Journal de géologie et géophysique
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Recognition of Lithostratigraphic Breaks in Undifferentiated Rock Units Using Well Logs: A Flow Chart

Shaaban FF, Al-Rashed AR

Location of subsurface lithostratigraphic boundaries (breaks) in a drilled sequence requires high precision in identification of the faunal and lithofacies of the sediments or rocks above and below them. Occasionally, difficulties face well site geologists and stratigraphers in locating these boundaries due to the absence of guide faunal assemblages and/or major lithological changes. A new approach is established to delineate breaks between undifferentiated subsurface rock units and to verify the most likely cause for their occurrence using Natural Gamma Ray Spectrometry and High Resolution Dipmeter logs. This depends on the physical response of these tools to the overall characteristics of the rocks penetrated below and above the concerned break. Abrupt changes in the SGR, CGR, Th/K and Th/U of the NGS logs indicate variations in the contents of the radioactive minerals, and consequently changes in geological conditions of deposition of the rocks penetrated i.e. lithostratigraphic breaks. Changes in the quality, density, and regularity of the Dipmeter log patterns of strata are also detected below and above break. A flow chart, illustrating the steps followed for the recognition of lithostratigraphic breaks, is suggested. Ambiguities that may arise in differentiation between unconformities and faults have been discussed.