تفسير بي‌هنجاري‌هاي ميدان پتانسيل به‌ منظور بررسي ساختارهاي زمين‌شناسي و اكتشاف تله‌هاي نفتي احتمالي، بررسي موردي: جنوب سمنان

Interpretation of potential field anomalies to investigate geological structures and oilfield exploration: A local study in the south Semnan

براي تعيين موقعيت منابع مولد بي ­هنجاري ­هاي ميدان پتانسيل نياز به پردازش و تفسير دقيق نقشه ­هاي بي­ هنجاري حاصل از برداشت داده ­هاي ميدان پتانسيل است. تفسير بي­ هنجاري­ هاي ميدان پتانسيل با استفاده از روش هاي مختلفي انجام مي­ گيرد. در اين مقاله جهت تفسير و جداسازي بي­ هنجاري ­هاي ميدان پتانسيل از فيلترهاي ژئوفيزيكي مانند روش روند سطحي، زاويه تيلت و ادامه فراسو استفاده گرديد. در نهايت نقشه ­هاي هر يك از اين روش ­ها تهيه و نتايج با هم مقايسه شد. نتايج حاصل از اين نقشه­ ها نشان مي­ دهد كه هشت ساختمان تاقديسي، پنج ساختمان ناوديسي، تعداد زيادي گسل و گنبد نمكي در ناحيه مورد بررسي وجود دارد. از بين تاقديس ­هاي تفسير شده، تاقديسB از عمق و ابعاد قابل توجه ­تري برخوردار است، به‌طوري‌كه با ادامه فراسوي 3000 متر اثر اين تاقديس باقي مانده است. همچنين هيچ‌گونه توده آذريني بر روي نقشه مغناطيسي آن مشاهده نشده است. از آنجا كه هيدروكربورها در سنگ­ هاي رسوبي يافت مي­ شوند و در سنگ ­هاي آذرين وجود ندارند، بنابراين مي­توان اين تاقديس را از نظر تجمع منابع هيدروكربوري، تله مناسبي تلقي كرد. اين نقشه­ ها يكي شدن گسل جنوب تاقديس كوه سرخ با گسل جنوب تاقديس سياه كوه را تأييد مي­ كنند. همچنين اين نقشه ­ها نشان مي‌دهند كه سياه كوه ادامه كوه سرخ است كه به‌ وسيله فعاليت گسل­ هاي منطقه، فرسايش و نهشته شدن رسوبات جديد از هم جدا شده ­اند.

South of Semnan province is an important region in Iran for the formation of oil traps, which has been received much attention for several decades. Many sulfur mines have also been discovered in this area. The sulfur is most probably derived from natural gases that are guided by regional faults. Numerous anticlines and salt domes are also present in this area, playing an important role in the formation of oil traps. As a consequence, this region seems to have a great potential to form hydrocarbon traps. Since the area is covered by Tertiary, Quaternary to current sediments, gravity, and magnetic studies are very useful to investigate and explore the geological structures in this region. Gravity and magnetic studies, in a trapezium grid, were performed to investigate the underground structures, sediment thicknesses, plutonic and volcanic igneous rocks and also hydrocarbon traps. Data acquisition was carried out on 86 profiles with a northwest-southeast trend. The distance between profiles and acquisition stations were selected to be 1000 meter. Due to the presence of swamps, mountainous areas, river, terrain, etc., some stations were removed from the survey plan. In addition, some transects measured in a direction deviating from the straight line. As a result, in some of the profiles, a number of missing stations can be observed. In some of the transects, profiling was not conducted in a regular 1000 m line spacing too. The average magnetic inclination and declination are 53.8 and 4.2 degrees, respectively, and the average total magnetic field is estimated 48181 nT as well. The gravity and magnetic data were collected simultaneously. In this study, to interpret and discern potential field anomalies, we applied geophysical filters such as surface trend, the tilt angle, and upward continuation. After producing a geophysical map for each method, the results were jointly interpreted. Joint interpretation demonstrated that eight anticlines, five synclines, several faults and salt domes could be recognized. Among these anticlines, anticline B has a considerable depth and dimension so that with 3000 m upward continuation, the gravity field can still be seen. Meanwhile, no igneous rocks were observed on the magnetic maps. Therefore, this anticline can be considered as an appropriate trap for the accumulation of hydrocarbon. These maps confirm the joining fault to the south of Kohe- Sorkh anticline with the fault to the south of Siah-Koh anticline, which are located in the south of Abulabad. The results show that the potential methods appear to be promising to characterize subsurface structures for the initial phase of hydrocarbon explorations. These maps also show that the Siah-Kuh is a continuation of Kuh-Sorkh, which are separated by regional fault activities, erosion, and new sedimentation. Consequently, it is suggested to use numerical modeling to define the shape, dimension and depth of anomalies, especially for the interpreted anticline B. Finally, a seismic survey can be performed over the potential anomalies that might have hydrocarbon accumulation.