Mud Volcanism in the South East Caspian, Gorgon Plane, Iran

Abstract:

Although numerous studies have been completed on the Western Caspian side (e.g. mainly Azerbaijan), very little is known about the hydrocarbon plumbing system of the deepest and southernmost basin. This region has great potentials for hydrocarbon exploration and the study of mud volcanoes located here represents the opportunity to access to an open window to better understand the stratigraphy and the mechanisms ongoing at great depth as well as the origin and signature of the seeping fluids.

Three so far unexplored mud volcano structures (Sofikam, Gharniarigh, and Naftliche) have been mapped and sampled in the Golestan region in the south eastern Caspian Sea. All the structures have negative morphology (i.e. “pockmark like”) with caldera collapse. A multidisciplinary workflow of analyses is being conducted including gas and water geochemistry, incubation of microbial colonies, petrography of the seeping mud and erupted mud breccia clasts.

Sofikam consists of 5 distinct pools up to 4-5 m in diameter that forms an E-W oriented alignment. All of the pools display vigorous seepage of fluids and are either water- or denser mud-dominated. Gharniarigh is a large mud volcano up to ~600 m in diameter with a bulging island in the internal part of the crater where eroded gryphons ridges witness a palaeo vigorous activity. The outskirts of the “island” are almost entirely flooded with water and/or covered with salt crusts in the summer. Here are distributed several small water and gas seeps. Naftliche (~400 m wide) is filled with water with a main seep in the centre of the lake.

Preliminary gas geochemistry indicates the seepage of methane-dominated gas in all structures with additional small portions of ethane and propane as well as iC₄ in Gharniarigh and Naftliche.

All samples collected for microbial colonies incubation reveal strong activity with CO₂ production under aerobic and anaerobic conditions as well as production of biogenic methane. In particular, samples from Gharniagirgh show high rates of sulfate-reducing microbes in agreement with the strong H₂S smell. Here is also detected methane oxidation activity as the microbial community seems to have adapted to methane seepage. Water geochemistry shows enrichment in all elements that is typical of brines seeping at mud volcanoes.