welllog

A welllog is a continuous record of the physical properties of geological formations encountered in a borehole. It is created by lowering sensors into the well and recording measurements as a function of depth. Welllogs are used to characterize lithology, porosity, fluid content, and rock mechanics, and they play a central role in formation evaluation for hydrocarbon exploration, groundwater studies, and engineering projects.

Welllogs are obtained in two main contexts: open-hole logging, where measurements are made before casing, and cased-hole logging, where tools are run in a well that has been cased and cemented. Common logging tools include gamma-ray detectors (for lithology), density and neutron porosity tools (for porosity estimation), resistivity tools (for pore-fluid content and hydrocarbon indicators), and sonic or acoustic tools (for rock mechanics and porosity through travel time). Caliper logs measure borehole diameter. Logging can be performed on wireline (reels lowered into the borehole) or during drilling (measurement-while-drilling or logging-while-drilling).

Data from welllogs are typically presented as depth versus value curves and stored in digital formats such as LAS (Log ASCII Standard), which supports multiple log types and quality metadata. Interpretation combines individual logs and cross-plots (for example, gamma-ray versus density-neutron) to identify lithology, estimate porosity, determine fluid content, and infer formation tops and potential hydrocarbons. Quantitative assessments may employ empirical relationships and models to estimate permeability, saturations, or pore pressure.

Limitations of welllog interpretation arise from borehole conditions, mud filtrate effects, borehole rugosity, and casing in later logs. The quality and resolution of a log depend on tool calibration, borehole size, depth of investigation, and data processing. Despite limitations, welllogs remain a foundational tool for subsurface characterization.