University of Isfahan Journal of Stratigraphy and Sedimentology Researches 2008-7888 42 1 2026 03 21 Investigation of the productive zone and hydrocarbon characteristics of the Fahliyan reservoir using online mud gas analysis in the Yadavaran Field, Abadan Plain Investigation of the productive zone and hydrocarbon characteristics of the Fahliyan reservoir using online mud gas analysis in the Yadavaran Field, Abadan Plain 15 28 29747 10.22108/jssr.2025.145709.1317 FA Ehsan Hosseiny Assistant Professor, Department of Mining and Metallurgical Engineering, Yazd University, Yazd, Iran Sayed Kazem Mousavi Shavar MSc, Department of Mining and Metallurgical Engineering, Yazd University, Yazd, Iran Farhad Mohammad Torab Associate Professor, Department of Mining and Metallurgical Engineering, Yazd University, Yazd, Iran Journal Article 2025 06 18 Abstract Online evaluation of gas in drilling mud provides valuable information about reservoir horizons and the type of fluid during drilling. This study investigates the productive horizons of the Fahliyan Formation reservoir in the Yadegaran Field located in the Abadan Plain using geochemical evaluation of hydrocarbon gases in the drilling mud. The recording of mud gas data was conducted by a gas chromatograph with a flame ionization detector, and the hydrocarbon ratios(Pixler, Wetness, Balance, and Character) were calculated for two wells in the Fahliyan reservoir. The results of these ratios indicated that the Fahliyan Formation has reservoir quality, and the fluid is light oil. The relationship between the Wetness and Balance ratios divides the Fahliyan reservoir into two reservoir horizons. The lower horizon contains light oil, where the difference between these two ratios is slight, , while the greater difference in the upper horizon is due to low production capacity accompanied by residual oil. Keywords: Mud gas, Fahlyian Formation, Yadavaran Field, Productive Zone, Reservoir continuity Introduction During drilling, valuable data is obtained that can be used to characterize the productive zones of hydrocarbon reservoirs. One of these data is the information on gas associated with drilling mud. By identifying the trends in gas composition and changes in their ratios, the petrographic changes and fluid content of the drilled formations can be examined (Farouk et al. 2014). The measurement of hydrocarbon gas amounts in the mud is performed by gas chromatography, which includes quality control of sampling at specified times and analysis of gas contents (Ferroni et al. 2012). The more accurate the identification of hydrocarbon gases and the broader the spectrum of hydrocarbons included, the higher the quality and clarity of formation evaluation during drilling, determination of reservoir fluid levels, and identification of the productive zone (Arief & Yang, 2020). The purpose of this research is to investigate the productive zones of the Fahliyan Formation in the Yadavaran Field located in the Abadan Plain, using geochemical evaluation of hydrocarbon gases in the drilling mud. Materials & Methods The record of mud gas information during drilling in two wells of the Yadavaran Field was carried out using a gas chromatograph equipped with a flame ionization detector. The gases are separated from the mud by gas trap motors and introduced into the gas chromatograph, where they are recorded based on the amount of gas and the time it takes to enter the detector. It is worth noting that the device is calibrated daily using standard samples. The quality of the recorded data is evaluated after the gas is analyzed by the device. For this purpose, the Gas Quality Ratio (GQR) index is used. This index is obtained from the ratio of the total gas amounts to the sum of hydrocarbon component amounts multiplied by their respective carbon atom numbers (Wiersberg & Erzinger 2007; Newton et al. 2014). A GQR value within the range of 0.8 to 1.2 indicates good data quality. The recorded results from the drilling mud in the two studied wells demonstrate the appropriate quality of these data. Discussion of Results & Conclusion Upon entering the Fahliyan Formation at a depth of 4050 meters, the concentration of hydrocarbon gases in the drilling mud, which was below 1000 ppm before this horizon, increases. This result is entierly consistent with the oil shows observed on the drilling cuttings. The