Qualitative and Quantitative Evaluation on the Fractured Granodiorite Reservoir by the Integration of Advanced Logging Techniques and Core Measures: A Case from East China Sea Basin

Abstract The exploration and exploitation of igneous rock reservoirs have been conducted in offshore oilfield, and CNOOC Ltd has achieved significant breakthroughs in igneous rock reservoir exploration in the Pearl River Mouth Basin, Bohai Bay Basin, and Qiongdongnan Basin, highlighting the vast exploration potential of these reservoirs in China's offshore oilfields. Since 2023, CNOOC Shanghai is leading multiple drilling campaigns to confirm the occurrence of gas and oil in East China Sea Basin. As the igneous rock reservoir is characterized by strong heterogeneity and complexity on reservoir properties, it is very difficult to perform accurate formation evaluation on heterogeneous igneous rock reservoir using conventional open-hole logs. To achieve these objectives, CNOOC shanghai utilized advanced wireline logging techniques and core measurements to qualitatively and quantitatively characterize the fractured reservoir and revealed the impact of the fractures and secondary pores on the producibility. The igneous rock reservoir is deep buried with the depth of more than 4000 meters, porosity less than 5% and permeability less than 0.1mD, which makes the reservoir more complex compared to the conventional reservoirs. The mineral assemblage is very important for igneous rock reservoir because it influences the development of fractures, secondary pores and fluid accumulation. The high-definition gamma-ray spectroscopy logs could provide significant information for geological description and evaluation of the downhole formations. Combined with core XRD data, the customized mineral model is established to characterize the lithologies of igneous reservoir. The igneous reservoir went through strong weathering process and was subjected to several tectonic movements, which lead to the formation of fractures and secondary pores. The high-resolution borehole resistivity image logs could provide abundant information to evaluate the fracture distribution, direction and intensity. It could also reveal the secondary pores and dissolution enhanced features from igneous rock reservoir. Stonely analysis and sonic anisotropy from sonic logs could provide valuable information to analyze effectiveness of the fractures along the borehole wall. Three-dimensional far-field (3DFF) acoustic imaging technology is used to quantitatively depict the lateral extension of fractures and secondary dissolution into the igneous rock reservoir for the first time. Furthermore, the lithofacies are determined by integrating the image log and sonic logging data, and the evolution of igneous rock reservoir is established. The integrated study has revealed the detailed characteristics of the igneous rock reservoir in lithology, lithofacies, fracture type, fracture effectiveness and extendibility, and secondary dissoluted pores, which shows that there is very promising potential in igneous rock reservoir in East China Sea Basin, China.