Abstract

A careful examination of the mud log and sidewall cores in certain interval of hydrocarbon anomaly, dry oil from low resistivity sands was surprisingly discoverded and confirmed afterward by well testing results. The problem with these sands is that the resistivity logs indicate high water saturation, but water free hydrocarbon will be produced.

This paper discusses the different reasons sandstone reservoirs can have low resistivity. The mechanisms resposibles for low resistivity phenomenon are described as being caused by the inclusion of clay or pyrite minerals and as being due to microporosity. Clean bearing sandstone has high resistivity, but when this rock contains clay, or heavy minerals such as pyrite, the resistivity can become low. 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. In this work, different shaly sand models will be discussed and applied in two field examples to correct the calculated water saturation from shale effect to get the true water saturation level. The contribution of NMR log in solving problems of low resistivity microporisity sandstone reservoirs was iluustrated by a third field example.

Introduction

The reasons for low resistivity phenomenon 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 the more reasonable levels associated with water free hydrocarbon production. High surface areas of certain inclusions e.g. clay minerals can cause high water saturation, although other mechanisms described as high capillarity can bind large amounts of water. In sandstones, high capillarity may be due to the existence of high to moderate surface clay minerals such as kaolinite or illite. In carbonates, high capillarity may be attributed to microporosity caused by recrstallization, dolomitization or oolites that creates seconday porosity.1,2

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: a) Electrolytic conductivity which is a property of for instance water containing dissolved salts and b) Electronic 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 (m. Pyrite's conduction is of metallic (electronic) nature and consequently any transfer of current between water and pyrites is based on conversion from ionic to electronic conduction and vise versa. This lead 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 the measuring the electrical current. The main problem with minerals such as pyrite is how to estimate their volume and the distribution from well logs.3,4

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