Logging-based identification and evaluation of karst fractures in the eastern Right Bank of the Amu Darya River, Turkmenistan

Abstract

Carbonate gas reservoir in the eastern area on the Right Bank of the Amu Darya River, Turkmenistan, are of low-porosity and with developed fractures. In this area, fractures control reservoir properties and natural gas production, and karst fractures are the most important kind of fractures, so their identification and evaluation are quite necessary. In this paper, fracture types were identified and their occurrence was extracted by using conventional logging and image logging data after core calibration. Then, the distribution characteristics of karst fractures and their controlling effect on reservoirs were studied according to the identification results. And the following research results were obtained. First, karst fractures are mainly of high angle with the characteristic of mono system and the interactive relation of genesis. Second, they are mainly distributed in the upper XVhp layer of Callovian–Oxford Stage and the lower XVa2–XVI layer. Third, they are the main effective fractures in this area. The dissolved pores are connected effectively through the expanded karst fractures by dissolution, and consequently reservoirs of high porosity and permeability are formed and they are the important reservoir type and high-yield gas reservoir in this area. Fourth, karst fractures are related to high-yield wells and high-yield layers in this area, and they also control the distribution of high-yield reservoirs in the lower part of Callovian–Oxford Stage. It is concluded that by virtue of imaging logging and conventional logging data, karst fractures, unfilled fractures, semi-filled fractures and fully filled fractures can be identified and evaluated better. Furthermore, the identification and evaluation of karst fractures deepen the understanding on fractured reservoirs in this area, improve the reservoir evaluation effect, and provide the basis for the target horizon and azimuth optimization of horizontal wells and highly deviated wells. And it is also indicated that the reservoirs with developed karst fractures are the subsequent important drilling targets.