RESERVOIR QUALITY ASSESSMENT AND CHARACTERIZATION OF SANDSTONE UNITS IN AFIKPO AREA, SOUTHEASTERN NIGERIA

ABSTRACT

Reservoir quality assessment and characterization of sandstone units was carried out across the Afikpo area of the Southern Benue Trough. The study involved field investigations and laboratory studies/analyses. Field samples were subjected to Grain Size Analysis (GSA), Sand Equivalent test and Methylene Blue test. Studies revealed that the sandstones are friable, cross-bedded and show coarsening upward motif. Results from lithofacies analysis indicate seven (7) lithofacies deposited in a low to high energy environment. They are (1) Dark Gray Shale facies (Micaceous Dark Gray Shale Facies and Fossiliferous Dark Gray Shale Facies), (2) Bioturbated Sandstone Facies, (3) Wave Rippled Sandstone Facies, (4) Cross Stratified Sandstone Facies, (5) Horizontal/Laminated Sandstone Facies (6) Heterolithic Sandstone Facies, and (7) Pebbly/Conglomeritic Sandstone Facies. The Micaceous Dark Gray Shale Facies belongs to the Eze-Aku Group while other liithofacies are of the Nkporo Group. Granulometric analysis of sand samples indicates sediments that are poorly to well sorted, generally positively to very positively skewed and mesokurtic to leptokurtic. Results from bivariate plots indicates river sand deposition while multivariate plots indicate the sediments are fluvial and deposited in a shallow marine environment. Out of 18 locations sampled, 13 locations had Sand Equivalent values ranging between 90% – 99%, 4 locations had values between 80% – 89% and one location had 77%. These results indicate that a high number of the samples contained little clay because the higher the Sand Equivalent values, the cleaner the sands. The Methylene Blue_F test revealed that the clays contained in the samples are not susceptible to moisture (do not swell) as they had Methylene Blue_F values ranges of 1.7g/kg and 3.3g/kg. These values do not exceed the Methylene Blue_F limit which is 10g/kg. Permeability results range from 35.07mD to 4112.56mD indicating moderate to excellent reservoir while porosity values indicate poor to good with value range of 7.0 – 15.7%. Regression analyses showed a good correlation between porosity and permeability with R² (coefficient of correlation) being 74.6%. There was a weak correlation between Sand Equivalent and Methylene Blue_F Tests with R² being 39.5%. Thus, combining results derived from the various analyses, sandstones within the study area possess good reservoir qualities especially to house gas hydrocarbon.

TABLE OF CONTENTS

Title Page                                                                                                     I

Approval Page                                                                                                   II

Dedication                                                                                                       III

Acknowledgement                                                                                      IV

Abstract                                                                                                            V

Table of Contents                                                                                        VI

List of Figures                                                                                              X

List of Tables                                                                                                       XIII

CHAPTER ONE:     INTRODUCTION                                                             1

1.1       Background Information                                                                      1

1.2       Location and Accessibility of the Study Area                              3

1.3       Study Objectives                                                                                        3

1.4       Scope and Methodology                                                                              5

            1.4.1    Preliminary Study                                                                     5

            1.4.2    Detailed Mapping                                                            6

            1.4.3    Laboratory Analyses                                                             6

1.5       Literature Review                                                                                 7

CHAPTER TWO: GEOLOGICAL SETTING                                                9

2.1       Regional Geographic Setting                                                                     9

            2.1.1    Topography                                                                    9

            2.1.2    Drainage                                                                                        9

            2.1.3    Climate                                                                                      13

            2.1.4    Vegetation                                                                                   18

2.2       Regional Stratigraphic Setting                                                               21

            2.2.1    Tectonic Setting                                                                         21

            2.2.2    Stratigraphic Setting                                                                 24

CHAPTER THREE: FACIES DESCRIPTION                                          29

3.1       Rock Facies                                                                                                29

3.2       Dark Gray Shale Facies                                                                           33

3.2.1    Micaceous Dark Gray Shale Facies                                               33

3.2.1.1             Interpretation                                                       35

3.2.2    Fossiliferous Dark Gray Shale Facies                                        35

3.2.2.1             Interpretation                                                     35

3.3       Bioturbated Sandstone Facies                                                                    36

            3.3.1    Interpretation                                                                              36

3.4       Wave Rippled Sandstone Facies                                                                36

            3.4.1    Interpretation                                                                                   39

3.5       Trough Cross Stratified Sandstone Facies                                                39

            3.5.1    Interpretation                                                             39

3.6       Herring-bone Cross Stratified Sandstone Facies                                    39

3.6.1    Interpretation                                                                                                  41

3.7       Tabular Cross Stratified Sandstone Facies                                41

            3.7.1    Interpretation                                                                 41

3.8       Horizontal/Laminated Sandstone Facies                             41

3.8.1    Interpretation                                                                                                  41

3.9       Heterolithic Facies                                                                       41

3.9.1    Interpretation                                                                                                  45

3.10     Pebbly/Conglomeritic Sandstone Facies                                     45

3.10.1  Interpretation                                                                                                  45

3.11     Lithofacies Correlation                                                                 64

3.12     Lithofacies Association                                                        66

3.13     Depositional Model                                                               68

CHAPTER FOUR: SEDIMENTOLOGICAL ANALYSIS                             69

4.1       Introduction                                                                                     69

4.2       To prepare Methylene Blue                                                             70

4.3       Laboratory Tests                                                                                     70

            4.3.1    Samples Tested                                                                        70

            4.3.2    Sand Equivalent Test                                                               70

        4.3.2.1             Sand Equivalent Test Apparatus                            70

                        4.3.2.2             Procedure                                                               72

                        4.3.2.3             Calculations                                                                 74

                        4.3.2.4             Precautions                             &n