ارتباط تغییرات ویژگی‎‍های مخزنی با فرایندهای رسوبی سازند ایلام در میدان سوسنگرد، جنوب غرب ایران

به‌منظور توسعه و اکتشاف میادین، به شناخت چارچوب مدل زمین‌شناسی و تغییرات کیفیت مخزنی نیاز است. در این مطالعه، تغییرات کیفیت مخزنی سازند ایلام با ضخامت تقریبی145متر، در میدان سوسنگرد در فروافتادگی دزفول شمالی بررسی شد. مدل رسوبی این سازند، باتوجه‌به نبود ساختار ریزشی و ساخت‎‍های ریفی از نوع رمپ کربناته در نظر گرفته شد. در میدان موردمطالعه، دو سکانس کامل و یک سکانس ناقص رسوبی ردۀ سوم شناسایی شد. با نزدیک‌شدن به مرز سکانس‎‍ در بخش میانی سازند ایلام، با توسعۀ انحلال‎‍های حفره‎‍ای و قالبی، به‌ویژه در جلبک‎‍های سبز با جنس آراگونیت، توسط سیال‎‍های خورندۀ عبوری از استیولیت‎‍ها و شکستگی‎‍ها، شرایط مخزنی بهتر شده ‎‍است؛ اما در سکانس بالایی، با وجود توسعۀ تخلخل‎‍های بین دانه‎‍ای در اائیدها در نزدیکی مرز سکانس سیمان‌شدگی، مانع از ایجاد شرایط مخزنی مناسب شده ‎‍است. جنوب شرق میدان در سکانس‎‍ قاعده‎‍ای و قسمت اعظم دستۀ رخسارۀ پیش‌روندۀ سکانس میانی، ریزرخساره‎‍های کم‌عمق‌تری نسبت‌به شمال غرب میدان دارد. به نظر می‎‍رسد جنوب شرق میدان بر بلندای قدیمی قرار دارد و احتمالاً مقادیر کمتر نمودار تخلخل در شمال غرب میدان در اثر دوری از این بلندی قدیمی باشد؛ بنابراین برای توسعۀ میدان سوسنگرد و پی‌جویی‌های آتی، جنوب شرق میدان پیشنهاد می‎‍شود.

Investigating the relationship between reservoir properties and sedimentary processes in the Ilam Formation of Susangerd oilfield, SW Iran

