تعيين الگوي تراكمي شكستگي هاي مخزن آسماري در ميدان نفتي مارون

Determination of density pattern of fractures in Asmari Formation in Marun oilfield

ميدان نفتي مارون در مياني فروافتادگي دزفول در امتداد تاقديس هاي آغاجاري، اهواز و رامين قرار گرفته است. با توجه به نقش مهمي كه شكستگي ها در افزايش توليد ايفا مي كنند، در اين مطالعه سعي شده تا الگوي تراكمي شكستگي هاي مخزن آسماري در ميدان نفتي مارون مورد بررسي قرار گيرند. بدين منظور نتايج داده هاي نمودار هاي تصويرگر، مغزه داده هاي نمودار ترسيمي سرچاهي، روش هاي تحليل دايره محاطي و تغييرات انحنا هندسه تاقديس مورد تحليل قرار گرفت. سازند آسماري مهم‏ترين سنگ مخزن اين ميدان است كه به پنج زون مخزني تقسيم شده است. زون مخزني يك، دو، سه عمدتاً از آهك و دولوميت تشكيل شده اند. بنابراين تراكم شكستگي ها در اين زون ها (بخصوص در زون يك كه90 درصد دولوميتي است) بيشـتر است. همچنين در زون هاي مخزني چهار و پنج اين ميدان به دليل افزايش ميان لايه هاي شيلي و مارني و كاهش شكنندگي، شكستگي ها ازگسترش كمتر برخوردار بوده، بيشتر شكستگي هاي ميكروسكوپي ديده مي شود. طبق نتايج حاصله انطباق خوبي بين روش تحليل دايره محاطي، داده هاي نمودار تصويرگر، داده هاي مغزه، تغييرات انحنا و گسترش شكستگي ها در تاقديس مارون ديده مي شود. تحليل داده هاي مذكور بيانگر اين است كه بيشترين تراكم شكستگي ها در يال جنوبي مخزن در بخش مركزي و در يال شمالي بخش شمال شرقي متمركز است. بنابراين بخش غربي مخزن، تراكم شكستگي كمتري نسبت به قسمت هاي مركزي را دارد. باتوجه به نتايج به‏دست آمده مي توان استنباط نمود كه براي حفر چاه هاي توسـعه اي و توليـدي آتي بايد پارامترهاي ذكر شده مد نظر قرار گرفته، به قسمت شمال شرقي و يال جنوبي تاقديس مارون توجه بيشتري گردد.

Introduction: Because of the effects of fractures on fluid flow in the reservoir and the manner of the discovery of oil and gas studying the natural fractures in subsurface formation In recent years have become more frequent (Sisler 1971; Wu & Pollard 1991). By Studying the density pattern of fractures its possible to recognize the paleo-stress direction applied to the reservoir, furthermore the permeability of the reservoir also could be determined (Aydin 2000; Bouroz 1990; Pollard & Aydin 1988). However the fractures are one of the most important path of flow for the carbonate-reservoir fluids. An example of the role of fractures, is the effect of the fractures on porosity, permeability and higher productivity of the SW carbonate reservoir of Iran (Mcquillan 1969; Nemati & Pezeshk2005). Generally these fracture are tectonical in nature and were formed during the pressure phases of late-Alpine orogeny due to NNE movement of Arabia plate toward Iranian plate. Geographical location Marun oil field is located in the southern of Northern part of Dezful Embayment and is situated along with Aghajari, Ahvaz and Ramin anticline. Numerous studies were conducted on the Asmari reservoir of the Marun oil field leading to different suggestion for Zoning of the Reservoir. Methodology: Using the GEOLOG 6.6 software along with various chart of the well 240 the needed parameters were calculated. For this study 140 cutting samples from the well 240 were analyzed and the fracture densities were calculated. Furthermore the U.G.C map of the Asmari Reservoir was used and the 3900 contour line was used as a base depth. Conclusion: The results of the study of the cores showed that at the the central and northeastern sections because of more fragility the fracture density was greater, furthermore a significant increase in density from the edge to the hinge is recognizable. The maximum density of fractures was seen in layers 1, 2, 3 and majority of open fractures have been identified in southern flank. Cores study shows that the dominant lithology in the eastern section is dolomite, that reaches 90% in the center of the field, therefore it’s natural to assume greater presence of fractures in the eastern half realtive to the western half. Based on the inscribed-circle-map, two areas prone to fracture were recognized, first one is in the southern flank in the central region anticline and the other in the north flank in the northeast region of the northern anticline. And the two sets of fractures are related to folding and bending (Based on Core). Analysis of inscribed circles show good coordination in areas prone to fractures with bending and show the greater density of fractionation in the central-western parts of the southern flank and the north-eastern part of the field. The isocline and curvature maps of Asmari reservoir of Marun oil field show areas with high gradient and therefore greater radius of curvature in the south flank of the field. In such circumstances, the maximum fracture formed in southern flank of the reservoir and with increasing of the slope toward central areas, due to a decrease in the radius of curvature of fold, the extension of the fractures toward the northern flank of the reservoir is noticeable. Finally, it could be concluded that in order to drill for the future production and development wells, the mentioned parameters should be considered and more concern and attention should be paid to the northeastern and southern parts of the Marun oil field.