Iron speciation in bleached rocks by hydrocarbon leaching in Dushanzi Mud Volcano, NW China

Abstract

Mud volcano is a typical petroleum leaching system, which normally result in bleaching effect to surrounding rocks. The result of Mössbauer spectroscopy of rock samples collected from the Dushanzi mud volcano system revealed clear variations in iron species. Compared to the unbleached reddish sedimentary rocks, the bleached rocks are dominated by reducing iron species such as paramagnetic ferrous iron (para-Fe2+), ferrous iron in siderite (sid-Fe2+), sulphide and pyrite (pyr-Fe2+) whereas the original reddish rock is enriched in ferric iron including iron in hematite (hem-Fe3+) and paramagnetic ferric iron (para-Fe3+). A reduction of ferric iron species and hydrolysis of iron along with oxidation of hydrocarbons should be one of the main processes along bleaching by hydrocarbons to rocks.