Abstract
The properties of rocks can be greatly affected by seepage hydrocarbons in petroleum-related mud volcanoes. Among them, the color of sedimentary rocks can reflect the changes of sedimentary environment and weathering history. However, little is known about the microbial communities and their biogeochemical significance in these environments. In this study, contrasting rock samples were collected from the Aiqigou mud volcano on the southern margin of the Junggar Basin in Northwest China as guided by rock colors indicative of redox conditions. The physicochemical properties and mineral composition are similar under the same redox conditions. For example, the content of chlorite, muscovite, quartz, and total carbon were higher, and the total iron was lower under reduced conditions compared with oxidized environments. High-throughput sequencing of 16S rRNA gene amplicons revealed that different functional microorganisms may exist under different redox conditions; microbes in oxidized conditions have higher diversity. Statistical analysis and incubation experiments indicated that the microbial community structure is closely related to the content of iron which may be an important factor for color stratification of continental sedimentary rocks in the Aiqigou mud volcano. The interactions between organics and iron-bearing minerals mediated by microorganisms have also been hypothesized.
Highlights
Due to the existence of natural gas seepage in the mud volcano systems, the reduction of minerals and/or elements likely became the dominant process, which has received a great deal of attention [1]
The strata exposed by the Aiqigou mud volcano are Cretaceous rocks [18]; natural gas and oil in underground reservoirs diffuse to the surface and react with the considered to be under oxidized environments, whereas samples AQG1/5/6 were considered to represent reduced environmental conditions
The difference in the color of these rocks has previously been shown to be caused by the bleaching effect of crude oil and natural gas, as well as the activity of previous mud volcanoes [1,9]
Summary
Due to the existence of natural gas seepage in the mud volcano systems, the reduction of minerals and/or elements likely became the dominant process, which has received a great deal of attention [1]. Mud volcanoes are the most extensive surface expressions of hydrocarbons fluid migration in petroleum bearing sedimentary basins [3]. Surface seepage of geological fluids, such as crude oils and natural gases from mud volcanoes can transform the color of rocks from bright reddish to green, deep gray, or black, and is known as the bleaching effect [7,9]. The dominant presence of ferrous iron (generally visible as gray and green colors) is indicative of a reduced environment, conducive for the formation of potential petroleum source rocks, whereas iron-bearing minerals rich in ferric iron (visible as red or yellow colors) are indicative of oxidized conditions [8]
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