Origin of the Gharnyaregh and Neftelijeh mud volcanoes in Gorgan plain, Iran

Mud volcanoes are natural phenomena that provide an essential source of information about subsurface sediments and deep structures. The phenomenon is usually associated with hydrocarbon accumulations in deep-seated strata. More than 30% of all mud volcanoes in the world are concentrated in the South Caspian Basin. Gorgan plain, as a part of the south Caspian lowland, is a transition zone between uplifting zones of the Alborz and Kopet Dagh and the subsiding basin of the south Caspian. Four active mud volcanoes in the western parts of the Gorgan Plain have extrusion of gas and mud. The existence of the possible hydrocarbon contents in this plain caused a reflection seismic survey and exploratory wells were performed in this area by the National Iranian Oil Company. In this paper, we used 2-D reflection seismic data and well top data for the investigation of stratigraphy and structural setting of the Neftlijeh and Gharnyaregh mud volcanoes. According to the Well Report, the area sediments are composed of the Tertiary to Quaternary sediments of the SCB and underlying pre-Tertiary sequence of the Kopet-Dagh basin and a missing Oligocene, Eocene, Paleocene and upper Cretaceous deposition are observed in the well. The interpretation of the seismic section shows that the Caspian basin deposits reveal a partial deformation and increasingly sloping toward the depocenter, while the sequence of Kopet Dagh is folded. The Neftlijeh and Gharnyaregh mud volcanoes occurred on subsurface anticline axes of the Kopet Dagh and a cluster of conjugate normal faults developed into the Cenozoic formations above the hinge zone of these anticlines. The subsidence pattern is observable at the summit of the anticlines. The structure suggests the presence of a chamber within the crest of the anticline. It seems extrusion and material removal from this chamber caused subsidence and forming of the structure. The Oligo-Miocene Maykop Formation is the significant source of both the extruded mud and the petroleum in the south Caspian basin, which is not observed in the studied well due to later regional erosion. However, the Cheleken and Apsheron deposits, known as reservoir rocks, have a large thickness in the area. The Kashafrud Formations are considered as the main source rock for the Kopet Dagh basin and the Mozduran and Tirgan formations are known as reservoir rocks. The occurrence of the Late Eocene compressional tectonic phase and following the folding of the Mesozoic sedimentary created structural traps at the strata. In addition, continued diapirism and uplifting of the deep-seated strata led to the growth of extensional faults and fractures over the hinge zones. These structures provide effective pathways for gas and fluid ascent from deep reservoirs to the surface. Generally, the results suggest that these mud volcanoes have a spatial and genetic relationship with buried folds. It seems the location of Gharenyaregh and Neftelijeh mud volcanoes is controlled by subsurface anticlines and their feeding source originated from both the Tertiary (SCB) and Pre-Tertiary (Kopet Dagh) units.