, palaeoenvironment, vegetation distribution, climatic changes and hydrocarbon potentials of both conventional and unconventional source rocks have positioned it as a topical research area in earth and natural sciences. The palynofacies model of the outcropping sections of the Imo Formation, Niger Delta demonstrates the efficacy of kerogen analysis in both conventional and unconventional oil and gas. Zelenin and Ozerov (1983) and Zhao et al. (1991) attempted a definition of oil shale. Although there is no general accepted oil shale definition but any good definition must take into cognizance, quantity and quality of kerogen (organic matter), textures and depositional environment. However, oil shale was initially viewed by Colorado School of Mines Research as a rock that contains kerogen, that breaks down into combustible liquids and gases at 900°F. Palynological studies in the Niger Delta basin were mostly in the southern section (subcroping formations) (Fig.3) towards the offshore (Oboh-Ikuenobe, 1992, Nwachukwu and Chukwura, 1986, Bustin, 1988 and Batten, 1982, Okeke et al. 2014) due to its potential as a petroliferous province in conventional and unconventional resources alike. Works on the up-dip Niger Delta basin (outcrop sections) were concentrated in the southern part around Bende and Umuasua area (Oboh-Ikuenobe et al., 2005; Germeraad et al. 1968) mostly on palynostratigraphy. Although palynological studies were carried out in the northern portion around Oba and Onitsha axis (Okeke and Umeji, 2016; Umeji, 2003 and Jan du Chêne et al., 1978.), this work illustrates detailed palynofacies characterization, source rock evaluation, organic thermal maturity and oil shale potential of the formation. Many researchers, (Combaz, 1964; 1977; Bujak et al., 1977; Masran and Pocock. 1981; Tyson 1993, 1995; Whitaker, 1982; 1984) proposed different palynofacies classification schemes to establish a high resolution palynofacies model. The diversity in classification schemes is as a result of different processing techniques and specific scope of study (Okeke, 2017). The scheme used here is that proposed by Tyson (1993, 1995) which described palynofacies in a practical sense as the quantitative and qualitative study of the total particulate organic matter present in a sedimentary rock. Palynofacies analysis reflect depositional environment and cycles associated with progradation and retrogradation. Nwachukwu and Chukwura, (1986) reported that amorphous organic matter of Agbada Formation constitutes 63% of the kerogen, 30% of terrestrial organic matter (woody remains, herbaceous material), recycled or coaly organic matter consists of 7%. The organic matter in the Niger Delta is mainly a mixture of type II and III kerogen (Bustin, 1988, Nwachukwu and Chukwura, 1986). The organic matter content of sedimentary rock is dependent on the environmental control prevalent during the sediment deposition and diagenetic processes. The quantity and quality of the organic matter constituents are very pivotal in the interpretation of palynofacies and oil shale prospects of the Formation. This is because classification and quality of the individual organic matter present in clastic sediments is key to effective palynofacies analysis of any clastic rock. The palynofacies model of the Imo Formation will enhance the understanding of palynofacies events and oil shale prospects of the formation which can be tied to the development of oil fields and subsequent exploration in the area. PREVIOUS WORK Several palynological studies were carried out in the northern portion of the Niger Delta basin (Germeraad et al., 1968; Jan du Chêne DOI: 10.1007/s12594-018-1048-x | 0016-7622/2018-92-4-498/$ 1.00 © GEOL. SOC. INDIA