SEDIMENTOLOGY AND RESERVOIR QUALITY OF OUTCROPPING SEDIMENTS

ABSTRACT

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.

CHAPTER ONE: INTRODUCTION

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).