AbstractTo developing and explore hydrocarbon fields, it is necessary to know geological models and reservoir quality changes. Therefore, the Ilam Formation with approximate 145m thickness was examined in the Susangerd oilfield. The sedimentary model was considered carbonate ramp type due to the lack of slump and reef structures. Two complete and one incomplete third-order sedimentary sequences were identified. Close to the sequence boundary in the middle part of the Ilam Formation, reservoir conditions improve with the development of vug and mold dissolutions, especially in green algae by corrosive fluids passing through stylolites and fractures. But near the sequence boundary in the upper part of the Ilam Formation, however intergranular porosity develops between peloids and ooids but almost destroyed with cementation. The southeastern domain of the oilfield contains shallower microfacies in the lower sedimentary sequence and most part of transgressive system tract of the middle sequence in comparison with the northwestern domain. It seems that the southeastern part of the oilfield is located on paleo high and the lower values ​​of porosity log in the northwestern part are probably due to the distance from this paleo high. Therefore, the southeast of the oilfield is suggested for the development of the Susangerd oilfield and future explorations.Keywords: Sedimentary environment, Sedimentary sequence, Reservoir quality, Ilam Formation, Susangerd oilfield  IntroductionA large part of the Cretaceous carbonate reservoirs belongs to the Bangestan Group, including the Sarvak and Ilam formations in the Zagros and the Arabian platform. The Ilam Formation and its equivalents contain important reservoirs in the south and southwest of Iran (Dezful Embayment) and the Middle East.In this study, the sedimentary environment, sequence stratigraphy, heterogeneity and reservoir quality of the Ilam Formation in the Susangard oilfield in the southern part of the North Dezful Embayment have been investigated to develop and explore hydrocarbon reservoirs.The Susangard oilfield is located at 45 km in the west of Ahvaz oilfield and adjacent to the Juafair, Abteymour and Band-E-Karkheh oilfields. The predominant lithology of the Ilam Formation is carbonate and sometimes argillaceous limestones and shales in the studied area. The upper boundary of the Ilam Formation with the Gurpi Formation and its lower boundary with the Lafan Formation are transitional.In the past, many studies have been done on the sequence stratigraphy, sedimentary environment and diagenetic processes of the Ilam Formation in the southern part of the North Dezful Embayment (Adabi & Asadi Mehmandosti 2008; Hassani Giv & Abarghani 2009; Soleimani 2011; Mehrabi et al. 2013; Khanjani 2015; Omidi 2020; Khodaei 2020). Previous studies demonstrate that the sedimentation of the Ilam Formation is in a homoclinal ramp consisting of three inner, middle and outer ramp belts, while tidal flat facies has not been reported in most studies. Material & MethodsThe studied wells are named A, B and C from the northwest to the southeast of the oilfield, respectively. The thickness of the Ilam Formation is 140 m in well A, 150 m in well B and 141 m in well C. In this research, the 84 and 70 thin sections prepared from drilling cuttings of well A and well B, respectively and the 96 thin sections prepared from the core (from 3356 to 3438m) and the drilling cuttings of well C have been studied. The percentage and type of grains and matrix in these thin sections were examined and microfacies were named based on the classification of Dunham (962) for carbonate rocks. In this study, the lateral distribution of sedimentary facies has been reconstructed using standard Flugel (2010) facies models in and Burchette and Wright (1992) carbonate ramp sedimentary models.For investigation of the vertical distribution of microfacies, sequence stratigraphy of the Ilam Formation has been studied using petrography studies, petrophysical gamma and neutron logs and the sedimentary sequence stratigraphy model IV (Catuneanu 2006). Discussion of Results & ConclusionsBased on petrography studies, one petrofacies (Anhydrite) and 15 microfacies were identified in the Ilam Formation. According to the results of this study, the Ilam Formation has been deposited in a carbonate ramp in three facies belts, including the inner ramp (tidal flat, lagoon and shoals), middle ramp and outer ramp. Two complete and one incomplete third-order sedimentary sequences were identified in the Ilam Formation.Reservoir conditions in the middle part of the Ilam Formation (the middle sequence) near the sequence boundary improve due to the development of vug and mold dissolutions, especially in green algae with aragonite mineralogy by corrosive fluids passing through stylolites and fractures. However, in the upper part of the Ilam Formation -- the upper sequence -- the presence of intergranular porosity in ooids and peloids is the main factor in increasing the reservoir quality and their cementation is a main factor in reducing the reservoir quality. In this oilfield, almost intergranular porosities of the upper part of the Ilam Formation were destroyed by cementation. The lower part of the Ilam Formation, the lower sequence, does not have a suitable reservoir quality due to the high content of mud in the matrix.Upon the onset of subduction of the oceanic crust of the Neo Tethys under the Central Iran Plate during the middle Cretaceous, the inactive and stable margins of the Neo Tethys became active margins and began the Alpine orogenic phase. At this time, due to the reactivation of basement faults and the movements of the Hormoz salt, several paleo highs have been formed in different parts of southwestern Iran and the Dezful Embayment. The presence of these paleo highs has caused heterogeneity and lateral facies changes and fluctuations. The Paleo highs in southwestern Iran are known in several oilfields, including the Gachsaran, Rag-E-Safied and Abteymour (Nazar Aghaei 1986; Afsharzadeh 1999; Van Buchem et al. 2001; Mehrabi 2013). The southeast of the Susangerd oilfield has a shallower sedimentary environment than the northwest of the oilfield at the time of deposition of the lower sedimentary sequence and almost transgressive system tract of the middle sequence which can indicate paleo high in the southeast of the oilfield. The lower values ​​of the porosity log in the northwest of the oilfield compared to the southeast are probably due to distance from the paleo high. Therefore, the southeast of the oilfield is suggested for the development of Susangerd oilfield and future explorations.