A workflow to the identification of key sequence stratigraphic surfaces from GR log interpretation; Case studies of the Sarvak and Fahliyan formations in the Abadan Plain and Northwest of the Persian Gulf

Gamma Ray (GR) log is widely used to overcome the lack of continuous core samples and thin sections in many subsurface reservoirs, significantly reducing the uncertainty of presenting a robust sequence stratigraphic framework. The Lower Cretaceous Fahliyan Formation and Late Cretaceous Sarvak carbonates host important hydrocarbon accumulations in SW Iran and the Persian Gulf. This study addresses the application of Gamma Ray Dynamic Integrated Prediction Error Filter Analysis (GR D-INPEFA) and Normalized Cumulative Gamma Deviation Log (NCGDL) curves to discriminate and correlate sequences of the studied carbonates in two giant oil fields in the Abadan Plain and Northwestern Persian Gulf. In the first stage, depositional sequences were differentiated using the results from the core description and petrographic analysis. In this respect, four and three third-order depositional sequences were identified in the Sarvak and Fahliyan formations respectively. In the second stage, according to the turning points of the D-INPEFA and NCGDL curves, they were picked as positive or negative breaks in all studied wells. Based on calibrating the results from the analysis of D-INPEFA curve with the identified depositional sequences, positive changes (PB) occur at sequence boundaries (SB) and negative trend (NB) changes are associated with maximum flooding surface (MFS). The trend of changes in the NCGDL curve compared to the D-INPEFA curve is quite opposite. In other words, Positive Surfaces (PS) correspond to MFS and Negative Surfaces (NS) occur at SB. The results demonstrate that the proposed integrated method is feasible and effective to identify sequence stratigraphic key surfaces in similar carbonate and siliciclastic reservoirs of Iran.