بررسی کنترل ویژگی‌های رخساره‌ای و فرایندهای دیاژنزی بر کیفیت مخزنی سازند فهلیان در ناحیۀ زاگرس مرکزی

سازند فهلیان شامل توالی کربناتۀ نسبتاً ضخیمی از گروه خامی به سن نئوکومین بارمین است. مطالعه‌های رسوب‌شناسی انجام‌شده روی این سازند در دو برش از زون ایذه و دو چاه از بخش جنوبی فروافتادگی دزفول به شناسایی 12 ریزرخساره در چهار کمربند رخساره‌ای شامل پهنۀ جزرومدی، رمپ درونی (لاگون، پشته‌های لیتوکودیومی جلبکی و شول‌های بایوکلستی پلوئیدی)، رمپ میانی و رمپ بیرونی منجر شد. محیط رسوب‌گذاری سازند فهلیان در ناحیۀ مطالعه‌شده، رمپ کربناتۀ هم‌شیب پیشنهاد می‌شود. مهم‌ترین فرایندهای دیاژنزی شناسایی‌شده شامل انحلال، میکرایتی‌شدن، تراکم، دولومیتی‌شدن، سیلیسی‌شدن، پیریتی‌شدن، سیمانی‌شدن و شکستگی است که طی مراحل مختلف دیاژنز دریایی، جوی و دفنی کم‌عمق تا عمیق ایجاد شده‌اند. مطالعه‌های چینه‌نگاری سکانسی به تفکیک چهار سکانس رسوبی در این سازند منجر شدند که با‌توجه‌به سن نسبی آنها بر اساس بایوزون‌های تفکیک‌شده، ضخامت سکانس‌ها و تطابق آنها با نواحی مجاور به‌طور محتمل سکانس‌های ردۀ سوم هستند. مطالعه‌های کیفیت مخزنی گویای تاثیر ویژگی‌های رخساره‌ای و فرایندهای دیاژنزی بر ویژگی‌های مخزنی سازند فهلیان است. رخساره‌های مهم مخزنی این سازند شامل رخساره‌های ریف‌ها/پشته‌های کومه‌ای لیتوکودیومی جلبکی (در میدان گرنگان) و رخساره‌های پرانرژی متعلق به زیرمحیط پشته‌های اُاُئیدی، پلوئیدی و بایوکلستی (در میدان گچساران) هستند؛ این رخساره‌ها اغلب در سیستم تراکت تراز بالا (hst) گسترش یافته‌اند و زون‌های مهم مخزنی سازند فهلیان را ایجاد کرده‌اند. تخلخل‌های اولیه از نوع بین‌دانه‌ای و درون‌دانه‌ای همراه با تخلخل‌های انحلالی (حفره‌ای و قالبی) مهم‌ترین حفره‌ها در این رخساره‌ها هستند؛ علاوه‌بر‌این، رخساره‌های متعلق به رمپ میانی برخی افق‌های مخزنی باکیفیت را در سیستم تراکت پیش‌رونده (tst) ایجاد کرده‌اند که در این افق‌ها، تخلخل‌های بزرگ‌مقیاس (ماکروسکوپی) گسترش چندانی ندارند و ریزتخلخل‌ها مهم‌ترین عامل ایجاد کیفیت مخزنی‌اند.

Controls of depositional facies and diagenetic alterations on reservoir quality of the Fahliyan Formation in the Central Zagros area

