Understanding the Complicated Fracture Network in a Fractured Granite Reservoir Using the Borehole Image and Geochemical Log, Case Study from Songliao Basin

Abstract

Granite basement reservoir has become a very important exploration target in Songliao Basin of North-east China. Several exploration gas wells were drilled in the study area, and it shows strong heterogeneity and complexity in reservoir properties. Since it is very difficult to perform accurate formation evaluation in fractured granite reservoir using the conventional open-hole logs, the advanced wireline logging such as borehole resistivity image and geochemical logs were acquired to understand the impact of the fractures on the producibility in the tight granite reservoir. The objective of this research is to understand the complicated natural fracture system in the tight granite reservoir. The high-resolution borehole resistivity image logs could provide abundant information to evaluate the fracture distribution, direction, and intensity. It can also reveal the secondary pores and dissolution enhanced features in the granite basement reservoir. The high-definition spectroscope logs provide significant information for geological description and evaluation of the downhole formations. One parameter "K/Na (potassium/sodium) ratio" was developed to differentiate the composition of the minerals in the formation. It was found that the intervals with high K/Na ratio have higher fracture apertures than other intervals in the case. The reservoir quality is controlled by fracture density and secondary porosity. Different sets of nature fractures have a different angle range or strike, and it will create a complex natural fracture network. The fracture zones with complex fracture network have suffered stronger weathering process than the zones with simple fracture network. The case study shows that the high porosity zone with complex natural fracture network has strong dissolution. The K/Na ratio from geochemical log also have clear anomaly in that zone. It indicates that there has strong chemical weathering process in that zone which has high K/Na ratio than other zones. These zones have complex fractures network and strong dissolution than other zones. Better understanding the controlling factors of the fractured granite reservoir could be helpful for fracturing design and well positioning. The integration of borehole image and geochemical log in the fractured granite reservoir was proven to be effective method in this case study. It could be used as a reference for similar reservoir fracture evaluation.