Mineralogical and geochemical characteristics of hydrocarbon-bleached rocks in Baiyanggou mud volcanoes, Xinjiang, NW China

Abstract

A total of 11 rock samples were systematically collected along the main flow of geofluids running on the slope of the Baiyanggou mud volcanic system and analyzed for their mineralogical and chemical compositions as well as for iron species using XRD, XRF and Mössbauer spectroscopy, respectively. The analytical results clearly indicated spatial variations for most geochemical parameters caused by redox fluctuation processes around the Baiyanggou mud volcanic system. Close to the mud volcanic craters, the bleached greyish rocks mainly contained ferrous iron in chlorite and muscovite (para-Fe2+) with some portion of ferric hydroxide and ferric iron in clay minerals (para-Fe3+), suggesting a strong reducing environment. Further away from the craters, the relative contents of reducing iron species (Fe2+) in the rocks gradually decreased. The reddish rocks did not appear to be greatly influenced by erupting geofluid and contained relatively high contents of para-Fe3+ and ferric iron in hematite (hem-Fe3+). A redox front occurred at a certain distance from the active mud volcanic craters, in which sharp changes of iron-bearing minerals could be tracked. The reducing and weakly alkaline medium contributed to the precipitation of secondary carbonate minerals such as calcite and siderite. Geofluids composed of brackish waters, gases and oils tended to migrate through sandy veins in a clay sequence, faults and fracture networks with relatively high porosity and permeability. Iron speciation of the solid rocks in the seepage areas could therefore be considered a potential indicator for the migration of reducing geofluids including hydrocarbons located underground.