SEDIMENTOLOGY AND RESERVOIR QUALITY OF OUTCROPPING SEDIMENTS
Field studies were carried out on outcropping sediments exposed in Lokoja and its environs to gain insight into reservoir characteristics and sedimentological characteristics. Sieve analysis and petrographic analysis were carried out on fresh samples of the sediments to determine their statistical parameters, paleodepositinal environment and their reservoir quality. Granulometric analysis of the Filele sands reveals that the sediments are medium to coarse, moderate to poorly sorted, near symmetrical to positively skewed and mainly leptokurtic, While the granulometric analysis of the Mount Patti sands reveals that the sediments are fine to medium, well sorted to moderate sorting, negatively skewed to positively skewed and mesokurtic to leptokurtic. The petrographic analysis of the Filele sands reveals that the sediments consist averagely of 78% Quartz, 17% Feldspar, and 5% Rock fragment, while the Mount Patti sands consist of 90% Quartz, 6% Feldspar, and 4% Rock Fragment. Ternary diagram also reveals that the sediments are mainly sub-arkose. The average mineralogical maturity index (IMM) for the Filele sands is 3.8 indicative of sub-maturity, while the Patti sands with 11.7 maturity index are indicative of mature sands. Using Field evidences and Bivariate plots of skewness versus sorting and mean versus sorting, a fluvial depositional setting was interpreted for the sediments.
1.1 BACKGROUND TO THE STUDY
Sandstones are sedimentary rocks formed by the cementation of sediment by material cements and they show a great deal of variation in mineral composition, degree of sorting and roundness and they possess quality reservoir characteristics and mineralogy. Sedimentological studies help to determine sediment characteristics of the rocks (sorting, sedimentary structures, grain size, shape, lithology, porosity, texture, maturity etc.). Similarly, petrographic studies are very useful in understanding and interpreting the mineralogical (petrographic) details of rocks – their composition, abundance, and morphology among others. Together, sedimentology and petrography are widely used in the reconstruction of the geological history of rocks – their distances from provenance, paleoenvironmental and conditions of formation and/or deformation, diagenetic processes acting on rocks, tectonic history as well as the stratigraphy.
The need to combine sedimentological and petrographic analyses for paleoenvironmental analyses is borne out of the fact that relying on textural analysis alone may result in gross interpretational errors, especially if diagenetic or disaggregation processes have significantly altered textural properties of such sediments (Wilson and Pittman, 1979).
The sedimentological investigation of some outcropping sediment in southern Bida basin and its environs employed the field mapping and the laboratory studies approach of the sediments sampled and it was used in deducing the porosity and permeability of the outcropping sediments in the study area.
This study involves the determination of the lithology, mineralogy and textural characteristics of the sandstone facies in order to predict the reservoir quality of the sandstone using Sedimentological and petrographic data as obtained from field evidence of some outcropping sediment of southern Bida basin and its environs.
1.2 AIM AND OBJECTIVES
This study is aimed at investigating the Sedimentological and petrographic attributes of outcropping sediments in Lokoja and its environs.
The objectives of this study include:
i. To determine the lithofacies characteristics of the sediments of the study area;
ii. To ascertain the mineralogical composition of the sediments;
iii. To determine the paleodepositional environment in which the sediments where formed using available sedimentological and petrographic data sets; and
iv. To characterize the reservoir properties of the sediments by inferring the porosity and quantitatively determine the permeability.
1.3 LOCATION AND ACCESSIBILITY
The study area is located in Kogi State. It is bounded by latitudes 070 30’ N – 080 30’ N and longitude 006000’E- 0070 00’ E. The area visited include outcrops on road cuts along the Lokoja-Abuja Express Road and include; Filele and Mount Patti.
The study area was accessible by main roads and footpaths. The main is the Lokoja-Abuja express road [(fig. 1)Ojo, 2009].
Figure 1 Geologic map of Southern Bida basin showing (Ojo, 2009).
1.4 RELIEF AND DRAINAGE
The River Niger is present in the study area as the main hydrological element. The Niger River runs in an ESE direction in the southern marginal area/part of the basin. Its floodplain is broad and locally up to 20km wide and marked in most areas by a series of elongated ponds running parallel to the river channel. The main tributaries of the River Niger are River Kaduna (wuya) and Gurara which drain the Northern Nigeria Basement Complex.
The area of study has various elevations above sea level as recorded by GPS and shown below:
Filele (169m), Mount Patti (255m) and Agbaja plateau (407m).
1.5 CLIMATE AND VEGETATION
The study area has an average maximum temperature of 33.20C and an average minimum temperature of 22.80C. It is generally hot throughout the year with an average relative humidity of 68-70 percent. The study area has two distinct weathers, dry and wet seasons. The dry season occurs between November and February while the rainy season is between March and October.
