Normalized and color-filled logarithmic gamma-ray logs to enhance subsurface stratigraphic interpretation of carbonates and siliciclastics

Abstract

Modern petrophysical software has broad capabilities for the display and manipulation of subsurface digital log data and for its integration with core data. Color and scale are two of the most important display attributes that can be used to enhance the visualization and interpretation of rock properties. The gamma-ray (GR) log, the most important log used in subsurface interpretation, is conventionally displayed on a linear scale of American Petroleum Institute (API) units. This makes it difficult to interpret in very clean lithologies with low API values or where the range of values is very large. We determine how displaying GR values on a logarithmic scale enhances the recognition of cyclicity in lithofacies with low GR values concurrently with rocks with very high API values. We further enhance the GR curve with a color-fill pattern that is intuitive and suggests the lithofacies. We calibrate the core-derived lithofacies data to the color-fill pattern, interactively “tuning” it to match lithofacies boundaries, further increasing the value of the methodology. Because of the many factors that cause variation in recorded API values, we normalized the GR curves, either by bulk shifts or statistical means, so that they display the same colors for equivalent lithologies in all wells in a cross section. We have developed two integrated studies demonstrating techniques to improve the display and interpretation of borehole logs, specifically those of Mesozoic carbonates and evaporites of the Middle East and Cenozoic siliciclastic fluvial and marginal marine systems of the Llanos Foothills of Colombia. Many examples of custom color-fill patterns for petrophysical logs could be suggested, the possibilities being limited only by the data available and the interpretation being presented.