تعیین رخساره‌های دیاژنزی و کیفیت مخزنی ماسه‌سنگ‌های سازند گدوان در چاه‌هایی از میادین غربی دشت آبادان

ماسه‌سنگ‌های سازند گدوان (بارمین آبتین) در دشت آبادان، طیف وسیعی از تخلخل و تراوایی را با توجه به تاریخچه‌ دیاژنزی پیچیده‌ خود دارند. بر همین اساس مطالعه‌ پتروگرافی با استفاده از میکروسکوپ نوری، الکترونی و نیز آنالیزهای روتین تخلخل تراوایی مغزه روی این نهشته‌‌ها برای بررسی تاثیر دیاژنز بر کیفیت مخزنی انجام شد. فرایندهای دیاژنزی از قبیل تراکم، انحلال، سیمانی‌شدن و تبدیل و درجازایی کانی‌های رسی، مهم‌ترین فرایندهای مرتبط با کیفیت مخزنی در نمونه‌های مطالعه‌شده ارزیابی شدند. به این ترتیب ماسه‌سنگ‌های گدوان به 5 رخساره‌ دیاژنزی شامل ماسه‌سنگ متراکم با سیمان اندک، ماسه‌سنگ با سیمان کلریت، کائولینیت و کلسیت، ماسه‌سنگ با پوشش کلریتی دانه‌های کوارتز، ماسه‌سنگ با سیمان کربناته‌ فراگیر و ماسه‌سنگ با سیمان انحلال‌یافته تفکیک شدند. پوشش کلریتی با جلوگیری از گسترش سیمان، رشد اضافه‌ سیلیسی و تشکیل چارچوب پایدارتر، به حفظ بهتر تخلخل اولیه کمک کرده است. سیمان‌های کربناته‌ ائوژنزی با نسبت حجمی 9% از کل سنگ، کیفیت مخزنی را بهبود می‌بخشند؛ در حالی که ماسه‌سنگ‌های با سیمان کربناته‌ مزوژنزی که در مراحل پایانی دیاژنز (با درصد حجمی بین 25 تا 32%) گسترش یافته‌اند، اغلب به‌عنوان سدی دربرابر حرکت هیدروکربور عمل کرده‌اند. پس از تعیین میزان تخلخل و تراوایی در بخش‌های دارای مغزه، برای روشن‌شدن بهتر ارتباط این دو پارامتر با رخساره‌های دیاژنزی، واحدهای جریانی به روش نشانگر زون جریان (fzi)، تعیین شد. با بررسی ارتباط بین پارامترهای مخزنی و دیاژنزی مطالعه‌شده، به نظر می‌رسد واحدهای جریانی 1 و 2 معادل با رخساره‌های دیاژنزی 3 و 5، بهترین کیفیت مخزنی را در چاههای مطالعه‌شده دارند.

Diagenetic facies determination and reservoir quality evaluation of the Gadvan Formation sandstones of some wells in the western fields of Abadan Plain

