Study of Porosity Loss Due to Compaction in the Cretaceous Upper Bima Sandstone, Upper Benue Trough, N.E. Nigeria

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1 Open access e-journal Earth Science India, Vol. 3 (II), April, 2010, pp ISSN: Study of Porosity Loss Due to Compaction in the Cretaceous Upper Bima Sandstone, Upper Benue Trough, N.E. Nigeria N.K. Samaila 1 and G.P. Singh 2* 1 Geology Programme, Abubakar Tafawa Balewa University, Bauchi, P.M.B Bauchi, Nigeria. 2 Department of Geology, Gombe State University, P.M.B. 127, Gombe, Gombe State, Nigeria. nsamaila@yahoo.com; gps1961@rediffmail.com*corresponding Author Abstract Present paper entails the study carried out on the fluvial Cretaceous Upper Bima Sandstone of the Upper Benue Trough. Compositional and textural changes due to diagenesis have altered the reservoir properties of the formation. Modal analyses reveal that these sandstones comprise of 71.21% quartz, 18.66% feldspars and 10.05% rock fragments. The average compactional porosity loss (COPL), cementational porosity loss (CEPL), and compaction index (ICOMPACT) in the Upper Bima Sandstone have been computed as 13.77%, 3.05% and 0.82 respectively. The sum of these values subtracted from the original porosity indicates that the initial porosity of the formation has been reduced to 18.18%. A cross plot of COPL versus CEPL shows that compaction is responsible for reducing the initial porosity of the sediments. Key Words: Bima Sandstone, Benue Trough, diagenesis, porosity loss, compection Introduction The Benue Trough is a NE-SW trending sedimentary basin extending for over 1000 km from the Niger Delta to the Chad Basin (Wilson and Guiraud, 1992). It consists of up to 5000m of Cretaceous sediments and is contiguous with the rift basins of the Niger, Chad and Sudan Republics (Akande, 2004). Benkhelil (1989) gave detail description of tectonic setting, sedimentary fill and tectonic phases of the Benue Trough. Accounts on the geology of the Benue Trough have been given by Carter et al. (1963), Cratchley and Jones (1965) amongst many other authors, indicating that the trough is flanked on either side by crystalline rocks of the Nigerian Basement Complex composed mainly of Precambrian to Early Paleozoic granites and gneisses. The Bima Sandstone, an entirely continental formation throughout the Upper Benue Trough (Fig. 1), is the basal part of the sedimentary succession (Table- 1) in the basin. The sandstone has been interpreted to be of alluvial fan to braided river origin (Benkhelil, 1989; Guiraud, 1990). It lies unconformably on the Precambrian Basement Complex rocks. The Bima Sandstone is a sequence that shows an upward maturation reflecting change from periods of tectonic activity to stable condition (Guiraud, 1993). Lithostratigraphic terms (the lower Bima member, middle Bima member, and upper Bima member) were given to the formation to informally denote major units of sediments formed in direct relationship with tectonic events that took place during the Early Cretaceous times (Guiraud, 1990). The upper Bima member, referred to in this paper as the Upper Bima Sandstone, consists of sandy deposits 105