ratio of methane to heavier gases such as ethane, propane, and butane can indicate gas, oil, and water intervals (Pixler 1969). A C1/C2 ratio between 2 and 15 indicates an oil zone, while a ratio between 15 and 65 indicates a gas zone. The higher this ratio, the richer the gas or the lower the hydrocarbon density. If the C1/C2 ratio is less than 2, it indicates residual oil, and if it is above 65, it signifies a non-productive zone (Pixler 1969). Interpretation of the Pixler C1/C2 ratio for the samples is between 2 and 15, indicating oil fluid in the Fahliyan reservoir, and the deviation of some data towards the gas suggests a higher API gravity of the oil. The Wetness ratio increases with increasing gas density; the Balance ratio is, in fact, a direct ratio between light and heavy hydrocarbons, used alongside the Wetness ratio for interpretation, and has an inverse relationship with it (Mode et al. 2014; Sahu 2018). Practically, a straightforwardrelationship between Wetness and Balance ratios is used to determine fluid type and fluid contact during drilling. If the Balance ratio is greater than the Wetness, it is predicted that gas exists in the layer, whereas if the Wetness ratio is greater than the Balance, oil is predicted in the layer. The closer the curves are to each other, the lighter the oil. The greater the distance between the curves, the heavier the oil or the presence of residual oil (Mode et al. 2014). Higher ratios of Wetness than Balance also confirm the oil fluid for the Fahliyan reservoir. The trend of these two ratios differs between the upper and lower horizons of the reservoir. These two ratios have little difference in the lower horizon, indicating a productive zone with light oil. In the upper horizon of the Fahliyan reservoir, the Wetness and Balance ratios differ more, which can be due to the presence of a heavier oil composition or a non-productive zone. Based on previous geological and petrophysical studies (Mohseni et al. 2016; Ramezani Akbari et al. 2017; Tavoosi Iraj et al. 2023), the reservoir quality of the upper horizon is low, and the hypothesis of low production capacity accompanied by residual oil seems more plausible. Moreover, the C1/C4+C5 ratio is used to determine the amounts of heavy hydrocarbons, and a high value of this ratio indicates low amounts of heavy hydrocarbons. The high value of this ratio indicates low amounts of heavy hydrocarbons in the Fahliyan reservoir. This ratio also divides the Fahliyan reservoir into two different reservoir horizons, with the lower horizon showing higher ratios. Abstract Online evaluation of gas in drilling mud provides valuable information about reservoir horizons and the type of fluid during drilling. This study investigates the productive horizons of the Fahliyan Formation reservoir in the Yadegaran Field located in the Abadan Plain using geochemical evaluation of hydrocarbon gases in the drilling mud. The recording of mud gas data was conducted by a gas chromatograph with a flame ionization detector, and the hydrocarbon ratios(Pixler, Wetness, Balance, and Character) were calculated for two wells in the Fahliyan reservoir. The results of these ratios indicated that the Fahliyan Formation has reservoir quality, and the fluid is light oil. The relationship between the Wetness and Balance ratios divides the Fahliyan reservoir into two reservoir horizons. The lower horizon contains light oil, where the difference between these two ratios is slight, , while the greater difference in the upper horizon is due to low production capacity accompanied by residual oil. Keywords: Mud gas, Fahlyian Formation, Yadavaran Field, Productive Zone, Reservoir continuity Introduction During drilling, valuable data is obtained that can be used to characterize the productive zones of hydrocarbon reservoirs. One of these data is the information on gas associated with drilling mud. By identifying the trends in gas composition and changes in their ratios, the petrographic changes and fluid content of the drilled formations can be examined (Farouk et al. 2014). The measurement of hydrocarbon gas amounts in the mud is performed by gas chromatography, which includes quality control of sampling at specified times and analysis of gas contents (Ferroni et al. 2012). The more accurate the identification of hydrocarbon gases and the broader the spectrum