نگاره انحراف معيار گاما: ابزاري جديد جهت جدايش واحدهاي سكانسي در توالي‏هاي كربناته با مثالي از سازندهاي كنگان و دالان، ميدان گازي پارس جنوبي

The gamma-ray deviation log (GDL): a new tool for sequence stratigraphic analysis in carbonate reservoirs, an example from Kangan and Dalan formations in South Pars gas Field

استفاده از نگاره گاما در تعيين سكانس‏هاي رسوبي در مخازن كربناته به سبب دامنه تغييرات كم اين لاگ در اين مخازن كمتر مورد توجه قرار گرفته است. در اين مطالعه از نگاره گاماي مربوط به سه چاه براي تعيين مرزهاي واحدهاي سكانسي سازندهاي كنگان و دالان بالايي (پرمين- ترياس) در ميدان گازي پارس جنوبي استفاده شده است. هر داده نگاره گاما از متوسط داده‌ها در كل چاه كسر گرديد و سپس مقادير به دست آمده با يكديگر جمع شد تا مقدار تجمعي انحراف از معيار به دست آيد. با توجه به اينكه رسوبات مربوط به هرسكانس يك واحد زايشي را تشكيل مي‌دهند، تغييرات نگاره گاما كه تاثير بسيار كمي از فرآيندهاي دياژنزي مي‌پذيرد، بايد همواره در يك سكانس نسبت به يك مبدا مشخص افزايش يا كاهش داشته باشد. اين آناليز در مرحله اول مرز بين سازندهاي كنگان و دالان را در چاه‌ مورد مطالعه نشان داد. در مرحله بعد از متوسط نگاره گاما در هر سكانس تعيين شده به عنوان مبنا استفاده گرديد و در مراحل دوم و سوم مرزهاي سيستم‌ تراكت‏هاي اين سكانس‏ها نيز مشخص گرديد. در مجموع در مرحله سوم، چهارسكانس رده سوم در سازندهاي مورد مطالعه تشخيص داده شد كه منطبق با مطالعات پيشين سكانسي در اين ميدان بود. اين فرآيند بر روي دوچاه ديگر از اين ميدان نيز اجرا گرديده و نتايج مشابهي را نشان داد. اين مطالعه نشان مي‌دهد كه استفاده از نگاره گاما و آناليزهاي ساده و اوليه رياضي مي‌تواند منجر به شناخت دقيق مرز واحدهاي سكانسي در سازندهاي كربناته گردد.

Introduction Sequence stratigraphy is one of the new and revolutionary branches of stratigraphy. This branch of science invented based on the need of petroleum geologist for an accurate reservoir zonation. Core data as the most reliable source of information is not available in all wells and all intervals and so wire line logs are usable for determination of sequence stratigraphic surfaces. Due to the low response value of GR log, this log has a limited usage for such purpose in carbonate rocks. This study use a new synthetic log calculated based on GR data for separation of sequence stratigraphic surfaces. The new tool, called gamma-ray deviation log (GDL) has been tested in Kangan and Dalan carbonate formations in Persian Gulf area. Materials and methods The GR log of three wells has been used for this study. These data cover the entire Kangan and Dalan formations in studied wells. A total of 443 m of cores have been studied in well A and a this section prepared each 30 cm from plug trims. The facies classification has been done based on Dunham nomenclature and repeated in well B and C. Discussion of results and conclusions The new comprehensive studies of Kangan and Dalan formations by the author show that four sequences are present in these formations: KS1 and KS2 in Kangan and KS3 and KS4 in upper Dalan formation. The sequence boundary of KS4 and KS3 (from bottom to top) is characterized by an anhydrite layer. The KS3 and KS2 are separated with Permian-Triassic boundary (PTB) that is characterized by negative carbon isotope shift, meteoric dissolution, Sr isotope peak and presence of intraclast grainstone facies. The KS2-KS1 boundary is characterized again with an anhydrite layer. The GR distribution within the three studied wells show no sharp change and so determination of sequence boundaries and maximum flooding surfaces is not possible based on the original GR data. This is routine for carbonate reservoirs and rocks; as they have no considerable natural gamma radiation. The principles of GDL calculation is change in GR log in the same conditions of the sea-level change. In other words, when sea-level is changing toward one side of the sea-level curve (decreasing or increasing), the GR value is always more (or less) than the average. So, if the deviation of each GR data is calculated from the average in the same way of the sea-level conditions, their summation will follow an increasing or decreasing trend (not both of them). In turnovers, the GR value is near the average and so the deviation is zero. As the result, the turnover of the GDL will be consistent with the sea-level turn overs. The important point is that the deviation should be recalculated for each systems tract after the first order calculation. The GDL of each well calculated in this study based on the mentioned method and compared with sequences derived from core data. The results show a main turnover over the entire interval that is exactly coincident with the PTB. The GDL calculated then in each of these two sections and then in each new cycles. The results show that the GDL has the negative excursion at the base of K4 unit. This unit is the LST of KS4 that is characterized by the presence of considerable anhydrite amount. The log continues with the positive change and a negative shift again. The positive change marks the TST of KS4 and the negative change separate the HST of this sequence. The same change is seen in the KS3, KS2 and KS1. As mentioned earlier, the main change that is clear on all GDL orders, is PTB between Kangan and Dalan formations. This study shows that an effective sequence stratigraphic analysis could be done using the GR log and simple mathematical calculations in carbonate reservoirs. Key words: Gamma-ray log, deviation, sequence stratigraphy, Permian-Triassic, South Pars