2 Study of Porosity Loss Due to Compaction in the Cretaceous Upper Bima Sandstone, Upper Benue Trough, N.E. Nigeria: N.K. Samaila and G.P. Singh containing ubiquitous cross bedding and diversity of soft-sediment sedimentary structures (Samaila et al., 2005, 2006; Samaila, 2007). Fig.1: Simplified geological map of the Upper Benue Trough. (1) Quaternary alluvium; (2) Tertiary volcanics; (3) Kerri kerri Formation; (4) Gombe Sandstone; (5) Pindiga Formaion; (6) Yolde Formation; (7) Bima Sandstone; (8) Burashika Complex (Mesozoic volcanism); (9)Undifferentiated Basement Complex. Main Fault zones; BL: Burashika Fault; KL: Kaltungo Fault; TL: Teli Fault (After Benkhelil, 1989). Study of porosity and permeability of rocks is of prime importance in relation to the search for oil, gas and underground water since the pore system is the channel for movement as well as the storage of fluids. Permeability of a rock, which is the property of allowing the passage of fluids through its pore spaces, is related to grain size; it increases with increase in pore-throats size (Beard and Weyl, 1973). Scherer (1987) explained further that porosity and permeability generally decreases in poorly sorted sands. Indurated samples of the Upper Bima Sandstone were cut into thin sections and studied to determine composition of the samples by carefully counting individual grains, analyzing their shapes and inter-granular relationships. Percentages of framework, non-framework and non-granular components of the grains in thin sections were calculated. Parameters used for the estimation of diagenetic processes (compaction and cementation) that affected porosity in the formation were determined from the modal analysis data. The data generated from each thin section include the volumetric proportion of minerals in the rock (modal composition), macro-porosity, grain sizes, sorting, roundness and sphericity. This paper is aimed at showing the effects of diagenesis on reservoir potential of the Upper Bima Sandstone.

3 Open access e-journal Earth Science India, Vol. 3 (II), April, 2010, pp ISSN: Table-1: Stratigraphic succession of the Upper Benue Trough (From Samaila et al., 2008) AGE PALEO- ENVIRONMENT GONGOL A BASIN YOLA BASIN LAMURDE- LAU BASIN Quaternary Pliocene Miocene Conntinental Biu Basalts Longuda Basalts Oligocene Eocene Paleocene Kerri-Kerri Fm Maastrichtian Continental/ Transitional Gombe Sandstone Campanian Santonian Lamja Sandstone Lamja Sandstone Coniacian Numanha Fm Numanha Fm Turonian Sekuleye Fm Sekuleye Fm Cenomanian Marine Pindiga Jessu Fm Jessu Fm Formation Dukul Fm Dukul Fm Upper Albian Yolde Formation Bima Sandstone (B 3 ) Late Aptian Bima Sandstone (B 2 ) Early Aptian Late Jurassic? Continental Bima Sandstone (B 1 ) Pre-Cambrian Basement Complex 107

4 Study of Porosity Loss Due to Compaction in the Cretaceous Upper Bima Sandstone, Upper Benue Trough, N.E. Nigeria: N.K. Samaila and G.P. Singh Petrography of the Upper Bima Sandstone Modal study of 17 thin sections of the Upper Bima Sandstone shows quartz, feldspars and rock fragments constituting the main components of the Upper Bima Sandstone. Raw values from the modal analysis showed that 25-50% volume of the samples is quartz, 5-25% feldspar, % rock fragments, 1-3% mica, % heavy minerals, 0.2-5% opaque minerals, 2-20% cements and 10-23% microporosity. The framework components were recalculated and tabulated (Table-2). Results show an average framework composition of 71.21% quartz, 18.66% feldspars and 10.05% rock fragments (Table-2). Overgrowth of quartz was not observed in thin sections of the Upper Bima Sandstone (Samaila, 2007). Some granular but non-framework components (void fillers including cement and matrix) (Tables-3) as well as non-granular components (including grain size, sorting, rounding and sphericity) (Table-4) were also determined from the thin section study. Roundness, which is a function of grain composition, grain size, and type of transport process and distance of transport, shows that the representative samples of the Upper Bima Sandstone are generally angular to sub-angular grains with a few that are sub-rounded (Fig. 2). Fig.2: Thin section photomicrographs of some representative samples of the Upper Bima Sandstone (A and B) under cross polarizer, showing highly fractured medium- to coarse-grained, poorly sorted arkose (x60).