The vegetation type in the study area is Guinea savanna or parkland savanna with tall grasses interspersed shrubs and some trees. The shrubs shed their leaves during the dry seasons in order to withstand some amount of drought. Apart from shedding of leaves, some shrubs have stems with thick bark, thick leaves etc. as adaptive features by the trees or shrubs in low rainfall regions to check excessive transpiration.
1.6 SETTLEMENT AND LAND USE
The study area is the headquarters of Kogi state with various settlements ranging from mainly urban to rural. Main towns in the study area include Ahoko, Kotonkarfi, Idu and Ozi. The main occupation of the residents is Agriculture. Trade and other cottage industries are also common in the area.
1.7 SCOPE OF PRESENT STUDY
This study is focused on determining the reservoir quality from the sedimentological and petrographic attributes of outcropping sediment in Lokoja and its environs in southern Bida basin.
The scope of this study includes:
I. Field Mapping and sample collection of outcropping sediments in the study area.
II. Laboratory studies involving sedimentological and petrographic analyses of collected samples
III. Quantitative determination of permeability of the sediments.
IV. Determination of provenance; and
V. Interpretation of results
1.8 LITERATURE REVIEW
The Campanian Maastrichtian Lokoja Formation, which is chiefly, if not wholly, non-marine clastics, comprises of conglomerates, massive, pebbly to coarse grained sandstones, claystones, siltstones, ironstones and lateritic capping (Adeleye, 1971, 1973 and Braide 1992). Lokoja sandstone is the oldest formation in the Southern Bida Basin being 90-280m thick, overlain by 70-100m thick Maastritichian Patti Formation and about 5-20m thick of Agbaja ironstones overlaying the middle Patti Formation (Braide,1992, Ladipo 1994). Lokoja sub-basin of the Bida Basin is a NW-SE shallow, downwarped trough which resulted from the wrench fault movement associated with the tectonic framework of the Nigerian sedimentary basins (Jones, 1953 and Braide, 1992) which was filled with Campanian-Maastritichian sediments as confirmed by the palaeontological and sedimentological studies (Ojo, 1992, Abimbola 1993). Alluvial fans and deltaic facies that interfinger with lacustrine facies provide an amount of reservoir quality for the formation.
The alternating sandstone and conglomeratic facies of the Lokoja Formation indicate fluvial sediment dominated by mass flow and bed-load deposits within an alluvial and braided stream complex (Braide, 1992; Akande et al., 2006). The immature clast to matrix supported conglomerates are interpreted as debris flow deposit, similar to the poorly sorted conglomerates interpreted as gravity induced alluvial fans of the Upper Benue Trough (Nwajide, 1980). Interbedded sandstones probably represent braided stream channel deposits whereas the claystones are probably flood plain deposits (Pettijohn, 2004). The massive matrix supported conglomerate corresponds to a debris flow in an alluvial fan setting. The conglomeratic unit with graded bedded is interpreted as an abandoned channel deposit. The alternating conglomerates and sandstones of the Lokoja Formation in the lower part of the section reflect continental deposition influenced largely by alluvial and braided stream processes (Akande et al., 2006). The stratified nature of the shales and sandstones provides likely favourable pathways for migration of fluids into potential reservoir rocks made up mainly of fluvial, shelf and flood plain sandstones in the Lokoja and the Patti formation (Obaje, et al. 2011). Outcrop studies have been used by Osokpor et al., (2013), to determine the sequence stratigraphy and environment of deposition for the Bida Basin, and inferred that the Lokoja Formation enabled the discrimination of various subfacies formed in continental paleodepositional settings of dominantly fluvial systems during the Late Cretaceous out-building of sedimentary sequences in the Southern Middle Niger Basin and signaled as lowstand systems tract architecture. The relatively well-sorted sandstones unit of the tidally influenced facies of the Patti Formation has been observed as better characterized reservoir rock compared to the fluvial Lokoja sandstones with the prevalence of alluvial fans, containing poorly sorted massive conglomeratic sandstone, matrix supported conglomerate [(fig, 2), Omali, et al., 2011], proximal to the basement. These continental alluvial fans with obvious clogging of the pore throats by clays and clay-filled minerals will be responsible for the inferred reduced porosity and permeability in the Lokoja sandstone than in the tidally influenced facies (obvious from observed herringbone and cross-stratifications) of the Patti Formation which may provide better reservoir possibilities at greater depths (Obaje, et al., 2011).
Figure 2 Lithologic section of the Lokoja Formation at Filele along Lokoja – Abuja Highway (Omali, et al.,2011).