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Large earthquakes can cause both casualties and economic losses, but they also provide invaluable opportunities for earthquake scientific research. Geofluids, due to their wide distribution, sensitive response to underground conditions, and ease of observation, are widely applied in field investigations after earthquakes. Analyzing the origin of fluids, energy transfer processes, temperature/pressure conditions, and the spatial-temporal evolution of geofluids can provide valuable information concerning the mechanism of earthquake precursor anomalies, short-term prediction methods, identification of the seismogenic faults, determination of earthquake risk, and the environmental impact of post-earthquake fluids. This article details post-earthquake scientific expeditions and research on fluid geochemistry in China and abroad, aimed at providing ideas and guidance for future scientific expedition work and geochemistry-related earthquake studies. [ABSTRACT FROM AUTHOR]

A series of samples including natural gas, formation water, and rocks were collected from volcanic rock reservoirs in the Niudong area of the Santanghu Oilfields and analyzed for their mineral and/or chemical compositions and sulfur and carbon isotopes in order to investigate the occurrence and origin of hydrogen sulfide (H2S). H2S was mostly dissolved in the formation water along with petroleum production in the study area. The δ34S values of on-well H2S samples varied in a range of 9.2‰ to 20.5‰, probably indicating thermochemical sulfate reduction (TSR) and/or thermal decomposition of organic sulfur-bearing compounds (TDS) as the genetic process for H2S. However, the chemical composition of formation waters from the Kalagang Formation (C2k) and their coefficient of desulfurization also revealed that TSR could be the main principle for H2S formation. Considering the regional geological background, especially the tectonic structures and thermal evolution features of the basin, it was concluded that H2S in the study area was dominantly produced by thermal genesis with TSR as a domain through interactions between hydrocarbons and aqueous sulfate dissolved from sulfate minerals. [ABSTRACT FROM AUTHOR]

Geofluids from natural springs connect with the crust and/or mantle in many cases, and their geochemical anomalies could be significant for the study on faults activity and even earthquakes. Several natural springs are distributed along the Lenglongling fault zone (LLLFZ) in the northeastern margin of the Tibetan Plateau, where the M s 6.9 Menyuan earthquake occurred on January 8th, 2022. Based on chemical and isotopic compositions (δD, δ18O, δ13C, and 3He/4He) of water and gas samples, the origin of geofluids and their potential correlation with fault activity even including earthquakes are preliminarily assessed in this paper. The δ13C CO2 values and 3He/4He ratios showed that the gas originating from the crust was associated with the metamorphism of carbonate rocks, whereas the δ18O and δD values of water samples indicated that the natural springs were predominantly infiltrated with precipitations from local mountains ranging 3.7 - 5.5 km in height. Obvious changes of Ca2+ and HCO 3 − concentrations in SZK spring waters in the surface rupture zones were observed in a short period (about three months) after the main shock, in contrast to those of the GSK springs far from the surface rupture zones. Such variations might be correlated with the stress increase prior to the 2022 Menyuan M s 6.9 earthquake. The mechanical fracturing of surrounding limestone rocks during the slipping movement of LLLF could facilitate the water-rock interactions. Compared to three-month observations after the main shock, relatively higher concentrations of HCO 3 − and heavier δ18O H 2 O values of the LHG springs were also observed in the short-term period. The shallow stored formation water might be squeezed along the cracks and rose to the surface during earthquake tremors, causing a sandblasting water phenomenon. • Ionic concentration of spring water showed anomalies with Menyuan M S 6.9 earthquake. • Two distinct causes of hydro-chemical anomalies revealed. • Bubbling CO 2 originated mainly from metamorphic and weathering carbonate in the crust. [ABSTRACT FROM AUTHOR]

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. Image 1 • Iron speciation in response to redox conditions was used for the first time as a tracer of geofluid migration. • Geofluids containing hydrocarbons modify mineralogical and chemical features of host rocks. • Heterogeneous alteration reveals geofluids tend to migrate through sandy veins within clay sequence. [ABSTRACT FROM AUTHOR]