5 Open access e-journal Earth Science India, Vol. 3 (II), April, 2010, pp ISSN: Some parameters from Tables-3 & 4 were substituted into the three equations developed by Ehrenberg (1989) and Lundergard (1992) to determine the porosity loss due to compaction and cementation using the initial porosity (P i ) of the formation assumed as 35%. Total optical porosity or macro-porosity (P o ), and volume-percent pore filling cement (C) attributed to compaction and cementation, was calculated (Table- 4). The sum of P o and C is equal to intergranular volume (IGV). Cement was not distinguished into primary and secondary because of equipment limitation, however, iron oxide cement, undifferentiated authigenic and detrital clays were detected from the petrographic study (Fig. 2). The compactional porosity loss (COPL), cementational porosity loss (CEPL), and compaction index (ICOMPACT) were clearly shown in Table-5. Discussion and Conclusions The Upper Bima Sandstone has shown varying degrees of diagenetic effects that contributed in reducing the reservoir properties of the formation. The sediments were subjected to severe diagenetic processes as a result; most of the feldspars, rock fragments and other detrital grains were partly dissolved or removed (Fig. 2). From both petrological and petrographical studies, the Upper Bima Sandstone is texturally and mineralogically immature (Samaila, 2007). Although development of secondary minerals (kaolinite, goethite, hematite, gibbsite, etc.) in the formation is capable of decreasing the porosity and permeability through pore-space occupancy (Samaila, 2007), this study is focused on the effect of compaction and cementation on diagenetic processes that significantly altered the original grain matrix ratio in the Upper Bima Sandstone. Detailed diagenetic analyses have not been carried out to ascertain whether different phases of dissolution, diffusion and precipitation of silica cements existed during the evolution of the Upper Bima Sandstone. However, the drastic decrease in grain size and sorting observed in some thin sections around the extensional fault zones in the formation is an indication that great amount of pressure solution was promoted (Samaila et al., 2008). Overgrowth of quartz, generally related to silica derivation from pressure solution, and usually signified by line of very small clay or dust particles or ferruginous coatings was not observed in thin sections of the Upper Bima Sandstone (Samaila, 2007). Sippel (1968) observed that light microscopy alone does not reveal all of quartz overgrowth; it takes the use of cathodoluniscence to show all of it. The undetected/ and or absence of overgrowth of quartz in the thin sections of the Upper Bima Sandstone is an indication that pressure solution due to compaction is not the most important source of silica in the entire formation. Compaction induced fracturing in form of microcracks on brittle quartz grains is fairly common in the Upper Bima Sandstone (Fig. 2). These fractured grains are mostly associated with outcrops that are adjacent to the extensional fault zones in the formation. Healing of the microcracks by silica was observed in the field, characterized by several sets of faults and microfaults of more resistant deformed sandstone (Fig. 3). The phenomenon is associated; with cataclasis along breccia-gouge zone induced by tectonic pressure solution (Samaila, et al., 2008). 109

6 Study of Porosity Loss Due to Compaction in the Cretaceous Upper Bima Sandstone, Upper Benue Trough, N.E. Nigeria: N.K. Samaila and G.P. Singh Fig.3: Microfaults (deformation bands) affecting the Upper Bima Sandstone, appearing as series of light colored, more resistant strands with slip surfaces on the order of few millimeters to a few centimeters. Fig. 4: Cross plot of compaction porosity loss versus cementation porosity loss for the Upper Bima Sandstone (Using Lundergard s, 992, scheme). The disaggregated samples of the Upper Bima Sandstone used for sieve analyses are generally poorly sorted, therefore its initial porosity was assumed to be 35% (Samaila, 2007), although Manus and Cogan (1974) showed that initial porosity of freshly deposited sands is 40-45%. The average values of COPL, CEPL and ICOMPACT in the Upper Bima Sandstone have been computed as 13.77%, 3.05% and 0.82 respectively (Table-5). The sum of these values was subtracted from the original

