The effect of tectonic geomorphology in the exploration of hydrocarbon resources in Fars Zagros (case study: Khesht and Nora anticline)

Introduction Nowadays, oil is considered as one of important energy resources because its storage for consumers is simple compared to many other energy resources. Furthermore, oil resources are of exhaustible and non-renewable energy sources and hence for compensation of used oils, we should only proceed with the prospecting of new resources. One may say, apart from geologist, geomorphologist also can be effective in determining oil fields by identifying anticlines morphology. Therefore, the study of relations between geomorphology and hydrocarbon resources is important. In this research, we have tried to evaluate the effect of structural geomorphology on the exploration of hydrocarbon resources in Khesht (hydrocarbon rich) and Nora (hydrocarbon-less) anticlines.
Methods The purpose of this research is to evaluate the morphometric characteristics of Khesht and Nora anticlines and the applications as well as the applications of these studies in exploration of hydrocarbon resources. To achieve the purpose of this study, at first the geologic maps from National Iranian Oil Company, at the scale of 1:250000, were prepared and the geological cross-sections of anticlines were obtained. The topographic maps of study area at a scale of 1:50000 were digitized in Arc-GIS software and then the hypsometry and drainage system maps were prepared. After stream ordering based on Strahler’s ordering system, the length and numbers of streams with different orders were obtained and thereby the Bifurcation Ratio (Rb), Drainage Frequency (Fs), Drainage Density (Dd) indexes and and drainage patterns of studied anticlines were calculated. The calculation of Fold Front Sinuosity (FFS), the Length of Anticline Divide (AD), anticline height, Fold Symmetry Index (FSI) and Aspect Ratio (AR) indexes as well as shape and trend of anticlines were made by means of LANDSAT satellite images and geological maps. Finally the relations of geomorphic-tectonic parameters with the formation of hydrocarbon resources in hydrocarbon rich and hydrocarbon-less anticlines, as well as the effects of mentioned parameters in the escapement of hydrocarbon resources were analyzed.
Discussion and Conclusion Results of this study show that here are close relationships between the geomorphology of studied anticline and the dimension of hydrocarbon resources as well as the escapement of hydrocarbon resources so that petroleum resources have been preserved in young, lower amplitude, less-eroded and less-faulted anticlines whereas petroleum resources have been depleted in older more eroded and faulted anticlines. Data analysis shows that the morphometry of petroleum- rich and petroleum-less anticlines is different. The value of Fold Symmetry Index (FSI) is closer to 1 in Nora anticline implying that symmetry and lower tectonic activity in mentioned anticline. The rate of Fold Front Sinuosity (FFS) is same in both anticlines showing the lower performance of FFS index in determining tectonic activities. The lower performance of mentioned index can be attributed to the presence of geologic Fars formations and its high mobility against tectonic activities. The value of Length of Anticline Divide (AD) is 1.12 for Nora anticline showing its older age and application of higher erosion in its hinge. The higher value of Aspect Ratio (AR) in Nora anticline is indicative of its high tectonic activity. Also, the higher rate of bifurcation ratio (Rb) in Nora shows the structural disturbance as well as low development of drainage system and hence high tectonic activity of mentioned anticline. The lower rate of drainage density (Dd) and drainage frequency (Df) in Nora anticline compared with Khesht one is indicative of older age and higher erosion of Nora, and this is a main cause of petroleum resource escapement in this anticline. The drainage patterns in Khesht anticline are parallel, sub-parallel and dendretic that are indicatives of tectonic youthful of mentioned anticline. The young age of mentioned anticline results in petroleum resource in the hinge, on the one hand, and the tectonic activities are not so intense to form profound faults and petroleum escapement, on the other hand. The drainage patterns in Nora anticline are subdendritic and and rectangular (in center) demonstrating the effect of tectonic parameters, such as faults and fractures, on mentioned anticline, and these parameters have resulted in escapement and depletion of its petroleum resources during time. Evaluation of shape and trend of anticlines reveals that Khesht anticline has straight and long trend while Nora anticline has sinuous trend. The more sinuous and deformed anticline hinge, the more probability of petroleum escapement and leakage.
Based on quantitative indexes calculated in this research, it can be expressed that although Nora anticline contains all necessary tectonic conditions to have hydrocarbon resources, the development of erosion in the central part of anticline has resulted in the escapement of hydrocarbon materials from its reserves. If the exploitation of anticline having hydrocarbon resources will not take place in future, the continuation of tectonic activity will result in the development of joints and fractures and hence in the intensification of erosion in the anticline hinge in future , resulting in escapement of resource before exploitation. Therefore, it is necessary to study all country’s anticlines having the potential of hydrocarbon resources formation based on geological history, in order to have an economic and on time exploitation of resources.