Spectral and geochemical characterization of onshore hydrocarbon seep-induced alteration in the Dezful Embayment, SW Iran

The presence of hydrocarbon seeps at the surface is indirect evidence of the presence of mature source rocks within a geologi- cal system at depth. Chemical changes in the environment of sur- face rocks caused by hydrocarbon seeps cause mineralogical alterations. To determine the nature of the alterations and the influences of lithology and type of seep, rock samples were col- lected from altered and unaltered evaporite and marly limestone formations in the Dezful embayment, southwest Iran. Reflectance spectroscopy, bulk rock/wet chemical analyses, and sulfur, carbon, and oxygen isotopic analyses were used to delin- eate surficial alterations and relate alterations to hydrocarbons seeping from underlying reservoirs. In addition, the boosted regression trees (BRT) method was used to predict the presence of alterations from spectral indices. Comparisons of geochemical data and spectral data of altered evaporites and altered marly lime- stones showed that the minerals within alteration facies have dis- tinctive spectral, chemical, and isotopic signatures. Gas-induced alterations were characterized by the formation of gypsum and native sulfur and depletion in 34S. The released H2S in natural gas reacted with gypsum in the evaporite sediments and calcite in the marly limestone formations, which led to precipitation of secondary gypsum and native sulfur. Oil-induced alterations were characterized by formation of secondary calcite and depletion in 13C. The oxidation of seeping oil and reactions between this oil and host rocks caused precipitation of secondary calcite within both formations. The combination of fieldwork data and spectral-geochemical data showed a connection exists between surficial alterations and underlying petroleum reservoirs, which can be used in exploration campaigns.