7 Open access e-journal Earth Science India, Vol. 3 (II), April, 2010, pp ISSN: porosity. The result showed a reduction of the initial porosity of the Upper Bima Sandstone to 18.18%. This value is in the range of values for some giant and super giant oil and gas fields found in siliclastic reservoir rocks of the world (14-32%) (Morse, 1994). It is clear from the COPL-CEPL diagram, that apart from the isolated cases around the major faults, where cementation played bigger role in reducing the initial porosity of the sediments, compaction was generally responsible for the porosity reduction (Fig. 4). The result agrees with work done on other sandstone formations (Ehrenberg, 1989; Houseknecht, 1987; Lundergard, 1992). The diagonal line in Fig. 4 indicates a divide between porosity loss due to compaction or cementation (Lundergard, 1992). Compaction index equals 1.0 when all porosity loss is by compaction but equals 0.0 when porosity loss is by cementation. Appendix: Table Nos. 2-5 at the end of the paper References Akande, S.O. (2004) Cretaceous source rocks and thermal maturation in the Nigerian sedimentary basins: Implication for offshore petroleum prospects. NAPE news, v.3 (5) Sept-Oct Akande, S.O. and Erdtmann, B.D. (1998) Burial metamorphism (thermal maturation) in Cretaceous sediments of the Southern Benue Trough and Anambra Basin, Nigeria. American Association of Petroleum Geologists, Bulletin, v. 82 (6), pp Benkhelil, J. (1989) The origin and evolution of the Cretaceous Benue Trough (Nigeria). J. African Earth Sciences, v. 8, pp Benkhelil, J. and Robineau, B. (1983) Le fosse de la Benoue est-il un rift? Bulletin des Centres de Recherches Exploration- production Elf- Aquitaine, v. 7, pp Carter, J.D., Baber,W., Tait, E.A., and Jones, G.P. (1963) The Geology of Parts of Adamawa, Bauchi, and Borno Provinces in North Eastern Nigeria. Geological Survey of Nigeria Bulletin. v. 30, 99p. Cratchley, C.R. and Jones, G.P. (1965) An interpretaion of the geology and gravity anomalies of the Benue valley, Nigeria. Overseas Geological Survey and Geophysics Paper, v. 1, 24p. Ehrenberg, S.N.(1987) Assessing the relative importance of compaction processes and cementation to reduction of porosity in sandstone: discussion; compaction and porosity evolution of Pliocene sandstone Venture Basin, California: discussion. American Association of Petroleum Geologits, Bulletin, v. 73, pp Guiraud, M. (1990) Tectono-sedimentary framework of the Early Cretacous Continental Bima Formation (Upper Benue Trough, N.E. Nigeria). Journal of African Earth Sciences (Special Publication), v. 10 (1/2), pp Guiraud, M. (1993) Late Jurassic rifting Early Cretaceous rifting and Late Cretaceous transpressional inversion in the Upper Benue Basin (N.E. Nigeria). Bulletin Centres Recherches Exploration et Production Elf Aquitane, v. 17, pp Houseknecht, D.W. (1987) Assessing the relative importance of compaction processes and cementation to reduction of porosity in sandstones. American Association of Petroleum Geologists, Bulletin, v. 71, pp Manus, R.W. and Coogan, A.H. (1974) Bulk volume reduction and pressure-solution derived cement. J. Sedimentary Petrology, v. 44, pp Morse, D.G. (1994) Siliclastic reservoir rocks. In: L.B. Magoon, and W.G. Dow (eds.) The Petroleum system- from source rock to trap. American Association of Petroleum Geologists Memoir, v. 60, pp Samaila, N.K. (2007) Reservoir Potentials of the Upper Bima Sandstone in the Yola and Lau- Lamurde Basins, Upper Benue Trough, Northeastern Nigeria. Unpublished Ph.D. thesis, 201p. Samaila, N.K. Dike, E.F.C. and Obaje, N.G. (2008) Microstructures in the Cretaceous Bima Sandstone, Upper Benue Trough, N.E. Nigeria: Implication for hydrocarbon migration. Journal of African Earth Sciences v. 50 (1), pp