AbstractThe Fahliyan Formation is a member of the Khami Group with Neocomian–Barremian age. In two surface sections in the Izeh Zone and two wells of the southern Dezful Embayment, it includes 12 microfacies deposited on four facies belts as tidal flat, inner ramp (lagoon, Lithocodium–algal mounds and bioclastic–peloidal shoals), middle ramp and outer ramp. A homoclinal ramp model is proposed as its depositional environment. Important diagenetic alterations are dissolution, micritization, compaction, dolomitization, silicification, pyritization, cementation and fracturing. They represent marine, meteoric and shallow to deep burial diagenetic realms. Sequence stratigraphic analysis resulted in the recognition of four depositional sequences that seem to be thirdorder sequences, according to their relative ages (biozones), thickness and correlation with neighboring areas. Reservoir quality evaluation indicates coupled imprints of facies characteristics and diagenetic alterations on reservoir properties of this formation. Important reservoir units are formed in Lithocodium–algal mounds/patchreefs (in the Garangan Field) and highenergy facies of shoal setting (in the Gachsaran Field). They include primary pore types such as intra and interparticle pores and mainly concentrated in highstand systems tracts (HSTs) of depositional sequences. Some secondary pores are also present that include dissolution vugs and molds. In addition, middle ramp facies provide some reservoir units in the transgressive systems tracts (TSTs). No macroscopic pores are recorded in these facies and micropores are responsible for high reservoir quality.Keywords: Fahliyan Formation, Depositional sequences, Diagenesis, Reservoir quality, Central Zagros  IntroductionThe Neocomian–Barremian Fahliyan Formation is a member of the Khami Group that form important hydrocarbon reservoir in SW Iran (James and Wynd 1965). Tectonic activities and eustatic sealevel changes have resulted in complex depositional and diagenetic characteristics in this formation (Hosseini et al. 2014, Jamalian and Adabi 2015, Noori et al. 2019). This study focuses on the controls of depositional facies and diagenetic alterations on reservoir quality of the Fahliyan Formation in a sequence stratigraphic framework. To achieve this goal, two surface sections of this formation were studied in the Izeh Zone along with two wells from the Dezful Embayment. Material & MethodsA total of 718 thin sections of the Fahliyan Formation were studied in the Gachsaran and Garangan fields along with 660 thin sections of this formation in the Lar and Eshgar sections. All sedimentological features are recorded in the macroscopic and microscopic scales. They include field observations together with core and thin section descriptions. A modified textural classification of carbonate rocks of Dunham (1962) by Embry and Klovan (1971) was used along with the standard microfacies of Flügel (2010). Well log data were used for electrofacies determination and reservoir quality evaluation. A transgressive–regressive (T–R) model (Embry and Johannessen 1992, Embry 1993, Catuneanu 2002) was used for sequence stratigraphic analysis. Discussion of Results & ConclusionsFacies analysis of the Fahliyan Formation has resulted in the recognition of 12 microfacies deposited on four facies belts including tidal flat, inner ramp, middle ramp and outer ramp. Frequency analysis of depositional facies are integrated with microfacies descriptions for paleoenvironmental reconstruction of the Fahliyan Formation. Consequently, a ramplike carbonate platform model is proposed as depositional setting. Lithocodium–algal and shoal facies are reported as the best reservoir facies. Frequency analysis and isopachous maps are adopted for these reservoir facies in the study area.Diagenesis study resulted in the identification of different processes including cementation, micritization, pyritization, dissolution, neomorphism, silicification, dolomitization, compaction and fracturing. They represent marine, meteoric and shallow to deep burial diagenetic realms. Diagenetic sequence of the Fahliyan Formation is presented and controls of diagenetic processes on reservoir quality (porosity) is evaluated. Accordingly, dissolution, dolomitization and fracturing were the most important diagenetic features with positive effects on reservoir quality. In contrast, compaction and cementation were the notable diagenetic processes that reduced the reservoir quality.Results of sedimentological studies are integrated with petrophysical logs and field observations to differentiate the depositional sequences of the Fahliyan Formation. Accordingly, four sequences were defined and correlated throughout the study area and with the reference sequences of the Zagros (van Buchem et al. 2010) and the Arabian Plate (Sharland et al. 2001).Cluster analysis of neutron (NPHI), sonic (DT) and density (RHOB) logs has resulted in the differentiation of 10 electrofacies in the Fahliyan Formation. They are integrated with sedimentological studies for reservoir evaluation of this formation in a sequence stratigraphic framework. Consequently, Lithocodium–algal facies, in the regressive systems tract, and middle ramp facies, in the transgressive systems tract, were the best reservoir units of the Fahliyan Formation in the Garangan Field. While in the Gachsaran Field, highenergy facies of shoal complexes provide the best reservoir properties in the regressive systems tracts.