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

Abstract

AbstractGamma 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.Keywords: Sarvak, Fahliyan, Gamma Ray Log, Sequence Stratigraphy, Stratigraphic Key Surfaces IntroductionThe distribution of various components of the petroleum systems (source, reservoir and cap rocks), generally has a close relationship with the sequence stratigraphy pattern (Parvin et al. 2019; Makled et al. 2020). Data from various sources with different scales, such as microscopic thin sections study, cores interpretation, petrophysical log signature, and seismic sections, can be employed in sequence stratigraphic investigations (Van Buchem et al. 2011; Kadkhodaie and Rezaee 2017; Tavakoli 2017; Guo et al. 2021; Yong et al. 2021; Hassan et al. 2022). The identification of key sequence stratigraphic surfaces is typically done accurately by information and results provided by cores and microscopic thin sections study. Nevertheless, cores are only available in limited and sparse wells as a discontinuous nature due to the high cost, lengthy process, and limited cores preparation (Hassan et al. 2022). Hence, it is crucial to identify a method that, in conjunction with the core data, can provide a precise and continuous comprehension of the sequence stratigraphic framework at the well scale.Petrophysical well log data especially GR log has long been used in sequence stratigraphic interpretation (Catuneanu 2006; 2017). Due to its low impact from environmental factors, stability against diagenesis processes, and availability in the majority of drilled wells in a field, the GR log is effectively used in identifying sequences and interpreting key sequence stratigraphic surfaces (Moghaddasi et al. 2020; Falahatkhah et al. 2021). The possibility of a comprehensive understanding of the relationship between key sequence stratigraphic surfaces, and a numerical interpretation of the GR log in recent studies is highlighted (Falahatkhah et al. 2021). The curve of D-INPEFA and NCGDL are effective in identifying and matching key sequence stratigraphic surfaces (Behdad 2019; Mayhoub et al. 2019; De Jong et al. 2020; Moghaddasi et al. 2020; Khalili et al. 2021; Yong et al. 2021; Kassem et al. 2022).In this study, the application of GR D-INPEFA and NCGDL curves to discriminate and correlate the sequence surfaces of the Sarvak and Fahliyan formations in two giant oil fields in the Abadan plain and Northwestern Persian Gulf was investigated. This study was conducted to reveal the response of GR D-INPEFA and NCGDL curves to key sequence stratigraphic surfaces including SB, MFS, and systems tracts. Material & MethodsBy integrating the results of core intervals description, thin sections study and GR well log data from the Sarvak and Fahliyan formations in two giant oil fields in the Abadan Plain and Northwestern Persian Gulf, it was possible to identify and present sequence stratigraphic framework. For sequence stratigraphic analysis in this study, we employed the van Wagoner model (Van Wagoner et al. 1990). In this model, a depositional sequence is discriminated by identifying two key surfaces including SB and MFS, as well as the transgressive system tract (TST) and high-stand system tract (HST). The integration of petrography analysis and core descriptions was employed to identify the main facies in the studied carbonate succession. Afterward, SB and MFS have been interpreted by integrating interpreted facies and main diagenesis features related to disconformity surfaces. Finally, the identified depositional sequences based on geological studies were compared to the discriminated turning point in cyclostratigraphy via INPEFA and NCGDL curves. Ultimately, the efficiency of this numerical method was determined based on correspondence and agreement of the results with the discriminating of the depositional sequences via geological interpretation. Discussion of Results & ConclusionIn the D-INPEFA curves, the upward trend is called the PB and the downward trend is called NB. The positive trend represents transgressive and the negative trend represents regressive. The positive changes in PB occur at SB and negative trend changes in NB show MSF. On the other hand, the trend of the NCGDL curve completely shows the opposite trend of the D-INPEFA curve. The turning points of the NCGDL curve, including PS and NS, correspond to some levels of maximum inundation and SBs, respectively.The following findings were obtained after investigating the sequence stratigraphic framework of the Sarvak and Fahliyan formations in two giants of fields:1- Four and three third-order sequences were identified in the Sarvak and Fahliyan formations respectively, based on facies interpretation and the pattern of shallowing and deepening facies trends. The disconformity surfaces which are recognized as an SB are discriminated by some evidence, including karstification and brecciation.2- In general, GR log interpretation and calculating INPEFA and NCGDL curves show that PB and NS occur at SB, NB and PS show MFS. As a result, such picked breaks and surfaces are timelines that can be calibrated with facies changes and key sequence stratigraphic surfaces and correlation in the field scale.3- The relationship between the turning points of the D-INPEFA and NCGDL curves with sequence stratigraphic surfaces is not absolute, and in some cases, this relationship can be ambiguous, inverse, or unrelated. In other words, to reduce the uncertainty in the presenting sequence stratigraphy correlation with these curves, the sequences should first be identified through the results of the direct data including core and thin section, and use these curves for correlation after presenting the sequence framework.

Keywords

• sarvak
• fahliyan
• gamma ray log
• sequence stratigraphy
• stratigraphic key surfaces