8 Study of Porosity Loss Due to Compaction in the Cretaceous Upper Bima Sandstone, Upper Benue Trough, N.E. Nigeria: N.K. Samaila and G.P. Singh Samaila, N.K. Abubakar, M.B., Dike, E.F.C. and Obaje, N.G. (2006) Description of soft-sediment deformation structures in the Cretaceous Bima Sandstone from the Yola Arm, Upper Benue Trough, Northeastern Nigeria. J. African Earth Sciences, v. 44, pp Samaila, N.K. Braide, S.P., Dike E.F.C. and Suh, C.E. (2005) Study of soft-sediment structures in the Cretaceous Bima Sandstone of the Yola Arm, Upper Benue Trough, northeastern Nigeria. J. Mining and Geology, v. 41(1), pp Sippel, R.F. (1968) Sandstone petrology, evidence from luminescence petrography. J. Sedimentary Petrology, v. 38, Wilson, M. and Guiraud, R. (1992) Magmatism and rifting in Western and Central Africa, from Late Jurassic to Recent times. Tectonophysics, v. 213, pp About the authors Dr. Nuhu Kadai Samaila is Associate Professor of sedimentology/petroleum geology at Abubakar Tafawa Balewa University, Bauchi, Nigeria. He received his Masters degree in geology from the Ahmadu Bello University, Zaria and Ph.D. in sedimentology/petroleum geology from the Abubakar Tafawa Balewa University, Bauchi. Major themes of his research have been related to reservoir potentials, depositional environments and tectonic settings of frontier basins in Nigeria. Dr. Samaila is member of various professional bodies including the Nigerian Mining and Geosciences Society (NMGS) and Council of Nigerian Mining Engineers and Geoscientists (COMEG). Dr. G. P. Singh is Senior Lecturer of Geology at Gombe State University, Gombe, Nigeria. He obtained his Masters and Ph. D. Degrees in Geology from Banaras Hindu University, Varanasi, India. He completed numerous prestigious assignments, including Principal Investigator of independent Young Scientist Research Project of D.S.T. and post-doctoral research sponsored by C.S.I.R. His early research work focused on organic petrology and geochemistry, source rock evaluation and Petroleum Systems. Since 2004, he has also concentrated on Sedimentological applications to petroleum generation and production, palaeoenvironment and reservoir management.

9 Open access e-journal Earth Science India, Vol. 3 (II), April, 2010, pp ISSN: Appendix Table-2: Modal analysis data of the framework components of some representative samples of the Upper Bima Sandstone. Serial Sample Quartz Feldspars Rock Fragments Modal value Modal value (%) Potassium Feldspar Plagioclase Feldspar Modal value (%) Modal Value Modal Value (%) Q/F/L 1 SB /13/0 2 SB /25/17 3 SB /17/13 4 BSB /25/13 5 BST /0/18 6 BST /0/0 7 SB /31/23 8 SB /38/4 9 BSS /21/1 10 BSS /33/7 11 SB /15/12 12 BSB /0/10 13 BSS /7/27 14 SB /37/15 15 BST /0/10 16 BSB /19/8 17 SB /31/0 Average Table-3: Estimated percentages of non-framework components from modal analysis of some representative samples of the Upper Bima Sandstone. Serial Sample Mica Heavy minerals Opaque Minerals Cements Optical Porosity% Detrital clay (D) 1 SB SB SB BSB BST BST SB SB BSS BSS SB BSB BSS SB BST BSB SB

10 Study of Porosity Loss Due to Compaction in the Cretaceous Upper Bima Sandstone, Upper Benue Trough, N.E. Nigeria: N.K. Samaila and G.P. Singh Table-4: Parameters used in determining porosity reduction in the Upper Bima Sandstone from the initial porosity of 35% at the time of deposition Serial Sample *Grain size *Sorting **Rounding **Sphericity 1 SB SB SB BSB BST BST SB SB BSS BSS SB BSB BSS SB BST BSB SB Table-5: Modal analysis of non-granular components of some representative samples of the Upper Bima Sandstone (based on**pettijohn, 1975, *Beard and Weyl, 1953). Serial Sample # P i IGV IGV*100 P i*igv 100-IGV CEM COPL CEPL ICOMPACT 1 SB SB SB BSB BST BST SB SB BSS BSS SB BSB BSS SB BST BSB SB Average