Influences of depositional environment and diagenesis on geophysical log response in the South Carolina Coastal Plain: effects of sedimentary fabric and mineralogy

Abstract

Interpretations of depositional environments and hydrologic units are made routinely from the study of geophysical well logs. Spontaneous potential (SP) and resistivity logs can be used as indicators of textural parameters. Gamma-ray logs denote lithologic zones based on the presence of radioactive material, particularly in fine-grained sediments. On the South Carolina Coastal Plain, surficial fluvially derived cobbles, sands and clays are characterized by very low radioactivity. Weathered paleosurfaces enriched in hydroxy-interlayered vermiculite (HIV) and associated mica produce higher gamma-ray responses at discrete intervals. The underlying sequence of siliciclastic shelf sands, deposited under conditions of eustatic sea-level rise, has high concentrations of smectite and low gamma-ray signatures. Small-scale variability of gamma radiation reflects temporal changes of sub-environments; higher values are recorded in finer-grained, organic-rich nearshore sediments. Calcareous Ecocene sediments are characterized by highly variable signals on the gamma-ray, SP and resistivity logs. Phospharic material, smectite and disseminated organic matter produced very high gamma-ray values; corresponding low SP readings are a function of low macro- to microscale porosity resulting from the fine grain size and authigenic pore-filling cement. Intervals that resemble massively bedded chalks at the macroscale are characterized by low gamma-ray and resistivity values and high SP readings. High SP values result from microposity preserved in biogenic tests rather than intergranular porosity that would be expected for a permeable sand. These observations indicate the importance of incorporating sedimentologic techniques with well-log data for a comprehensive evaluation of subsurface lithologic units.