مجله زمین ساخت
پیاپی 21 (بهار 1401)

Geometric analysis of fractures in calcareous reservoirs containing hydrocarbon resources is of particular importance. Knowing the location and distribution of fractures in such reservoirs can play an effective role in providing field development plans, determining suitable sites for new drilling and reducing exploration costs. This study was carried out to evaluate the relative frequency of fractures method to identify regional fractures in the Bibihakimeh oilfield. The required data have been extracted by analyzing the underground countor maps in the Bibihakimeh oilfield. Then, the accuracy of the fracture map prepared based on this method was evaluated with the relevant data of reservoir parameters in the studied oilfields. Data on mud loss, thermal gradient, overburden thickness, total organic carbon and production index confirm results obtained from the relative frequency of fractures in the center of the oilfield. Comparison of the results with the map of basement faults in the study area shows that the performance of these regional fractures has caused curvature development and fractures concentration in some parts of the Bibihakimeh oilfield. These regional fractures act as a conduit for heat transfer from the depth to the surface, which in turn has led to a sharp increase in thermal gradient and production in wells in the central parts of the studied oilfield.

The Permian period is associated with an extensional tectonic regime during which the Neotethys oceanic basin developed between the central Iranian blocks and the Arabian Plate. In this study, the effects of this extensional regime on developing the normal faults and associated syndepositional sequences have been investigated in the Abgarm region at NW Iran. There is a huge gap in the stratigraphic sequence of the region from the Cambrian to the Devonian. This discontinuity is seen in some areas as concordant and some areas as angular. In addition to the Cenozoic and younger thrust and strike-slip faults system, faults with the major northwest-southeast trends are seen that extend only to Paleozoic units and are not observed in units of Triassic or younger age. Field observations document the normal kinematics of these faults. They have cut and displaced pre-Permian sequences. The Permian rock units including the Dorud and the Ruteh Formations have generally thickened toward these normal faults that document the syndepositional normal activity of faults. The normal faults have been covered by the Upper Permian and Triassic and younger deposits including Doroud, Ruteh, and Elika formations document the pre-Permian timing of the extensional regime in this part of the central Iranian basin.

Calculation of geological and seismic moments is one of the methods in evaluating the level of tectonic activities in different regions. In this study the seismic potential in eastern Iran was assessed using geological and seismic moment rates. Results show that the maximum geological moment for the Sabzevaran fault is 2.61544 × 1018 and the minimum value, 9.91367 × 1015, belongs to the Jazmourian fault. Based on the obtained iso-acceleration map, the average and maximum acceleration values (0.747423) have been observed around the important Saravan, Bam posht, Asagi, Pishgiram, Zahedan and Gowk. The calculated cumulative geological moment rate for the studied region is 1.4166 × 1018. According to the cumulative geological moment rate map, the maximum value of the cumulative geological moment rate of 2.26248 × 1020 was observed in the vicinity of the Sefidabeh-Zahedan, Kahurak, Narreh Now, Gowk, and Shahdad fault systems. The minimum cumulative geological moment rate, 1.77828 × 1019, was detected in the northern region of the Narges fault.

Galleh Chah area is located in the 1:100000 geological map of Ayesk, in the northwestern part of the Lut zone, in a sub-shear zone (the Galehchah shear zone) along the northwest-southeast direction. By studying satellite images along with field observations and drawing rose diagrams, fractures in the region are placed in three groups in order of occurrence time. 1. Almost north-south faults that are dextral strike-slip with a reverse component that are cut by other faults. These faults lack mineralization. 2. Northwest-southeast faults as dextral strike-slip with reverse component. These main shear zone faults (Y) and conjugate faults that have a smaller angle with the shear zone (R and P) have provided conduits for the rise and deposition of ores. 3. The northeast-southwest faults are left-lateral with a normal component that cut off the previous two groups. The paleostress patterns indicates two evolutionary stages in the Galleh Chah region. In the first stage, the stress in the northeast-southwest direction has caused the creation of an almost north-south shear zone with dextral strike-slip faults with a reverse component. In this stage, folds with a northwest-southeast axial trend have been created. Then, as a result of a local stress in the direction of north to northwest-south to southeast, Galleh Chah shear zone with northwest-southeast trend and with dextral strike-slip faults with reverse component has been formed. With the continued activity of Galleh Chah shear zone, antithetic Riddle fractures with the northeastsouthwest trend have caused the displacement of previous faults and mineralization veins.

In this paper effect of the strike-slip deformation on the Neogene structural evolution of the Qom sedimentary basin, western area of Qom city is investigated. For this purpose, after preparing geological map, structural study has been performed. Two stages of deformation are identified in the study area as the effect of NW trending strike-slip zone parallel to the Qom fault. In the first stage of deformation the main folds such as Mil and Kamarkuh anticlines and Yazdan syncline are formed and rotated counterclockwise. Then the Mill-Dochah fault and relevant sinistral fault are formed and folded. In the second stage, NNW to NW dextral strike-slip faults were initiated which in turn created restraining zone where the Mil-Dochah fault reactivated reversely and limbs of the Mil anticline were overturned. The syn-sedimentation normal folds are also reactivated normally in this stage. Sinistral and dextral strike-slip deformation of the zone is compatible with the first and second stage of deformation respectively. According to the angular unconformity that is formed as the effect of syn-sedimentation folding in the Upper Red formation, it is proposed that the age of first stage deformation is Miocene and second stage of deformation, which cut the first stage structures and reactivated the faults, is initiated in Pliocene.

Sefidabe and Hyderabad antimony ore deposits are located in the southwest of Sefidabe and in the Sistan Suture Zone .In order to structural analysis of these areas, at first, using satellite images, tectonic lineaments were extracted and the areas with the highest density of length and direction were determined. Then, in the field surveys, veins, faults and mineralization occurred along them were taken. By using stress analysis software, the direction of the principal stresses in the area and the relationships between these structures, tectonic faults and mineralization were analyzed. Compressive shear zones with a northwest-southeast trend are caused by a constraining bend formed between the two of Neh and West-Asagi main faults, is the main factor of mineralization in the study area. This positive flower structure has strongly affected the Paleogene sedimentary and igneous rocks of the region. The Moore's circle indicate a compressional to wrench tectonic regime with the maximum stress direction (03/215) in the northwest-southeast. In this stress regime, in addition to thrust faults, a group of conjugate faults with an acute angle and tensile fractures aligned with the direction of maximum principal stress have formed, which have provided a suitable environment for fluid injection by creating weak zones.