Petrophysical Evaluation of Low Resistivity Sandstone Reservoirs
Authors: Hamada G.M. , Al-Awad M.N.J. 
Published: July 2010

There are many reasons for low resistivity pay zones. It is of crucial importance to know the origin of this phenomenon. The problem with these zones is that the resistivity data interpretation indicates high water saturation, but oil or even dry oil will be produced.

This paper discusses the different reasons sandstone reservoirs can have low resistivity. Clean bearing sandstone has high resistivity, but when this rock contains shale, or heavy minerals such as pyrite, the resistivity can become low. This resistivity change depends on the nature, the volume of clay and heavy minerals. Pyrite shows a good electrical conductivity, that is usually comparable to or even higher than the conductivity of formation water, and can therefore have a larger effect than shale. Low resistivity pay zones are not necessarily shaly sand problems.

In this study, different shaly sand models will be discussed, to propose a suitable shale sand model. Field examples were analysed using different shaly sand models.

It has been found that the modified total shale sand model gives good results. In the case of low shale volumes, the normal saturation equation might give acceptable water saturation values.

Introduction

The reasons for low resistivity phenomena are classified mainly into two groups. The first consists of reservoirs where the actual water saturation can be high, but water free hydrocarbons are produced. The mechanism responsible for the high water saturation is usually described as being caused by microporosity. The second group consists of reservoirs where the calculated water saturation is higher than the true water saturation. The mechanism responsible for the high water saturation is described as being caused by the presence of conductive minerals such as clay minerals and pyrite in a clean reservoir rock. The resistivity data must be corrected for the effect of these conductive minerals to reduce the calculated water saturation to more reasonable levels associated with water free hydrocarbon production.

Most formations logged for potential oil or gas production consist of rocks which without fluids would not conduct an electrical current. There are two types of rock conductivity:Electrolytic conductivity which is a property of water containing dissolved salts andElectronic conductivity which is a property of solids such as graphite and metal sulfides such as pyrite.

Pyrite is a common heavy mineral associated with marine sedimentary rocks. It has a good electrical conductivity that is usually comparable to, or even higher than the conductivity of the formation water. The crystals of pyrite may form a continuous network even at low pyrite concentrations. Measured resistivity on dry pyrite ranges between 0.03 and 0.8 Ω m. Pyrite's conduction is of a metallic (electronic) nature and consequently any transfer of current between water and pyrites is based on conversion from ionic to electronic conduction and vice versa. This leads to polarization at the water-pyrite interfaces with the corresponding frequency dependent electrical properties. Thus the electrical properties of porous rocks with pyrites are strongly dependent on the amount and distribution of pyrite and the frequency of measuring the electrical current.