علل پیدایش نفت سنگین در خلیج فارس
محورهای موضوعی : زمین شناسی مخازن نفت
1 - شرکت ملی نفت ایران
کلید واژه: خلیج فارس نفت سنگین کمان قطر درز وشکاف شکشتگی میادین فرزاد فردوس وهامون ,
چکیده مقاله :
تعدادی از غنی ترین مخازن نفت سنگين و آسفالت طبيعي در بخش ايراني خليج فارس در جهت NW-SE شبه جزيره قطر و حواشی ان رخداده است. اغلب ساختمانهاي زمين شناسي كه در مسير کمان قطر قرار گرفته‌اند دارای نفت سنگين و آسفالت طبيعي در طبقات پسا ژوراسيك ان است . يكي از فاكتورهاي اصلي زمين شناسي كه باعث تجمع نفت سنگين در ميان تعدادی از ساختمانهاي زمين شناسي اين منطقه تاثیر گذار بوده عامل بالا زدگی کمان قطر را میتوان نام برد. این رخداد،باعث ايجاد تفاوتهائي بنیادی در واحدهاي زمين شناسي در بخش شمال شرق و جنوب غرب کمان قطر ( بالا زدگی قطر) گرديده است. بالازدگي باعث تقسيم ژئوسنكلينال خليج فارس بر دو بخش گرديده است. ليكن اين دو بخش همواره از دیدگاه رسوب گذاري يك حوضه مرتبط بحساب مي آيد. فعاليت كوهزائي هيماليا " آلپ " در پايان ميوسن ـ پليوسن تداوم بخش این رخداده است. در اين زمان چين خوردگي زاگرس بدون شك در شتاب بخشيدن به رشد بعضي از ساختمانهاي زمين شناسي اين ناحيه بسیار موثر بوده است. از طرفي ساختمانهاي زمين شناسي واقع در شبه جزيره عربستان و بخش عربي خليج فارس ظاهرا" كمتر دستخوش اين حركات كوهزائي قرار گرفته است.
Some of the most prolific petroleum reservoirs with high gravities of oil in the world occured in the Upper Jurassic and Cretaceous carbonate formations in the Persian Gulf area .Most of the reservoirs are composed of pelletal,oolitic ,or bioclastic grainstones and reefal limestone that have high primary porosity and permeability .These reservoirs are sealed either by tight limestone ,massive anhydrite, or by impermeable rocks. The seals are effective throughout most of the Persian Gulf and surrounding areas, Accumulations of heavy oil and natural asphalts on the Iranian side of the Persian Gulf extend NE to SW along the Qatar arch. some of the geological structures which are located along this trend contain heavy oil and natural asphalts within post- Jurassic formations. The major geologic factor which caused accumulations of heavy oil within some geological structures is the Qatar arch. As a result ,some differences appear in lithologic units in two sub-basins northwest and southeast of the Qatar Arch. In general , the occurrence of heavy oil in this trend can be explained as follows; • Fracturing and joints within formation ; • Reduction of thickness of post- Jurassic sediments; • Facies change of Arab reservoirs, cap rock (Hith anhydrite), in some structures from anhydrite to dolomite and also pinchout; The absence of proper environment for the generation of high gravity oil may also be of importance .It should be mentioned that "F" structure (Ferdows), with huge amount of oil in Ratawi and Sulaiy carbonate reservoirs(Lower Cretaceous), is one of the largest heavy oil fields along this trend .The Farsi "B" structure, ( Farzad), with a thick Jahrum formation (Eocene) which has excellent reservoir properties, is filled with natural asphalts.
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