Porosity and Lithology Prediction in Eve Field, Niger Delta Using Compaction Curves and Rock Physics Models

M T Olowokere,J.S Ojo

doi:10.4236/ijg.2011.23039

Abstract

The primary objective of this study is to investigate the porosity-depth trends of shales and sands and how they affect lithologies. Compaction curves from well logs of five wells were determined using interval transit time from sonic logs. The depth of investigation lies between 1087 m and 2500 m. Based on the shale and sand trend modeling, the study intends to determine the model to be used for lithology prediction at various depths given the interplay between shale and sand compaction. The improved understanding of the physical properties of shales and sands as a function of burial depth was demonstrated, in conjunction with a good understanding of how compaction affects lithology. The compaction curve for shale and sand lithologies varies with shale being parabolic in form, and sands with linear and exponential in nature. Plots of sonic porosity against depth show great dispersion in porosity values while plotting porosity values against depth for different lithologies produced well-defined porosity trends. This shows decrease in porosity with depth. The negative porosity trend is less marked in sandstones, and faster in shale which suggests that it is possible to make accurate porosity predictions using compaction trend. The porosity trend showed exponential relationship at small depth less than 2500m. The linear and exponential models are not dependable at large depth. The result shows that the compaction models applicable for sandstones do not necessary apply for shales.

Highlights

Various authors have proposed different theoretical models to show the fluid-solid interaction in reservoir rocks for the purposes of both porosity and lithology prediction and fluid substitution (e.g. Bjørlykke, 1998; and Ehrenberg, 1990)
Sclater and Christie (1980) used exponential curves for porosities in sandstone, siltstone and chalk in the Central Graben of the North Sea, but their exponential curve for shale was criticized by Baldwin and Hutler (1985) who proposed a power-law curve for shale compaction
This study showed that velocity-porosity and impedanceporosity relation for Agbada Formation in Eve Field is nonlinear

Summary

Introduction

Various authors have proposed different theoretical models to show the fluid-solid interaction in reservoir rocks for the purposes of both porosity and lithology prediction and fluid substitution (e.g. Bjørlykke, 1998; and Ehrenberg, 1990). These models can only be applied under certain conditions because the theories have some limitations. Athy, (1930); Magara, (1976a); Sclater and Christie, (1980), Magara (1980) Liu and Roaldset (1994) and Selley (1978)) have proposed a variety of models that shows the relationship between porosity and depth. Using exponential curves to describe normal compaction in shale were favoured by Korvin, (1984); Magara, (1980); Selley, (1978); and Haung and Gradstein (1990). Issler, 1992 used the time-average equation of Wyllie et al (1956) to estimate porosity and clay content in consolidated formations

Objectives

Findings

Discussion

Conclusion