Application of porosity logs for cyclostratigraphic analysis in the absence of gamma ray logs.

High-resolution cyclostratigraphic and sequence stratigraphic analyses are often compromised and lead to incorrect interpretations due to the unavailability or critical, proven errors in the reference Gamma-Ray (GR) and Density (RHOB) logs in hydrocarbon wells. This issue creates operational risks, delays, and costly re-logging procedures, directly impacting reservoir modeling and development decisions. This study presents an innovative, multi-Proxy methodology to address this critical problem by assessing the capability of the Sonic (DT), Neutron (NPHI), and Resistivity (RT) logs as reliable substitutes. The research focuses on three wells (A, B, and C) in the Oligocene to Miocene Asmari Formation of the Dezful Embayment, Southwest Iran. Available logs were analyzed to delineate third- and fourth-order sedimentary sequences based on Inflection Points. Subsequently, several advanced, distinct methods were used for comprehensive validation and the study of astronomical cycles: Spectral Analysis (SA) and Evolutionary Spectral Analysis (ESA) to isolate the influential Milankovitch cycles; Correlation Coefficient (COCO) and Evolutionary Correlation Coefficient (eCOCO) analysis to determine the Sediment Accumulation Rate (SAR); Wavelet Transform Scalogram analysis to identify intense frequency variations and interpret corresponding lithological changes; and Lag-1 Autocorrelation Coefficient (ρ1) to examine sedimentary noise and determine global sea-level trends. The outputs of the substitute logs were compared with the results of the reference GR and RHOB logs to establish their validity. The results strongly confirm the utility of the multi-proxy substitute logs. The DT and NPHI logs demonstrated a high degree of correlation across all analytical methods and reliably identified the third-order sequences. However, the RT log exhibited greater variability, particularly in high-frequency cyclicity (fourth-order) and SAR calculation, which underscores the necessity of a multi-proxy approach. This validated, multi-proxy methodology offers a definitive, cost-effective, and rapid solution for accurate stratigraphic analysis and reservoir characterization in data-scarce or error-prone environments. It allows the petroleum industry to make reliable geological and engineering decisions in the very early stages of interpretation, significantly enhancing informational certainty for exploration and development projects.