CHAPTER ONE
1.1 INTRODUCTION
One very important aspect in exploration geophysics which will complement previous data acquisition is the information from well log data (wireline data), this does not only gives information about the petrophysical properties of the subsurface formation but it is a major tool in linking stratigraphy, delineating reservoir properties of a formation, calibrating seismic data and in correlating lithology where more than one wells are available.
Formation evaluation is the practice of determining both the physical and chemical properties of rocks and the fluids they contain. The objectives of formation evaluation are to evaluate the presence or absence of commercial quantities of hydrocarbons in formations penetrated by the wellbore, to determine the static and dynamic characteristics of productive reservoirs, detect small quantities of hydrocarbon which nevertheless may be very significant from an exploration standpoint, and to provide a comparison of an interval in one well to the correlative interval in another well. It can be performed in several stages such as during drilling by mud logging, logging while drilling, during logging (quick look log interpretation) and after logging (detailed log interpretation), by core analysis in the laboratory, etc. Wireline logs are one of the many different sources of data used in formation evaluation.
Using wireline log data, formation evaluation and petrophysical analysis gives reservoir data that can be used for future reserve estimation and reservoir analysis.
1.3 RESEARCH PURPOSE AND OBJECTIVES
The aim of this study is to integrate petrophysical log data to qualify and quantify reservoirs in order to assess the production potential.
The objective includes;
- Knowing the lithology through the identification of sand units from chosen top sand to the last hydrocarbon bearing sand, using Gamma Ray Log.
- Estimation of shale volume and reservoir thickness.
- Assessment of effective porosity
- Determination of water saturation.
- Estimation of log derived permeability.
- Facies analysis by classifying reservoir sands and their depositional environment from the log motifs.
- Identification of hydrocarbon and gas-bearing sands and gas/oil contact from density log in combination with the neutron porosity log.
1.4 SCOPE OF STUDY
The scope of this work borders on using suites of wireline logs, to interpretthe properties of the formation and differentiate sand (reservoir) from shale (non-reservoir) by integrating other Petrophysical logs such as resistivity logs, porosity logs etc. to obtain lithologic sequence. Log cross plots such as compensated neutron log and formation density compensated log will be used to accurately determine the true formation porosity of the reservoir. Porosity determines the storage capacity for hydrocarbons and permeability determines the fluid flow capacity of the rock formation. Saturation is the fraction of the porosity that is occupied by hydrocarbons or by water. This method is also used to determine pore pressure and gas bearing zones within the reservoir. Finally, capillarity determines how much of the available hydrocarbons can be produced. Accurate evaluation of the formation are essential to access the economic viability of these reservoir wells in the Niger Delta oilfield.
1.5 SIGNIFICANCE OF STUDY
- This studywillhelp optimize reservoir characteristics and carry out reservoir monitoring & management of the wells.
- With advent of modern well logging tools with enhanced data analysis reservoir wells cannot be over emphasized. Therefore this study has the tendency to enhance the hydrocarbon potential of the Niger Delta basin.
- It would help to carry out detailed characteristics of the minor & major solid and fluid fractions both in reservoir and in shales (containing varying amounts of clay bound and capillary bound water), in Niger Delta.
1.6 GEOLOGY OF NIGER DELTA
1.6.1 REGIONAL SETTING
- The Niger delta is a Cenozoic sedimentary basin situated on the continental margin of the gulf of guinea in the Equatorial West Coast of Central Africa between latitude 30 and 6 0 N and longitude 50 and 80E (Doust and Omotosola, 1990; et al, 1997). It is situated at the intersection of the Benue Trough and the south Atlantic ocean where a triple junction developed during the separation of the continents South America and Africa in late Jurassic (Whiteman, 1982; Obaje 2009.) it covers an area of about 73,000Sq km extending more than 300km from Apex to mouth and is composed of an overall regressive clastic sequence which reaches a maximum thickness of 30,000 to 40,000 ft. (9,000 to 12,000m). (Evamy et al., 1978; Doust and Omatsola, 1990).The sediment deposited in Niger Delta is supplied by the Niger River which is 4,100km long and rises in the mountains of Sierra Leone to West. The largest tributary is the Benue River with which it has it confluence in central Nigeria. (Shannon and Naylor, 1990). During the Tertiary, the Niger Delta built out into the Atlantic Ocean at the mouth of the Niger-Benue river system, an area of catchment that encompasses more than million square Kilometers (about 1,200,000km2) of predominantly savannah-covered lowlands. (Doust and Omotsola, 1990).The Cenozoic Niger Delta is framed by a set of older, stable mega tectonic elements. At the eastern fringe of the Niger Delta, there is a similar but complex feature, the Calabar Flank is the subsurface continuation of the Oban Massif. The Calabar Flank breaks off along the Calabar hinge Line which trends in a SE/NW direction. To the north of the Cenozoic lie the SenonianAbakaliki Uplift and the post AbakalikiAnambra basin. These latter units were also stable elements throughout Cenozoic time. (Murat, 1970; Merki, 1972). The sedimentary basin of the Niger delta encompasses a much larger region than the geographical extent of the modern Delta constructed by the Niger Benue drainage systems. It includes the Cross River and extends eastwards into the continental margins of neighboring Cameroun and Equatorial Guinea (Reijers et al, 1997). The present day Niger and Benue valleys are developed along areas of Mesozoic and Cenozoic sediments which separate the massifs exposed basement rocks. Westward from Delta is Dahomey basin, a coastal and shelf continent sediment wedge of these areas, the Niger Delta is the only province with substantial oil production (1.29 billion barrels/day in 1987) (Shannon and Naylor, 1990).
