Geophysical Well-Log Evaluation in the Era of Unconventional Hydrocarbon Resources: A Review on Current Status and Prospects

Geophysical well-log evaluation in the era of unconventional hydrocarbon resources (mainly tight oil and gas, shale oil and gas) is complicated and challenging. This review aims to fill this gap between well-log evaluation and unconventional hydrocarbon resources by characterizing the source rock property, reservoir property and engineering property using petrophysical well logs. The advanced well-log series used for unconventional oil and gas evaluation include nuclear magnetic resonance (NMR) log, image logs, array acoustic logs, elemental capture spectroscopy (ECS) and LithoScanner logs. The source rock property in terms of total organic carbon content is predicted using conventional logs and LithoScanner log. Then petrophysical parameters including porosity, permeability and oil saturation are calculated, and the appearance of natural fracture is predicted from conventional, sonic logs, image logs and NMR logs. Additionally, the reservoir property is evaluated to optimize the favorable layers with high hydrocarbon bearing property and productivity. Brittleness index as well as in situ stress direction and magnitudes are characterized by the comprehensive use of density, sonic log, ECS log and image logs. Then, the engineering property (high brittleness index but low horizontal stress difference) is evaluated to screen out the prospected layers for hydraulic fracturing. The internal relationships between the three types of properties are unraveled, and the geological and engineering sweet spots are optimized by integrating lithology, reservoir quality, hydrocarbon bearing property, source rock property, brittleness and in situ stress magnitude and direction. This multidisciplinary approach provides a comprehensive method for optimizing sweet spots in unconventional play, and will support petroleum geoscientists’ and engineers’ decisions in exploration and exploitation of unconventional hydrocarbon resources.