A novel evaluation method of dolomite reservoir using electrical image logs: The Cambrian dolomites in Tarim Basin, China

The Cambrian dolomites in Tarim basin are important hydrocarbon exploration targets. However, these ancient dolostone reservoirs are deeply buried, and the lithology and reservoir space are complex, therefore the logging evaluation of reservoir effectiveness is difficult. This study analyzes the log response characteristics using core data and electric image logs, and divides the dolomite reservoir into four types: pore type, fracture type, vug type and fracture-vug type. Among them, the vug type and fracture-vug type are the main reservoir types with the highest industrial production. Then, this study innovatively proposes the method of point-by-point calibration of electrical image log, which avoids the difference in response characteristics of various electrical image log instruments, and reduces the uncertainty caused by human factors of conventional calibration. The point-by-point calibration method uses local data for comparison and calculates calibration coefficient automatically. The calibrated electrical image log data can maintain better matching and consistency with conventional resistivity in high and low resistivity simultaneously, and it is more truly reflect the borehole-wall resistivity, consequently, can establish a foundation for subsequent reservoir evaluation. Finally, according to the calibrated electrical image log data and the 360 degrees roll-scan core photos, the vugs in-plane porosity on electrical image, the vug connectivity and the porosity spectrum are used to identify reservoir types and evaluate reservoir effectiveness. The vugs in-plane porosity on electrical image can truly reflect the development of fractures and vugs, and quantitatively evaluate the vugs and fractures of the whole borehole-wall by calibrating with core. The vug connectivity index can indicate the isolated vugs and connected vugs, and quantitatively evaluate the connectivity of vugs and fractures, in addition the porosity spectrum can reflect the pore structure of reservoir and identify the reservoir with secondary porosity. The coupling of the three methods can accurately identify high quality dolomite reservoirs with vugs and good connectivity. This novel technology adopts the idea of classification and identification before quantitative evaluation and establishes a new bridge from qualitative analysis to quantitative evaluation. The method proposed helps improve the interpretation accuracy and coincidence rate and provides a reference for the comprehensive evaluation of ultra-deep dolomite reservoirs in the Tarim basin and worldwide.