مطالعه ژيوشيميايي سنگ هاي منشا نفت در ميدان نفتي آغاجاري بر اساس داده هاي راك ايول

Geochemical investigation of petroleum source rocks by using Rock- Eval data in the Agha-Jari oilfield

در اين مطالعه 40خرده حاصل از حفاري سازندهاي پابده، گورپي،كژدمي و گدوان ميدان نفتي آغاجاري مورد آناليز راك ايول قرار گرفته است. به منظور شناسايي شرايط محيط رسوب‌گذاري سازندهاي مورد مطالعه، به 4زون تقسيم شده اند كه زون هاي A (سازند كژدمي-187) و C (سازند كژدمي-140) بيانگر شرايط احيايي تر همراه با حضور مواد آلي دريايي و زون هايD (سازند گدوان-140)وB (سازند گدوان-187) بيانگر شرايط اكسيدي تر همراه با ورود مواد آلي قاره اي به حوضه است. براساس آناليز راك ايول سازند هاي پابده، گورپي، پابده و كژدمي از پتانسيل هيدروكربن-زايي متغيري برخوردار هستند. نمودار HI در مقابل OI نشان داد كه نوع كروژن در اين سازندها مخلوطي از II وIII مي‌باشد. ميزان اثر ماتريكس سازندهاي پابده، گورپي، كژدمي و گدوان، از طريق ترسيم نمودار S2در برابر TOCو تعيين معادله خط رگرسيون آنها مورد محاسبه قرار گرفت. نتايج نشان مي دهد كه مقدار قابل توجه S2 جذب شده توسط ماتريكس در سازند پابده (mg HC/gr rock96/6-98/ 4 در چاه‌هاي 113و 121) و گورپي (mg HC/gr rock33/4 در چاه 113) مي باشد كه به علت بلوغ حرارتي كم (430(Tmax < و نسبت پايين گاز به نفت اين سازندهاست. در سازند گدوان چاه 187مقدار S2 جذب شده توسط ماتريكسmg HC/gr rock13/0 مي باشد كه داراي بلوغ حرارتي نسبتاً بالايي مي باشد. كمترين تاثير ماتريكس مربوط به سازندهاي كژدمي و گدوان مي باشد، زيرا اكثر نمونه ها بالغ بوده، به مرحله نفت‌زايي رسيده اند و GORPبالايي را نشان مي دهند. مقدار كربن آلي فعال در سازندهاي پابده 64/1- 27/0(بر سنگ منشا ضعيف تا خوب)، سازند گورپي 64/0- 14/0 (سنگ منشا ضعيف تا متوسط)، كژدمي 27/2- 85/1 (سنگ منشا خوب تا غني) و سازند گدوان 51/0- 30/0درصد وزني (سنگ منشا ضعيف تا متوسط) دلالت دارند. مقدار پارامتر GORP براي سازندهاي مورد مطالعه (1-0) نشانگر محدوده متغير پتانسيل نفت زايي و گاززايي آنها مي باشد.

Introduction Rock-Eval device is one of the main tools that uses in geochemical studies. This has widely application for oil and gas exploration in the sedimentary basins (Behar et al., 2001). In this investigation, S2 vs.TOC diagramis used for quantitative determination of matrix effect, type of organic matter, active and inactive content of organic carbon and the gas-oil ratio of organic carbon in the Pabdeh, Gurpi, Kazhdumi and Gadvan Formations as possible source rocks in the Aghajary oilfield. The amount of S2 peak which is corrected with matrix effects can be considered as petroleum potential measure. Langford and Valleron in 1990 used S2 (hydrocarbons produced by pyrolysis) versus total organic carbon diagram to determine the matrix effect and type of organic matter. To determine oil and gas prone carbons coefficients are used from pyrolysis analysis- gas chromatography (PY-GC) and microscopic studies of kerogen (Mukhopadhyay et al, 1992). Dahl et al. (2004) used TOC vs. S2 diagram to determine the matrix effect on the hydrocarbon absorption, content of active organic carbon (TOClive) and inactive (TOCinert) and also the amount of oil prone (TOCoil) and gas prone (TOCgas) organic carbon. This method of Rock-Eval pyrolysis it can help to determine the percentage of oil and gas prone organic matter while the PY-GC and kerogen microscopy’s data are rarely used to geochemical data analysis (Dahl et al., 2004). Material and Methods In this investigation, 40 drilling cutting of the Pabdeh, Gurpi, Kazhdumi and Gadvan Formations were analyzed from the Aghajari Oilfield by using Rock-Eval pyrolysis. Rock-Eval pyrolysis data were utilized to determined type of organic matter, mineral matrix effect, the content of active and inactive organic carbon. Gas-oil ratio was examined from plotting S2 and TOC data. The type of organic matter was determined by using HI vs. OI diagram. To evaluate organic matter content diagrams of S2 vs. TOC and S1+S2 vs. TOC were employed. In order to determine the maturity of the studied formations HI vs. Tmax diagram was studied. The Jones diagram was used for determining organic facies. Qualitative determination of the amount of hydrocarbon absorption by mineral matrix was studied by using S2 vs. TOC diagram. Mineral matrix effect reduced the amount of S2 values derived from from Rock Eval data. The result of geochemical data analysis are presented in Table 1 & 2. Discussion of Results and Conclusions In the investigation, the kerogen type is a mixture of types II and III (dominantly type II). S2 vs.TOC diagram is a good method for interpreting the Rock Eval data. It was found to be a very effective method for the quantitative determination of mineral matrix role in hydrocarbon absorption and analysis of inactive and active organic carbon content (Table 1). Average active organic carbon indicates the studied formations have acted from poor to very good for hydrocarbon generation. The results of analyzing depositional environment of the formations show that A (Kazhdumi Formation in well#187) and C zones (Kazhdumi Formation in well #140) are related to more reduction conditions associated with the presence of marine organic matter. While D (Gadvan Formation in well#140) and B zones (Gadvan Formation in well#187) represent more oxidation conditions associated with presence of continental organic matter. The amounts of GORP parameter for the formations are 1-0 that indicates variable range of oil and gas generation potential (Fig. 1 & Table 2). The amount of GORP for the Pabdeh Formation is 0.22 that reveals slightly higher oil prone potential together with type II kerogen and rich organic matter. This parameter for the Gurpi Formation is about 0-0.85 indicating the higher gas prone potential.The GORP parameter value for Kazhdumi and Gadvan Formations are 0.55-0.6 and 0.83-1respectively. The maximum impact of matrix effect in TOC absorption is related to the Pabdeh Formation due to immature nature which have not reach oil window. Low GORP ratio (0.22) is another factor which enhanced the mineral matrix effect. Probably, samples during pyrolysis produced heavy hydrocarbons and also had less organic matter content; consequently, the matrix effect is high for the Pabdeh Formation. Minimal impact of the matrix effect is related to the Kazhdumi and Gadvan Formation. The reason for this mature nature of source rocks ehich have reached oil window and showed a higher GORP. It can probably be inferred most of studied samples were light and had more organic matter content.