Fracture Identification and Classification Using the Resistivity Image Logs in Tight Conglomerate Reservoir: A Case Study from Junggar Basin

The conglomerate reservoir with a very low porosity and permeability is one of the most important reservoirs in Northwest China. The pay zones are very difficult to be identified in a conglomerate reservoir which is more than 200 meters thick, because there is no any clear positive response in the open-hole logs. It is challenging to evaluate this kind of tight reservoir using the traditional methods. Identifying and evaluating fracture zones in a fractured conglomerate reservoir were the objective of this case study. The high-resolution resistivity image logs were widely acquired on the key wells in this research area. Based on image interpretation and core calibration, a fracture quality evaluation method was proposed and applied in this conglomerate reservoir. Traditionally, the natural fractures on wellbore resistivity image are divided into two types: conductive or resistive fracture depends on it is "white" or "dark". Typically, conductive fractures filled with mud are open fractures, while resistive fractures are half-open or sealed and filled entirely or partially with calcite cementations. Based on core observations and descriptions, the core images and wellbore images were well-matched. The fractures picked from wellbore images are all calibrated with the core images. In this case study, the fractures were classified in three types: open, half-open, and sealed. The half-open fractures were mainly identified based on the angle between the fracture strike and maximum horizontal stress direction. The fractures were mainly resistive fractures in the study area. Not all resistive fractures are sealed. They may be half-open when the fracture strike has a low angle with maximum horizontal stress. The test and production data validated the selection of formation "sweet spots" with good fracture quality and identified where stimulation of half-open fractures could improve production significantly. The production data showed that half-open fractures can be the target zone in tight conglomerate reservoirs.