of hydrocarbons included, the higher the quality and clarity of formation evaluation during drilling, determination of reservoir fluid levels, and identification of the productive zone (Arief & Yang, 2020). The purpose of this research is to investigate the productive zones of the Fahliyan Formation in the Yadavaran Field located in the Abadan Plain, using geochemical evaluation of hydrocarbon gases in the drilling mud. Materials & Methods The record of mud gas information during drilling in two wells of the Yadavaran Field was carried out using a gas chromatograph equipped with a flame ionization detector. The gases are separated from the mud by gas trap motors and introduced into the gas chromatograph, where they are recorded based on the amount of gas and the time it takes to enter the detector. It is worth noting that the device is calibrated daily using standard samples. The quality of the recorded data is evaluated after the gas is analyzed by the device. For this purpose, the Gas Quality Ratio (GQR) index is used. This index is obtained from the ratio of the total gas amounts to the sum of hydrocarbon component amounts multiplied by their respective carbon atom numbers (Wiersberg & Erzinger 2007; Newton et al. 2014). A GQR value within the range of 0.8 to 1.2 indicates good data quality. The recorded results from the drilling mud in the two studied wells demonstrate the appropriate quality of these data. Discussion of Results & Conclusion Upon entering the Fahliyan Formation at a depth of 4050 meters, the concentration of hydrocarbon gases in the drilling mud, which was below 1000 ppm before this horizon, increases. This result is entierly consistent with the oil shows observed on the drilling cuttings. The ratio of methane to heavier gases such as ethane, propane, and butane can indicate gas, oil, and water intervals (Pixler 1969). A C1/C2 ratio between 2 and 15 indicates an oil zone, while a ratio between 15 and 65 indicates a gas zone. The higher this ratio, the richer the gas or the lower the hydrocarbon density. If the C1/C2 ratio is less than 2, it indicates residual oil, and if it is above 65, it signifies a non-productive zone (Pixler 1969). Interpretation of the Pixler C1/C2 ratio for the samples is between 2 and 15, indicating oil fluid in the Fahliyan reservoir, and the deviation of some data towards the gas suggests a higher API gravity of the oil. The Wetness ratio increases with increasing gas density; the Balance ratio is, in fact, a direct ratio between light and heavy hydrocarbons, used alongside the Wetness ratio for interpretation, and has an inverse relationship with it (Mode et al. 2014; Sahu 2018). Practically, a straightforwardrelationship between Wetness and Balance ratios is used to determine fluid type and fluid contact during drilling. If the Balance ratio is greater than the Wetness, it is predicted that gas exists in the layer, whereas if the Wetness ratio is greater than the Balance, oil is predicted in the layer. The closer the curves are to each other, the lighter the oil. The greater the distance between the curves, the heavier the oil or the presence of residual oil (Mode et al. 2014). Higher ratios of Wetness than Balance also confirm the oil fluid for the Fahliyan reservoir. The trend of these two ratios differs between the upper and lower horizons of the reservoir. These two ratios have little difference in the lower horizon, indicating a productive zone with light oil. In the upper horizon of the Fahliyan reservoir, the Wetness and Balance ratios differ more, which can be due to the presence of a heavier oil composition or a non-productive zone. Based on previous geological and petrophysical studies (Mohseni et al. 2016; Ramezani Akbari et al. 2017; Tavoosi Iraj et al. 2023), the reservoir quality of the upper horizon is low, and the hypothesis of low production capacity accompanied by residual oil seems more plausible. Moreover, the C1/C4+C5 ratio is used to determine the amounts of heavy hydrocarbons, and a high value of this ratio indicates low amounts of heavy hydrocarbons. The high value of this ratio indicates low amounts of heavy hydrocarbons in the Fahliyan reservoir. This ratio also divides the Fahliyan reservoir into two different reservoir horizons, with the lower horizon showing higher ratios.

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