ریزرخساره‌ها، شرایط ته‌نشینی و چینه‌نگاری سکانسی توالی کربناته-تبخیری سازند کنگان، در بخش مرکزی خلیج فارس

خلیج فارس بزرگ‌ترین میدان‌های گازی دنیا را دارد. بخش مخزنی این میدان‌ها، سازندهای دالان بالایی و کنگان به سن پرمو-تریاس است که معادل سازند خوف در کشورهای عربی جنوب خلیج فارس شناخته می‌شود. این پژوهش بر پایۀ اطلاعات حاصل از مغزه و اطلاعات پتروفیزیکی ( لاگ گاما، لاگ صوتی و لاگ نوترون) در چاه a بخش مرکزی خلیج فارس، با ضخامت 174متر انجام شده است. یافته‌های حاصل از مطالعۀ 628 مقطع نازک، نشان‌دهندۀ 12 ریزرخساره در 5 گروه ریزرخساره‌ای (محیط رسوبی) شامل بالای پهنه‌کشندی، پهنه‌کشندی، لاگون، تپه‌های سدی و دریای باز (رمپ کم‌عمق میانی و رمپ عمیق خارجی) است. مجموع این رخساره‌ها و توالی آنها نشان‌دهندۀ تشکیل توالی کربناته- تبخیری کنگان در یک محیط رمپ با شیب ملایم است که در آب‌وهوای گرم و خشک تریاس پیشین نهشته می‌شوند. براساس تلفیق مطالعات ریزرخساره‌ای رسوبی و لاگ‌های پتروفیزیکی، 2 سکانس ردۀ سوم ds1 و ds2 شناسایی شدند. بررسی‌های انجام‌شده نشان داد سازند کنگان به 4 زون مخزنی تقسیم می‌شود. زون 1 و 3 غالباً ریزرخسارۀ تبخیری یا مادستونی و تخلخل بسیار کم دارند، بخش ناتراوا و برداشت‌نشدنی مخزن محسوب می‌شوند، منطبق با گروه‌های ریزرخساره‌ای بالای کشندی و لاگون‌اند و زون‌های 2 و 4 شامل ریزرخساره‌های دانۀ غالب ااییدی‌اند، تخلخل بسیار بالا دارند، بخش تراوا و برداشت‌شدنی این سازند محسوب می‌شوند و منطبق بر گروه ریزرخساره‌ای پشته‌های سدی‌اند.

Microfacies, depositional environment and sequence stratigraphy of the carbonate-evaporate successions of the Kangan Formation in the central part of the Persian Gulf

