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Geophysical well log data are widely used in the field of structural geology, sedimentary geology and petroleum geology. Gaps and misunderstandings are still existing in the scientific interpretation of geophysical well logs. Logging environments and log curves need correction and standardization before interpretation, additionally, there are some special geological phenomena that will mislead the well log interpretation. This review critically highlights the typical misunderstandings existing in the well log data interpretation, and proposes countermeasures as well as scientific interpretation of well logs when encounter these misunderstandings. The factors that affect the well log data acquisition are summarized in terms of types of drilling muds, borehole stability and logging instrument rotation. The vertical resolution of various log series spans a wide range from 5 mm to about 10 m. In the field of structural geology, well logs can be used for determination of stratum attitude, fault recognition, fracture and in situ stress characterization as well as unconformity identification. Lithology and depositional facies can be interpreted using well logs. Well logs aim at finding hydrocarbons, and are used for source rock characterization and logging reservoir evaluation in the petroleum geology field. Then the typical misunderstandings and countermeasures in solving geological issues using geophysical well logs are reviewed from published papers as well as from the authors' personal experiences. This review will provide insights into the scientific interpretation of geophysical well log data, and help solving geological issues for the petrophysicist and geologist. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]