AbstractThe sandstones of Gadvan Formation (Barremian–Aptian) in the Abadan Plain show a wide range of porosity and permeability due to their complex diagenetic history. Therefore, their petrography was investigated by polarizing and scanning electron microscopy. The porositypermeability was estimated by routine core analysis. Based on the results, Gadvan sandstones were divided into five diagenetic facies. Limitedvolume of chlorite has greatly reduced the effect of compaction by creating a coat on quartz grains and limiting the overgrowth of silica cement. Carbonate cements up to 9% of the total rock have improved the reservoir quality whereas mesogenetic carbonate cemented sandstones (25 and 32% cementation) have often acted as a barrier to the continued movement of hydrocarbons. After determining the porosity and permeability, in order to find the relationship between these two parameters with diagenetic facies, the flow units were determined by the flow zone indicator (FZI) method. It seems that HFU 1 and 2, which are equivalent to diagenetic facies 3 and 5, have the best reservoir quality in the studied wells.Keywords: Diagenesis, Diagenetic facies, Reservoir quality, Gadvan Formation, Abadan Plain  Introduction Throughout the Neocomian–Aptian aged siliciclastic Gadvan Formation deposited in SW Iran as the lateral equivalent of the lower Zubair Formation of S Iraq (e.g., Jassim et al. 2006). This potential reservoir is a clean and quartzrich sandstone (quartz arenite) that has a low diagenetic potential and is basically prone to cementation (quartz and carbonate) and compaction. These diagenetic processes were defined as differentiating diagenetic facies. Then, appropriate methods of evaluating porosity and permeability (such as Flow zone indicator: FZI) were utilized to establish the relationship between diagenetic facies and flow units.  This relationship can be a basis to make a reservoir model which is generalizable to a field, an area or another formations. Materials MethodsAfter the petrography analysis of 70 thin sections and SEM of 12 samples from 95m of cores, porositypermeability of 130 samples was estimated by routine core analysis and then HFUs were determined using Amaefule et al. (1993) parameters by FZI method. Discussion of Results ConclusionsAlthough Abadan Plain Cretaceous successions host a considerable part of the world’s total hydrocarbon reserves and numerous oil reserves of the Middle East (Alsharhan and Nairn 1993), siliciclastic reserves are neglected in this area. Throughout the Barremian–Aptian, Gadvan Formation– a siliciclastic interval– is deposited on the Arabian Plate platform in SW Iran as the lateral equivalent of the Zubair Formation (a giant siliciclastic reservoir) of S Iraq (Alsharhan and Nairn 1993).In this study, the identification of petrofacies, diagenetic processes and products, pore types, and reservoir properties led to recognition of five diagenetic facies and made a reservoir model based on relationship between these two facies attributes.Based on the modal analysis and classifications of sandstones, the studied interval is composed of three main petrofacies and five diagenetic facies as follows: PF1: Well sorted fine to mediumgrained quartz arenite, PF2: Moderate to well sorted very finegrained quartz arenite, PF3: Poor to moderately sorted very finegrained quartz wacke. DF1: Compacted and weakly cemented sandstone, DF2: ChloriteKaolinite and calcite cemented sandstone, DF3: Chlorite coatrich sandstone, DF4: Tightly calcitecemented sandstone and DF5. Calcitedissolved sandstone.The main diagenetic processes are compaction, cementation, replacement and dissolution, and the main diagenetic minerals identified within this diagenetic paragenesis are calcite, chlorite, kaolinite, and quartz. Mechanical compaction reduced the porosity throughout diagenetic steps. During eodiagenesis, the formation of chlorite coatings and precipitation of calcite are the main diagenetic reactions. The point and straight contacts between grains show that the chlorite coatings and calcite formed before significant compaction. Concurrently with this enhancement of alkalinity, calcite started to be precipitated and occupy intergranular pores prior to compaction. Chlorite coating is dominant in the tightly kaolinitecemented sandstone and chloritecoatrich sandstone, and porefilling calcite is dominant in the tightly calcitecemented sandstone. Organic acids and carbon dioxide derived from the decarboxylation of organic matter promoted the dissolution of feldspars and calcite, resulting in the development of secondary porosity and precipitation of authigenic minerals (Seewald 2003). Kaolinite was produced by feldspar dissolution and filled pores preserved by chlorite coating, as commonly observed in the DF2. The overgrowth of quartz as a result of feldspar dissolution is the dominant diagenetic process in the compacted and weakly cemented sandstones (DF1). Carbonate cement content shows a positive relationship with porosity at low contents (less than ~9%) but a significant negative relationship at high contents (greater than 9%) (Chi et al. 2003; Cui et al. 2017). After determining the porosity and permeability, in order to understand the relationship between these two parameters with diagenetic facies, flow units were determined by the flow zone indicator (FZI) method. Comparing of DFs and HFUs shows that HFU 1 and 2, which are equivalent to diagenetic facies 3 and 5, have the best reservoir quality in the studied wells.