Selection of low-resistant reservoir rocks in geological cross-section of Helvetian deposits at Letnianske field

Abstract

The main difficulties that arise in the selection of reservoirs in the geological section of the Helvetian deposits at Bilche-Volytska zone are due to the fact that they are characterized by low values of electrical resistivity. Under such conditions, very often, when interpreting the data of electrical and electromagnetic logging, gas-saturated reservoir rocks are characterized by increasing the penetration of filtrate into the formation and are mistakenly referred to as water-saturated rocks. Based on the analysis results of laboratory studies of the core material and data from geophysical well logging (GWL) in the geological section of Helvetian deposits at Letnianske field, two groups of reservoir rocks were identified. The first group of rocks belongs to the low-resistance, and the second - to the typical quartz sandstones. The formation of groups is due to the difference in electrical, radioactive and neutron attributes of reservoir rocks. According to the results of research it is established that the low resistance of Helvetian reservoir layers is caused by the electronic conductivity of pyrite and chlorite grains, which are part of the cementing material in rocks. Low indications of gamma radiation intensity radiation capture of thermal neutrons for low-resistance rocks are associated with high content of bound water in pyrite and chlorite minerals, and increased radioactivity is due to the content of zircon grains and high adsorption attributes of chlorite and clay masses. In order to increase the reliability of electric and electromagnetic logging data interpretation for the separation of reservoir rocks into typical and low-resistance quartz sandstones, it is proposed to use the comparison of the lateral logging curve (LL) with the normalized neutron gamma-ray logging (NGL) curve. The efficiency of proposed method for dividing reservoir rocks into groups was tested on actual material.