Abstract: The Persian Gulf possesses the largest gas fields in the world. The reservoir part of these fields includes upper Dalan and Kangan formations dating back to the Permo–Triassic age, equivalent to Arab countries’ Khuff Formation in the south of the Persian Gulf. The present study was based on the core and petro-physical information, including gamma, sonic, and neutron log, in well A in the central part of the Persian Gulf with a thickness of 174 meters. Findings obtained from examining 628 thin sections indicated 12 microfacies in five groups of microfacies (sedimentary environment). These five facies belts consisted of supratidal, intertidal, lagoon, shoal, and open marine (middle and outer ramp). The sum of these facies and their sequence indicated the formation of carbonate-evaporative successions of Kangan in a ramp environment with a gentle slope deposited in the hot and dry climate of the former Triassic. Regarding the combination of microfacies studies and petrophysical logs, two third-order depositional sequences named DS1 and DS2 were identified. Investigations indicated that the Kangan Formation could be divided into four reservoir zones. Zones 1 and 3 with evaporitic or mudstone facies and very low porosity were considered impermeable and non-production capacity parts of the reservoir. These zones were in correspondence to the supratidal microfacies and lagoon. On the other hand, zones 2 and 4 included ooid-bearing grain-supported microfacies with very high porosity. These zones were regarded as the permeable and production capacity parts of this formation and corresponded to the microfacies group of the back shoal.Keywords: Kangan Formation, Microfacies, Depositional Environment, Sequence Stratigraphy, Sedimentary Environment  Introduction The Persian Gulf has always taken researchers’ attention in the oil and gas basin due to its largest gas fields in the world. The gas part of this field is located in the Dalan (mainly the upper part) and Kangan formations dating back to the Permo–Triassic age (Esrafili-Dizaji and Rahimpour Bonab 2013). Sedimentary deposits in this field are related to the extension and evolution of the Neotethys Ocean (Szabo and Kheradpir 1978). According to many changes in the facies in the ramp environment and the creation of different sedimentary conditions in the sequences of this formation, a more detailed review and analysis of microfacies and the sedimentary environment is required. In addition, the reservoir potential from petrographic surveys based on the microfacies determination, the sediment environment analysis, and the sequence stratigraphy should be reviewed. Materials & MethodsThe present study was conducted based on cores taken from Kangan Formation and information obtained from petrophysical logs, including gamma, sonic, resistance, and neutron. In the studied well, coring of Kangan Formation was performed with a thickness of 174 meters. After coring, the samples were sent to the laboratory to examine the core and prepare thin sections. For a more comprehensive and accurate study, a thin section of the core was prepared every 30 cm (one foot). The samples were measured for porosity and permeability by conventional core tests. A total of 628 thin sections were taken from the plugs. These thin sections were stained with Alizarin Red solution to differentiate the calcite minerals from dolomite. These samples were evaluated with a microscope, and the facies were named based on the Dunham classification (Dunham 1962). Next, the sedimentary sequence was drawn, and the sedimentation environment of the microfacies was specified in a model. Based on the progradational plan of sedimentary facies, deep and shallow trends and petrophysical logs of the sequence stratigraphy framework were examined, and the third- and fourth-order sedimentary sequences were identified. Discussion of Result & ConclusionExamining the petrographic and sequence stratigraphy data of Kangan Formation in the studied area showed some conclusions as follows:1- Kangan Formation dating back to the former Triassic consisted of a series of carbonate-evaporative successions with thin layers of shale deposited in the ramp environment with a gentle slope in hot and dry climates (due to the presence of large masses of anhydrite-gypsum often in sabkha and supratidal environment).2- Deposits of Kangan Formation in the Persian Gulf were divided into five facies belts, including supratidal (sabkha and small evaporitic ponds), intertidal, lagoon, shoal, and open marine (middle ramp and outer ramp).3- Due to its evaporation, the supratidal facies belt had the lowest porosity with the role of caprock, separating the permeable and production capacity zones within the Kangan Formation. The intertidal facies belt was mostly dolomitic; however, it was observed with some mudstone and anhydrite facies. The predominant diagenetic phenomenon in these facies was dolomitization, and these dolomites resulted from the Seepage-Reflux model. In the absence of anhydrite in the dolomitic intertidal zone, these distances had high reservoir properties. They were considered the second type of reservoir in Kangan Formation in terms of quality. The lagoon facies belt mainly consisted of dolostone and anhydrite with interbedded shale. The lagoon belt corresponded to the sequence boundary of the second type; it was considered part of the impermeable part of the Kangan reservoir. The back shoal microfacies comprised fine facies with lithology, including dolostone and limestone with a small amount of anhydrite-gypsum diagenesis. In terms of reservoir quality, the back shoal belt had the highest reservoir quality. Two parameters played a significant role in improving the quality of the reservoir, the formation environment in which the ooidal grain-supported facies were formed and the process of dolomitic diagenesis. The shoal belt was used as the maximum flooding surface, and a progressive part of the sequence was formed in the center of the shoal in the final stages. The open marine facies (middle and outer ramp) are also composed of compacted limestone lithology. These facies could be known as the maximum flooding surface. There was a lack of reservoir quality in this section due to its mudstone being. This facies belt is very thin and was formed in a thin layer.4- Sequence stratigraphic studies showed that Kangan Formation has two third-order sequences, DS1 and DS2. Sequence DS1 consisted of three parasequences of fourth-order such as DS1a, DS1b, and DS1c. The DS2 sequence consisted of four parasequences of fourth-order, including DS2a, DS2b, DS2c, and DS2d. Studies demonstrated that sequential boundaries were formed in the supratidal zone and the central part of the lagoon. In addition, the progradational part of the sequences extended from the supratidal zone to the marine and deeper areas such as lagoons and back shoals. In the continuation of the cycle and at the highest water level, the environment was located in the center of the shoal or open marine. It should be noted that the highest amount of porosity and quality of the reservoir was observed in the center of the shoal and especially in the condition of dolomitization.5- Based on microfacies studies, sequence stratigraphic and petro-physical of Kangan Formation had two permeable and production capacity sections, including a) Zone No. 2 with a thickness of 11 meters related to the TST progressive section of sequence DS1 with lithology of 60% limestone and 40% dolostone was formed in the central part of the shoal, and the dominant microfacies was ooidal grainstone; b) Zone No. 4 with a thickness of 25 meters in the progressive part of the DS2 sequence with predominant dolomitic lithology (more than 90%) was formed in the central shoal and microfacies of dolomitic ooidal grainstone.